July
2017
Updated: 17 July 2017
Welcome to the night skies of Winter, featuring Virgo, Carina, Crux, Centaurus, Scorpius, Sagittarius, Jupiter and Saturn
Note: Some parts of this webpage may be formatted incorrectly by older browsers.
The Alluna RC-20 Ritchey Chrétien telescope was installed in March, 2016.
The telescope is able to locate and track any sky object (including Earth satellites and the International Space Station) with software called TheSkyX Professional, into which is embedded a unique T-Point model developed for our site with our equipment over the past year.
Explanatory Notes:
Rise and set times are given for the theoretical horizon, which is a flat horizon all the
way round the compass, with no mountains, hills, trees or buildings to obscure the view. Observers will have to make allowance for their own actual
horizon.
Transient phenomena are provided for the current month and the next. Geocentric phenomena are calculated as if the Earth were fixed in space as the ancient Greeks believed. This viewpoint is useful, as otherwise rising and setting times would be meaningless. In the list of geocentric events, the nearer object is given first.
When a planet is referred to as ‘stationary’, it means that its movement across the stellar background appears to have ceased, not that
the planet itself has stopped. With inferior planets (those inside the Earth’s orbit, Mercury and Venus), this is caused by the planet heading either directly
towards or directly away from the Earth. With superior planets (Mars out to Pluto), this phenomenon is caused by the planet either beginning or ending its
retrograde loop due to the Earth’s overtaking it.
Apogee and perigee: Maximum and minimum distances of the Moon or artificial satellite from the Earth.
Aphelion and perihelion: Maximum and minimum distances of a planet, asteroid or comet from the Sun.
A handspan at arm's length covers an angle of approximately
18 - 20 degrees.
mv = visual magnitude or brightness. Magnitude 1 stars are very bright, magnitude 2 less so, and
magnitude 6 stars are so faint that the unaided eye can only just detect them under good, dark conditions. Binoculars will allow us to see down to magnitude
8, and the Observatory telescope can reach visual magnitude 17 or 22 photographically. The world's biggest telescopes have detected stars and galaxies as faint as
magnitude 30. The sixteen very brightest stars are assigned magnitudes of 0 or even -1. The brightest star, Sirius, has a magnitude of -1.44. Jupiter can reach -2.4, and
Venus can be more than 6 times brighter at magnitude -4.7, bright enough to cast shadows. The Full Moon can reach magnitude -12 and the
overhead Sun is magnitude -26.5. Each
magnitude step is 2.51 times brighter or fainter than the next one, i.e. a magnitude 3.0 star is 2.51 times brighter than a magnitude 4.0. Magnitude 1.0
stars are exactly 100 times brighter than magnitude 6.0 (5 steps each of 2.51 times, 2.51x2.51x2.51x2.51x2.51 = 2.515 = 100).
The Four Minute Rule:
How long does it take the Earth to complete one rotation? No, it's not 24 hours - that is the time taken for the Sun to cross the meridian on successive days. (The meridian is an imaginary semicircular line running from the due south point on the horizon and arching overhead through the zenith, and coming down to the horizon again at its due north point.) This 24 hours is a little longer than one complete rotation, as the curve in the Earth's orbit means that it needs to turn a fraction more (~1 degree of angle) in order for the Sun to cross the meridian again. It is called a 'solar day'. The stars, clusters, nebulae and galaxies are so distant that most appear to have fixed positions in the night sky on a human time-scale, and for a star to return to the same point in the sky relative to a fixed observer takes 23 hours 56 minutes 4.0916 seconds. This is the time taken for the Earth to complete exactly one rotation, and is called a 'sidereal day'.
As our clocks and lives are organised to run on solar days of 24 hours, and the stars circulate in 23 hours 56 minutes approximately, there is a four minute difference between the movement of the Sun and the movement of the stars. This causes the following phenomena:
1. The Sun slowly moves in the sky relative to the stars by four minutes of time or one degree of angle per day. Over the course of a year it moves ~4 minutes X 365 days = 24 hours, and ~1 degree X 365 = 360 degrees or a complete circle. Together, both these facts mean that after the course of a year the Sun returns to exactly the same position relative to the stars, ready for the whole process to begin again.
2. For a given clock time, say 8:00 pm, the stars on consecutive evenings are ~4 minutes or ~1 degree further on than they were the previous night. This means that the stars, as well as their nightly movement caused by the Earth's rotation, also drift further west for a given time as the weeks pass. The stars of autumn, such as Orion are lost below the western horizon by mid-June, and new constellations, such as Sagittarius, have appeared in the east. The stars change with the seasons, and after a year, they are all back where they started, thanks to the Earth's having completed a revolution of the Sun and returned to its theoretical starting point.
We can therefore say that the star patterns we see in the sky at 11:00 pm tonight will be identical to those we see at 10:32 pm this day next week (4 minutes X 7 = 28 minutes earlier), and will be identical to those of 9:00 pm this date next month or 7:00 pm the month after. All the above also includes the Moon and planets, but their movements are made more complicated, for as well as the Four Minute Drift with the stars, they also drift at different rates against the starry background, the closest ones drifting the fastest (such as the Moon or Venus), and the most distant ones (such as Saturn or Neptune) moving the slowest.
Solar System
Sun:
The Sun begins the month in the constellation of Gemini the Twins. It leaves Gemini and passes into Cancer, the Crab on July 20.
Partial Lunar Eclipse, August 8:
This eclipse of the Moon is partial - only about one fifth of the Full Moon will be immersed in the umbra (the darkest part of the Earth's shadow). The eclipse will begin at 1:49 am when the Moon enters the faint penumbra. This part of the eclipse is only noticeable to careful observers, as the Moon's light is only slightly dimmed. The Moon will begin to enter the umbra at 3:22 am - this is easily seen as a 'bite' out of the Full Moon. The maximum phase of the eclipse will occur at 4:23 am, and the Moon will leave the umbra at 5:18 am Moonset will occur at 6:26 am, and the penumbral phase of the eclipse will end at 6:54 am. This event will be visible across the western Pacific including Australia and New Zealand, Asia, Europe and Africa. It is perfectly safe to watch lunar eclipses as they occur at night. Solar eclipses are the dangerous ones, for looking at the Sun without special protection can ruin your eyesight.
Total Solar Eclipse, August 21 (USA time):
This eclipse of the Sun is total over the whole of the United States but no other country. The path of totality begins in the North Pacific Ocean between the Hawaii and the Aleutian Islands of Alaska. It proceeds in an east-south-easterly direction, reaching the continental USA just south of Portland, Oregon. The Moon's shadow continues over Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina and then passes over the North Atlantic, the eclipse ending in mid-ocean.
The eclipse will be perfectly timed for observers, lasting a little over two and a half hours in mid-morning in Oregon, and a little over three hours inmid-afternoon in South Carolina. 12 million people live within the path of totality, and 25 million within a day's drive of it, so NASA has sent out traffic warnings. The totality of this eclipse is unfortunately fairly short, averaging between two and two and a half minutes. The centre of the eclipse track is near the town of Hopkinsville in Kentucky, which will enjoy 2 minutes 40 seconds of darkness at 1:24 pm. Four minutes later, Nashville in Tennessee will also experience totality. The further east in the United States you are, the more likely it is that clouds may interfere with the eclipse.
This event will be seen as a partial eclipse from Hawaii, Alaska, Canada, Central America, Equador, the Caribbean Islands, countries in South America that border the Caribbean, and north Brazil. Ireland, the United Kingdom, France, Portugal and Spain may catch a glimpse just before sunset. No part of the eclipse will be observable from Africa, most of Europe, Asia or Australia.
First Quarter: July 01
10:52 hrs diameter = 30.4'
Full Moon:
July
09 14:08 hrs
diameter = 29.7'
Last Quarter: July 17
05:27 hrs diameter =
31.8'
New Moon:
July
23 19:46 hrs diameter =
32.9'
First Quarter: July 31
01:24 hrs diameter = 29.9'
Full Moon:
August 08 04:12 hrs
diameter = 30.3' Lunar Eclipse
Last Quarter: August 15 11:16 hrs diameter =
32.2'
New Moon:
August 22 04:31 hrs diameter = 32.1'
Solar Eclipse
First Quarter:
August 29 18:14 hrs diameter = 29.6'
Lunar Orbital Elements:
July 06:
Moon at apogee (405 943 km) at 05:33 hrs, diameter = 29.4'
July 12: Moon at descending node at
15:16 hrs, diameter = 30.3'
July 22: Moon at perigee (361
246
km) at 03:21 hrs, diameter = 33.1'
July 25: Moon at ascending node at
10:46 hrs, diameter = 32.3'
August
03:
Moon at apogee (405 053 km) at 03:46 hrs, diameter = 29.5'
August
08: Moon at descending node at
20:54 hrs, diameter = 30.4'
August
18: Moon at perigee (366
120
km) at 22:56 hrs, diameter = 32.6'
August
21: Moon at ascending node at
20:35 hrs, diameter = 32.2'
August
30:
Moon at apogee (404 313 km) at 21:51 hrs, diameter = 29.6'
Moon at 9 days after New, as on July 03
The two photographs above show the Mare
Imbrium area in the Moon's northern hemisphere. They were taken a day apart, just after First Quarter. Mare Imbrium (the Sea of Rains) is a large lava flow
caused by the Imbrium Event - a cataclysmic collision of an asteroid with the Moon many millions of years ago. A comparison of the two photographs will show
how the appearance of lunar features changes with the angle of the Sun. In the first photograph, Mare Imbrium (left) is separated from Mare Serenitatis (right) by two ranges of mountains, the Alps to
the north and the Apennines to the south. Two large craters at upper right are Aristoteles and Eudoxus. The straight Alpine
Valley may be seen cutting through the Alps. Mt Piton (height 2000 metres) is visible as a bright spot with a shadow, due south of the southern end
of the Alpine Valley. Archimedes is the large crater at left. It is a walled plain 80 kilometres in diameter with a flat floor. To its right are two
bowl-shaped craters, Aristillus and Autolycus. These craters are all formed by impact with large meteors. Apollo 15 landed close by the
Apennines, in a small enclosed area to the right and below Archimedes, on the picture's central vertical axis. In the second photograph, the sunrise line (called the 'terminator') has moved to the left, revealing a large walled plain in
the Alps, known as Plato. South of Plato, an isolated mountain protruding through the lava flow is called Mt Pico. Ripples in the lava, called
'wrinkle ridges', are visible. The crater at lower left is Timocharis, 42 kilometres in diameter. A detailed map of the Moon's near side is available here. A rotatable view of the Moon, with ability to zoom in close to the surface, and giving detailed
information on each feature, may be downloaded here.
Click
Lunar Feature for this Month
Each month we describe a lunar crater, cluster of craters,
valley, mountain range or other object, chosen at random, but one with interesting attributes. A recent photograph from our Alluna RC20 telescope will illustrate the object. As all large lunar objects are named, the origin of the name will be given. This month we will describe the large walled-plain named Plato.
This area was photographed from Starfield Observatory, Nambour on October 10,
2017. East (where the Sun is rising) is to the right, north is to the top. The area is dominated by the
large oval formation at centre, which was named by Riccioli and Grimaldi in 1651
after the ancient Greek philosopher Plato.
The rectangle shows the location of Plato on the Moon.
Plato is a huge walled plain with a diameter of 104 kilometres. As it lies at lunar latitude 51 degrees North, we see Plato greatly foreshortened, but if we could see it
from directly above, it would be almost perfectly circular. It has struck the Moon in a rugged, mountainous area called The Alps, lying between the Mare Frigoris (Sea of
Cold) and Mare Imbrium (Sea of Rains). The floor of Plato is remarkably flat, and is evidently the solidified surface of a great lake of molten lava, which welled up and
half-filled the crater after the initial impact fractured the underlying bedrock. In the western half there is a very low lava dome or shield volcano that is 30 kilometres
wide and 40 kilometres long. The dark floor is dotted with dozens of small craterlets. The five largest of them can be detected above – they average two kilometres in
diameter and have interior shadows. The bright walls average 2000 metres in height, with some peaks being much higher. Under a low Sun, the peaks throw long shadows across the floor. The
western wall exhibits a huge landslip, where a large mass has slumped down and now intrudes upon the floor. Two smaller landslides can be seen in the
north-western wall. A narrow but prominent valley leads from the interior floor through the south-eastern wall, then immediately turns south-west and runs parallel to the
wall, eventually reaching the plain of Mare Imbrium.
To the south-west of Plato, in the lower left corner of the image, is a bright mountain range protruding from the cooled lavas of the Mare Imbrium. It is called the Montes Teneriffe or Teneriffe Mountains (after the volcanoes that rise out of the Atlantic Ocean to form the Canary Islands). Two of the mountains in the northern spur exceed 2000 metres in height, and the furthest one to the east reaches 2500 metres.
Naming the Moon
As far as we know, the first person to name lunar features was William Gilbert (1544-1603), an Englishman, who made a sketch in 1600 of the Moon, showing dark and light areas. He assumed that the dark areas were land, and the light ones were seas, an assumption that he shared with Leonardo da Vinci and the young Kepler, but which was the opposite to most other observers, who thought that the 'seas' were the dark areas. Gilbert invented thirteen names for features in his drawing, which actually are familiar places on the Moon today. One dark oval area he named "Brittannia" (sic), thinking it was an island. It was later named "Mare Crisium", as later observers considered it to be a 'sea'. Gilbert died of bubonic plague aged 59 in 1603, five years before the first telescopes made their appearance. His naked-eye drawing was not published until 1651, by which time it was merely a curiosity. However, Gilbert's real claim to fame is for his work on magnetism, and as one of the originators of the word 'electricity'.
The first realistic map of the Moon, made with a good quality telescope, was produced by Michael van Langren (Langrenus, 1598-1675) in 1645. He used the names of famous and historic people to identify lunar features, and some large dark areas were named after places on Earth, two being the Belgian Sea and the Caspian Sea. Only three of his names are still in use, for the craters Endymion, Pythagoras and Langrenus (which he named after himself).
Only two years later, 1647, Johannes Hevelius (1611-1687) in East Prussia produced a book, Selenographia, which contained maps of the Moon made with a 3.6 metres long telescope which could magnify 50 times. He also showed maps of the Moon at different phases. The maps were hand-tinted, and the colours chosen seem to indicate that his telescope was unable to convince him that the ‘seas’ on the Moon were not really water, for he has had them coloured blue. This project took four years, and entitles him to be called a founder of lunar cartography. He began this work before van Langren's map was published, and may not have been aware of its existence or of van Langren's names for lunar features.
Hevelius decided to name all of the lunar features after places on the Earth, but in their classical Latin and Greek forms. Why did he choose the names he did? People had argued since antiquity that the Moon is a mirror of the Earth. They had believed the patterns of dark and light on the Moon’s surface to be seas and continents that paralleled those on the Earth. Just as humans, animals and plants inhabit the Earth, so ‘selenites’ and other life-forms were presumed to inhabit the Moon. This parallel between the Earth and the Moon was important to Hevelius, and he compiled his lunar maps from multiple, detailed telescopic observations, adopting some of the symbolic conventions of geographical mapping. He did not seem to fully understand the interplay of light and shadow on the Moon’s surface, and mapped the shadow-casting ridges and crater rims as if they were chains of terrestrial mountains. For some reason, he called most hollowed-out depressions (craters) ‘Mons’, meaning ‘mountains’, e.g. the crater Tycho was named Sinai Mons (Mount Sinai), the crater Copernicus was named Mons Ætna (Mount Etna), and the crater Ptolemæus was named Mons Sipylus. The three terrestrial mountains just named are all volcanic in origin, but only Mount Etna has an actual crater, which may be why he chose it for the great formation of Copernicus. He called the dark crater-plain Plato illustrated above ‘Lacus Niger Major’ (Large Black Lake), which is an apt description given his 50x telescope.
On his map of the Moon's western side he shows ‘M. Ætna’ (Mt Etna) in the island of ‘Sicilia’ (Sicily), sitting in the middle of the ‘Mare Mediterraneum’ (Mediterranean Sea). The islands of Corsica and Sardinia are nearby, with Majorca and Minorca not far away. At the southern end of the Mare Mediterraneum, we see ‘Mare Ægyptiacum’ (Egyptian Sea), with ‘Fl. Nilus’ (‘Fluvius Nilus’, the Nile River), flowing into it as three streams. The islands of Crete and Cyprus are also shown. By applying familiar names to the lunar features, perhaps Hevelius aimed to make it a more understandable and recognisable place, and to reduce any strangeness the observer might experience.
Like van Langren before him, Hevelius exhorted other astronomers to adopt his system of naming lunar landmarks, and threatened litigation if anyone else should devise different systems of names in opposition to his. However, this was an empty threat, as there was no legal requirement for any competitor to comply with such requests.
Giovanni Battista Riccioli
(1598-1671) discovered the first binary star (Mizar in Ursa Major) in 1650. He worked with his former pupil Francesco Grimaldi (1618-1663) at Bologna. Both were Jesuit clerics in the Catholic Church. They shared an interest in astronomy, and decided to work together in producing an accurate map of the Moon, among other things. This map, or selenograph was the result of Grimaldi’s telescopic study of the Moon and was drawn by himself. It is fairly accurate, although for some reason the spectacular crater Cassini is missing.Riccioli, with some help from Grimaldi, contributed a new series of names for lunar features, in which mountains and valleys were named as such, and the bowl-shaped depressions (not yet called ‘craters’) were named after famous astronomers and philosophers, some still alive. The 'seas' ('maria') were named after various meteorological ideas of the time, for it was thought that the Moon influenced the weather. We still use Riccioli’s names today, and those of Langrenus and Hevelius are mostly dispensed with, but there are some exceptions.
Riccioli published the map in his two-volume work on astronomy entitled Almagestum novum, astronomiam veterem novamque complectens
or New Almagest, in 1651. By necessity, as an official of the Catholic Church, when writing about astronomical matters he had to comply with Church doctrine. He therefore opposed the Sun-centred world view of Copernicus, Kepler and Galileo, though he ventured to praise the heliocentric theory as a useful hypothesis for calculating planetary positions. Riccioli and Grimaldi experimented with falling bodies in an attempt to refute Galileo’s findings. Instead, they found that Galileo was correct, which impressed them greatly.It appears that, when they began assigning names to the lunar craters, Riccioli and Grimaldi had a hidden agenda. Despite their professed opposition to Copernicus’ theory, they named the Moon’s most spectacular crater after him, and a nearby prominent crater after Kepler. Another relatively close crater was named after Aristarchus, the originator of the heliocentric theory. Not far away, they named a crater after Galileo. Twelve craters were named after other Jesuit astronomers, such as Bettinus, Clavius, Curtius, Cysatus, Furnerius, Gruemberger, Kircher, Moretus, Scheiner, Schomberger, Simpelius, and Zucchius, but they are in a far-off part of the Moon, near its south pole and the crater Tycho, as the Society of Jesus supported the Tychonic world view. When questioned, they said that they had “flung the heliocentrists away into the Ocean of Storms”, far from the Jesuits.
But Riccioli and Grimaldi named two large adjoining craters near the Moon’s limb after themselves. Where are those craters? Not in the far south of the Moon with other Jesuits, but near its Equator, bordering the Oceanus Procellarum (Ocean of Storms), close to the crater Galilæus and in the same quadrant of the Moon as Copernicus, Kepler and Aristarchus. This is considered to be covert support for the Copernican theory, which as Jesuits they knew that they could not publicly express.
As telescopes improved, smaller and smaller features became detectable, and so later astronomers such as Schröter, Mädler, Birt, Lee, Neison, Schmidt, Franz, Krieger, König, Fauth, Lamèch and Wilkins applied new names to them. With the mapping of the far side by satellites, official bodies such as NASA were allowed to name features, as long as they abided by rules laid down by the International Astronomical Union or IAU.
Plato
Plato (ca. 429-348 BCE, a student and friend of Socrates, described the spherical nature of the Earth in one of his Dialogues, Timaeus. He also
taught that the world (universe) was composed of tiny atoms comprising four fundamental elements – earth, water, air and fire,
with the least heavenly element, earth, at the bottom. He said that water flowed over the earth, air floated above both, and fire rose through the air to the
sphere of the Moon, above which the planets and stars moved within a fifth element, the quintessence or æther.;
The word ‘æther’ in Homeric Greek means ‘ever-moving, pure, fresh air’ or ‘clear sky’, and was believed in Greek mythology to be the pure essence where the gods lived
and which they breathed, different from the impure air breathed by mortals which only reached as high as the Moon. On rare occasions the æther was seen as
a glow around the Sun’s disc during solar eclipses. Its name comes down to us today in the words ‘ethereal’ (‘heavenly’) and ‘ethernet’. Æther had no qualities at all (was
neither hot, cold, wet nor dry), was inviolate, everlasting and incapable of change (with the exception of change of place), and by its nature moved in perfect circles.
It was also personified as a god itself, Æther, the son of Erebus and Nyx.
Plato taught
that the seven planets (which included the Sun and Moon) and the fixed stars revolved around the Earth on eight crystalline spheres, as first postulated by
Pythagoras. The spheres were made of a solid, transparent form of quintessence or æther, and the planets and stars were denser, luminous nodules on the
spheres. Gaseous quintessence enclosed all the spheres and was perfect and immutable. The celestial bodies moved forever in perfect circles at constant speed. Like others of his time, Plato said that seeing was not necessarily believing: the eye could be deceived – optical illusions proved that.
Only the human mind could reveal how things really were, by the application of logic and reasoning to create mathematical models of reality. This was the basis for his
acceptance of a system of concentric, transparent spheres for carrying the planets. Plutarch, a student at Plato’s Academy some 500 years later, tells us that, in his old age, Plato changed his geocentric beliefs and
entertained some of the Pythagoreans’ latest ideas. He came to accept that the diurnal (daily) movement of the heavenly bodies was due to the axial rotation of the Earth
and was interested in Philolaus’ concept that the Earth orbited a central fire. Maybe he even thought that Heracleides’ idea that the Earth was one of a family of planets
orbiting a central Sun could be possible. All we really know for sure is that Plutarch quotes Plato as saying that he “repented of giving to the Earth the central place
in the universe, which did not belong to it.”
Geocentric Events for July and August:
July 1: Moon
2.8º
north
of Jupiter at 20:29 hrs
August 2: Mercury at
aphelion at 22:48 hrs (diameter = 8.2")
July 5: Earth at
aphelion (furthest distance from the Sun) at 11:13 hrs (diameter of Sun =
31.5')
July 6: Jupiter at
eastern quadrature at 12:30 hrs (diameter = 36.8")
July 7: Moon 3.8º
north
of Saturn at 12:41 hrs
July 9: Moon 2.4º
north
of the star Pi Sagittarii
(mv=2.88)
at 12:34hrs
July 9: Moon 2.8º
north
of Pluto at 15:05 hrs
July 10: Pluto at opposition at
14:09 hrs (diameter = 0.1")
July 14: Limb of Moon 13 arcminutes south of Neptune at
04:34 hrs
July 17: Moon 3.5º
south of Uranus at
12:46 hrs
July 20: Moon
1.2º
north of
the star Aldebaran (Alpha Tauri, mv= 0.87) at 10:25 hrs
July 20: Moon 2.6º
south
of Venus at
21:26 hrs
July 21: Uranus at western quadrature at 10:10 hrs (diameter = 3.5")
July 23: Moon 2.9º
south
of Mars at
22:38 hrs
July 25: Moon
1º
north of
Mercury at
20:54 hrs
July 25: Moon occults
the
star Regulus (Alpha Leonis, mv=1.36) between 21:45 and 22:14 hrs -
Moonset is at 19:16 hours on the Sunshine Coast.
July 26: Mercury 1º
south
of
the
star Regulus (Alpha Leonis, mv=1.36) at 11:54 hrs
July 27: Mars in conjunction
with the Sun at 11:22 hrs (diameter = 3.5")
July 27: Venus 23 arcminutes north of
the star Alheka (Zeta Tauri,
mv=
2.88) at 16:54 hrs
July 29: Moon 3.6º
north
of Jupiter
at 08:10 hrs
July 30: Mercury at greatest
elongation east (27º
10') at 07:24 hrs (diameter = 7.7")
August 3: Uranus at
western stationary point at 12:40 hrs (diameter = 3.6")
August 3: Moon 3.7º
north
of
Saturn at 16:20 hrs
August 5: Moon 1.9º
north of
the star Pi Sagittarii (mv=
2.88) at 18:31 hrs
August 5: Moon 2.5º
north of
Pluto at 21:38 hrs
August 10: Moon occults Neptune
between
9:43 and 10:18 hrs
August 13: Mercury at eastern
stationary point at 11:09 hrs (diameter = 9.8")
August 13: Moon 3.9º
south of Uranus at
16:52 hrs
August 16: Limb of Moon 21
arcminutes north of
the star Aldebaran (Alpha Tauri, mv= 0.87) at 17:22 hrs
August 19: Moon 1.9º
south
of Venus at
16:02 hrs
August 21: Moon 1.1º
south
of Mars at 15:38
hrs
August 22: Moon 6º
north
of Mercury at
19:58 hrs
August 25: Saturn at eastern
stationary point at 20:30 hrs (diameter = 17.1")
August 26: Moon 3.6º
north
of Jupiter at
01:39 hrs
August 27: Mercury at inferior
conjunction at 06:35 hrs (diameter = 10.8")
August 31: Moon 3.9º
north
of Saturn at
02:05 hrs
Mercury:
On July 1, Mercury will be in the west-north-western twilight sky, but is too close to the Sun to be observed safely. By mid-month it should be observable with binoculars, and then as a naked-eye object as the end of the month approaches. It will be reasonably easy to find on July 25, as it will be just two degrees above the thin crescent Moon, with the first magnitude star Regulus in the constellation Leo only one degree to the north-east.
Venus:
This, the brightest planet, is now dominating the pre-dawn sky as a 'morning star', and for the next five months will be noticeable to even the most casual observer, rising in the east before the Sun. At the beginning of July Venus will appear in a small telescope as a tiny 'gibbous Moon' with a magnitude of -4.1 and an angular size of 18 arcseconds. Its phase will be 62.5%. By the end of July its phase will have increased to 74.4% but its diameter will decrease to 15 arcseconds as it moves further away from us. Its brightness will drop slightly to about -4.0.On the mornings of July 20 and 21, the waning crescent Moon will be near to Venus in the sky.
(The coloured fringes to the first and third images below are due to refractive effects in our own atmosphere, and are not intrinsic to Venus. The planet was closer to the horizon when these images were taken than it was for the second photograph, which was taken when Venus was at its greatest elongation from the Sun).
April 2017 June 2017 December 2017
Click here for a photographic animation showing the Venusian phases. Venus is always far brighter than anything else in the sky except for the Sun and Moon. For the first two months of 2017, Venus appeared as an 'Evening Star', but on March 25 it moved to the pre-dawn sky and became a 'Morning Star'. Each of these appearances lasts about eight to nine months. Venus will pass on the far side of the Sun (superior conjunction) on January 9, 2018, when it will return to the evening sky and become an 'Evening Star' once again.
Because Venus was visible as the 'Evening Star' and as the 'Morning Star', astronomers of ancient times believed that it was two different objects. They called it Hesperus when it appeared in the evening sky and Phosphorus when it was seen before dawn. They also realised that these objects moved with respect to the so-called 'fixed stars' and so were not really stars themselves, but planets (from the Greek word for 'wanderers'). When it was finally realised that the two objects were one and the same, the two names were dropped and the Greeks applied a new name Aphrodite (Goddess of Love) to the planet, to counter Ares (God of War). We use the Roman versions of these names, Venus and Mars, for these two planets.
Mars:
Having passed through opposition on May 22 last year, the red planet continues to shrink and fade as the speeding Earth leaves it behind. At opposition it reached magnitude -2, rivalling Jupiter in brightness, but by July 1 it has faded to 1.7 (one thirtieth as bright). In the same period, its apparent size has shrunk from 18.4 arcseconds to 4 arcseconds. It is on July 1 only nine degrees (half-a-handspan) from the Sun, and lost in the solar glare. It is therefore impossible to observe safely. Mars is now at the far side of its orbit, about as far away as it can get, and will reach conjunction with the Sun on July 27.
In this image, the south polar cap of Mars is easily seen. Above it is a dark triangular area known as Syrtis Major. Dark Sinus Sabaeus runs off to the left, just south of the equator. Between the south polar cap and the equator is a large desert called Hellas. The desert to upper left is known as Aeria, and that to the north-east of Syrtis Major is called Isidis Regio. Photograph taken in 1971.
Mars
photographed from Starfield Observatory, Nambour on June 29 and July 9, 2016, showing two different
sides of the planet. The north polar cap is prominent.
Brilliant Mars at left, shining at magnitude 0.9, passes in front of the dark molecular clouds in Sagittarius on October 15, 2014. At the top margin is the white fourth magnitude star 44 Ophiuchi. Its type is A3 IV:m. Below it and to the left is another star, less bright and orange in colour. This is the sixth magnitude star SAO 185374, and its type is K0 III. To the right (north) of this star is a dark molecular cloud named B74. A line of more dark clouds wends its way down through the image to a small, extremely dense cloud, B68, just right of centre at the bottom margin. In the lower right-hand corner is a long dark cloud shaped like a figure 5. This is the Snake Nebula, B72. Above the Snake is a larger cloud, B77. These dark clouds were discovered by Edward Emerson Barnard at Mount Wilson in 1905. He catalogued 370 of them, hence the initial 'B'. The bright centre of our Galaxy is behind these dark clouds, and is hidden from view. If the clouds were not there, the galactic centre would be so bright that it would turn night into day.
Jupiter:
This gas giant planet is now a spectacular evening object as it passed through opposition (directly opposite the Sun in the sky) on April 8. This month it may be easily seen high in the sky, being about one-and-a-half handspans west-north-west of the zenith as darkness falls, at mid-month. It is in the constellation Virgo, north-west of the first magnitude star Spica. The crescent Moon will be close to Jupiter as night falls on July 1, 28 and 29.
Jupiter as
photographed from Nambour on the evening of April 25, 2017. The images were
taken, from left to right, at 9:10, 9:23, 9:49, 10:06 and 10:37 pm. The rapid
rotation of this giant planet in a little under 10 hours is clearly seen. In the
southern hemisphere, the Great Red Spot (bigger than the Earth) is prominent,
sitting within a 'bay' in the South Tropical Belt. South of it is one of the
numerous White Spots. Between the darker tropical belts is the light-coloured
Equatorial Belt. It is crossed with numerous dark festoons which, like the bands
themselves, make
ever-changing patterns. All of these are features in the cloud tops of Jupiter's
atmosphere.
Saturn:
The ringed planet is visible all night long this
month, as it reached opposition on June 15. It
will be visible about three handspans above the eastern horizon as soon as darkness falls, underneath the huge
S-shaped curve of Scorpius, the Scorpion. The almost-full Moon will be just
underneath Saturn on July 7.
Jupiter as
it appeared at 7:29 pm on July 2, 2017. The Great Red Spot is in a similar
position near Jupiter's eastern limb (edge) as in the fifth picture in the series
above. It will be seen that in the past two months the position of the Spot has drifted when compared
with the festoons in the Equatorial Belt, so must rotate around the planet at a
slower rate. In fact, the Belt enclosing the Great Red Spot rotates around the
planet in 9 hours 55 minutes, and the Equatorial Belt takes five minutes less.
This high rate of rotation has made the planet quite oblate. The prominent 'bay'
around the Red Spot in the five earlier images appears to be disappearing, and a
darker streak along the northern edge of the South Tropical Belt is moving
south. Two new white spots have developed in the South Temperate Belt, west of
the Red Spot. The five upper images were taken near opposition, when the Sun was
directly behind the Earth and illuminating all of Jupiter's disc evenly. The
July 2 image was taken just four days before Eastern Quadrature, when the angle
from the Sun to Jupiter and back to the Earth was at its maximum size. This
angle means that we see a tiny amount of Jupiter's dark side, the shadow being
visible around the limb of the planet on the left-hand side, whereas the
right-hand limb is clear and sharp. Three of Jupiter's Galilean satellites are visible, Ganymede to the left and Europa to the right. The satellite Io can be detected in a transit of Jupiter,
sitting in front of the North Tropical Belt, just to the left of its centre.
Saturn with its Rings wide open on July 2, 2017. The shadow of its globe can be
seen on the far side of the Ring system. There are three main concentric rings:
Ring A is the outermost, and is separated from the brighter Ring B by a dark gap
known as the Cassini Division, which is 4800 kilometres wide, enough to drop
Australia through.
Uranus:
This ice giant planet is an early morning object in July, as it reached conjunction with the Sun on April 14 and at mid-month rises just after midnight. Uranus shines at about magnitude 5.8, so a pair of binoculars or a small telescope is required to observe it. It is currently in the constellation Pisces, near the boundary with Aries. The Last Quarter Moon will be in the vicinity of Uranus on July 17.
Neptune:
The icy blue planet is a morning object this month. It reached western quadrature on June 5, which means that on that date it rose at midnight. Neptune is located in the constellation of Aquarius, between the magnitude 3 star Skat (Delta Aquarii) and the four-star asterism known as the Water-Jar. As it shines at about magnitude 8, a small telescope is required to observe Neptune. The waning gibbous Moon will be extremely close to Neptune in the early hours of July 14.
Neptune, photographed from Nambour on October 31, 2008
Pluto:
The erstwhile ninth and most distant planet can be observed almost all night this month, as it will reach opposition on July 10, when it rises at sunset. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth that of Neptune. Located inside the 'Teaspoon' which is north-east of the Sagittarius 'Teapot', it is a faint 14.1 magnitude object near the centre of Sagittarius. A telescope with an aperture of 25 cm or more is necessary to observe Pluto.
The movement of the dwarf planet Pluto in two days, between 13 and 15 September, 2008. Pluto is the one object that has moved.
Width of field: 200 arcseconds
This is a stack of four images, showing the movement of Pluto over the period October 22 to 25, 2014. Pluto's image for each date appears as a star-like point at the upper right corner of the numerals. The four are equidistant points on an almost-straight line. Four eleventh magnitude field stars are identified. A is GSC 6292:20, mv = 11.6. B is GSC 6288:1587, mv = 11.9. C is GSC 6292:171, mv = 11.2.
D is GSC 6292:36, mv = 11.5. (GSC = Guide Star Catalogue). The position of Pluto on October 24 (centre of image) was at Right Ascension = 18 hours 48 minutes 13 seconds, Declination = -20º 39' 11". The planet moved 2' 51" with respect to the stellar background during the three days between the first and last images, or 57 arcseconds per day, or 1 arcsecond every 25¼ minutes.
Meteor Showers:
Pegasids
July 10
Full Moon, 100%
sunlit ZHR = 8 S Delta Aquarids
July 29
Waxing crescent Moon, 33%
sunlit ZHR = 20 Alpha Capricornids
July 30
Waxing crescent Moon, 42% sunlit ZHR =
8 Use
Radiant: Near the star Markab
Radiant:
Between the stars Skat and Deneb Algedi
Radiant: Near the star Algedi
Although most meteors are found in swarms associated with debris from comets, there are numerous 'loners', meteors travelling on solitary
paths through space. When these enter our atmosphere, unannounced and at any time, they are known as 'sporadics'. Oan average clear and dark evening, an
observer can expect to see about ten meteors per hour. They burn up to ash in their passage through our atmosphere. The ash slowly settles to the ground as
meteoric dust. The Earth gains about 80 tonnes of such dust every day, so a percentage of the soil we walk on is actually interplanetary in origin. If a
meteor survives its passage through the air and reaches the ground, it is called a 'meteorite'. In the past, large meteorites (possibly comet nuclei or small asteroids) collided with the Earth and produced huge craters which still exist
today. These craters are called 'astroblemes'. Two famous ones in Australia are Wolfe Creek Crater and Gosse's Bluff. The Moon and Mercury are covered with such
astroblemes, and craters are also found on Venus, Mars, planetary satellites, minor planets, asteroids and even comets.
ZHR = zenithal hourly rate (number of meteors expected to be observed at the zenith in one hour). The
maximum phase of meteor showers usually occurs between 3 am and sunrise. The reason most meteors are observed in the pre-dawn hours is because at that time
we are on the front of the Earth as it rushes through space at 107 000 km per hour (30 km per second). We are meeting the meteors head-on, and the speed at
which they enter our atmosphere is the sum of their own speed plus ours. In the evenings, we are on the rear side of the Earth, and many meteors we see at that
time are actually having to catch us up. This means that the speed at which they enter our atmosphere is less than in the morning hours, and they burn up less
brilliantly.
Comets:
Comet Lulin
This comet, (C/2007 N3), discovered in 2007 at Lulin Observatory by a collaborative team of Taiwanese and Chinese astronomers, is now in the outer Solar System, and has faded below magnitude 15.
The
LINEARNearly all of these programs are based in the northern hemisphere, leaving gaps in the coverage of the southern sky. These gaps are the areas of sky where amateur astronomers look for comets from their backyard observatories.
To find out more about current comets, including finder charts showing exact positions and magnitudes, click
here. To see pictures of these comets, click here.
The 3.9 metre Anglo-Australian Telescope (AAT) at the Australian Astronomical Observatory near Coonabarabran, NSW.
Deep Space
Sky Charts and Maps available on-line:
There are some useful representations of the sky available here. The sky charts linked
below show the sky as it appears to the unaided eye. Stars rise four minutes earlier each night, so at the end of a week the stars have gained about half an
hour. After a month they have gained two hours. In other words, the stars that were positioned in the sky at 8 pm at the beginning of a month will have the
same positions at 6 pm by the end of that month. After 12 months the stars have gained 12 x 2 hours = 24 hours = 1 day, so after a year the stars have returned
to their original positions for the chosen time. This accounts for the slow changing of the starry sky as the seasons progress.
The following interactive sky charts are courtesy of Sky and Telescope magazine. They can simulate a view of the sky from any location on Earth at any time of day or night between the years 1600 and 2400. You can also print an all-sky map. A Java-enabled web browser is required. You will need to specify the location, date and time before the charts are generated. The accuracy of the charts will depend on your computer’s clock being set to the correct time and date.
To produce a real-time sky chart (i.e. a chart showing the sky at the instant the chart is generated), enter the name of your nearest city and the country. You will also need to enter the approximate latitude and longitude of your observing site. For the Sunshine Coast, these are:
latitude: 26.6o South longitude: 153o East
Then enter your time, by scrolling down through the list of cities to "Brisbane: UT + 10 hours". Enter this one if you are located near this city, as Nambour is. The code means that Brisbane is ten hours ahead of Universal Time (UT), which is related to Greenwich Mean Time (GMT), the time observed at longitude 0o, which passes through London, England. Click
here to generate these charts._____________________________________
Similar real-time charts can also be generated from another source, by following this second link:
Click here for a different real-time sky chart.The first, circular chart will show the full hemisphere of sky overhead. The zenith is at the centre of the circle, and the cardinal points are shown around the circumference, which marks the horizon. The chart also shows the positions of the Moon and planets at that time. As the chart is rather cluttered, click on a part of it to show that section of the sky in greater detail. Also, click on Update to make the screen concurrent with the ever-moving sky.
The stars and constellations around the horizon to an elevation of about 40o can be examined by clicking on
View horizon at this observing site
The view can be panned around the horizon, 45 degrees at a time. Scrolling down the screen will reveal tables showing setup and customising options, and an Ephemeris showing the positions of the Sun, Moon and planets, and whether they are visible at the time or not. These charts and data are from YourSky, produced by John Walker.
The charts above and the descriptions below assume that the observer has a good observing site with a low, flat horizon that is not too much obscured by buildings or trees. Detection of fainter sky objects is greatly assisted if the observer can avoid bright lights, or, ideally, travel to a dark sky site. On the Sunshine Coast, one merely has to travel a few kilometres west of the coastal strip to enjoy magnificent sky views. On the Blackall Range, simply avoid streetlights. Allow your eyes about 15 minutes to become dark-adapted, a little longer if you have been watching television. Small binoculars can provide some amazing views, and with a small telescope, the sky’s the limit.
The Eta Carinae Nebula is nearly a handspan to the right of the Southern Cross at sunset in mid-July.
The Stars and Constellations for this month:
These descriptions of the night sky are for 8 pm on July 1 and 6 pm on July 31. They start at the western horizon.
Close to the western horizon is the second magnitude star Alphard. This is an orange star that was known by Arabs in ancient times as 'The Solitary One’, as it lies in an area of sky with no bright stars nearby.
In the north-west, Leo the Lion is preparing to set. It will have completely disappeared by 10.30 pm. The bright star
Regulus (Alpha Leonis) marks the Lion’s heart. Mercury will be
about a degree from Regulus as darkness falls on July 26. A handspan to the right and above Regulus is
Denebola, a white star marking the lion's tail. It is about 30 degrees
above the north-western horizon. We see the lion upside-down from the Southern Hemisphere. Regulus is the western-most star in a pattern called 'The Sickle' (or
reaping-hook). It marks the end of the Sickle's handle, with the other end of the handle, the star
Eta Leonis, below and to the right. The blade of the Sickle
curves around clockwise from Eta Leonis to the horizon. The Sickle forms the mane and head of the lion, when observed right-way-up. The Sickle is just touching the
theoretical horizon at this time tonight.
The constellation Leo, as we see it from Australia. Regulus is above centre left, and Denebola above centre right. The Sickle curves down from Regulus.
High in the north, (about 43 degrees above the horizon, and about 10 degrees west of the meridian or north-south line) we can find the third brightest star in the night sky,
Arcturus. It is outshone only by Sirius and Canopus. Arcturus differs from those just named, for it is an obvious orange colour, a K2 star of zero magnitude.
It is a particularly beautiful star, and, as it is the brightest in the constellation of
Boötes, the Herdsman, it has the alternative name of Alpha Boötis.
(Boötes is pronounced 'Bo-oh-tees). Boötes is due north (culminating) at this time of night.
Arcturus
East of Boötes and above the north-north-eastern horizon is a fainter circle of fourth magnitude stars,
Corona Borealis, the Northern Crown. The
brightest star in the Crown is named Alphecca, and shines at magnitude 2.3.
East of the Northern Crown is Hercules, stretching from the north-north-eastern horizon upwards. Rising in the north-east is a bright white A0 star,
Vega, which is the fifth brightest star, after Sirius, Canopus, Arcturus and Alpha Centauri. Vega is the main star in the small constellation of
Lyra the Lyre, which
contains the famous Ring Nebula, M 57.
The Ring Nebula was ejected from the central star in a great explosion
About fifteen degrees (a little less than a handspan) to the right of Vega can be seen
Albireo, a beautiful double star with
contrasting colours. It is the highest star of the Northern Cross, Cygnus.
Rising above the eastern horizon is the great main-sequence star Altair. This A7 white star is the eleventh brightest in the heavens. Altair is also known as Alpha Aquilae, as it is the brightest star in the constellation of Aquila, the Eagle. It marks the heart of the eagle, and is flanked by two lesser stars marking each wing-tip, Gamma Aquilae and Beta Aquilae. This threesome, making a short horizontal line in the east, is easy to find.
Just to the west of the zenith is the next zodiacal constellation after Leo, Virgo, the Virgin. It is a large but fairly inconspicuous constellation, but it does have one bright star, Spica, which is an ellipsoidal variable star whose brightness averages magnitude 1. This star, also known as Alpha Virginis, is a hot, blue-white star of spectral type B2. It is the sixteenth brightest star, and the rest of the constellation Virgo lies to the north-west of it. Tonight, Spica is at an altitude of 65 degrees, between the zenith and Corvus. It is roughly halfway between Arcturus and the Southern Cross. For most of this year, Virgo is dominated by the presence of the brilliant planet Jupiter - it is much brighter than any of Virgo's stars. It is currently west of the star Spica, heading towards that star having completed its retrograde loop on June 9. On November 14 Jupiter will cross into Libra.
A handspan to the left of Jupiter is the constellation of Corvus the Crow. Corvus is a lopsided quadrilateral of four third magnitude stars. It is about three handspans above the western horizon. Directly overhead is the faint constellation of Libra, the Scales, the brightest stars of which are two of magnitude 2.7 with exotic names, Zuben Elgenubi and Zuben Eschamali.
Approaching the zenith is the spectacular constellation of Scorpius, the Scorpion (see below), which is very rich in objects to find with a small telescope or binoculars. This famous zodiacal constellation is like a large letter 'S', and, unlike most constellations, is easy to recognise as the shape of a scorpion. At this time of year, he has his tail down and claws raised. The brightest star in Scorpius is Antares, a red type M supergiant of magnitude 0.9. Antares is the fifteenth brightest star, and will be almost exactly overhead at 9:40 pm on July 1 (4 minutes earlier per night for succeeding nights).
Below or east of Scorpius is
Sagittarius the Archer, through which the Milky
Way passes. Sagittarius teems with stars, glowing nebulae and dust clouds, as it is in line with the centre of our galaxy.
Adjoining Sagittarius to the south (right), there is a beautiful curve of faint stars This is
Corona Australis, the Southern Crown, and it is very elegant and delicate. The brightest star in this constellation has a magnitude of only 4.1.
Below Sagittarius and above the eastern horizon is
a large constellation known as
Capricornus, the
Sea-Goat. This constellation is lacking in any bright stars, and is
fairly unremarkable.
The Trifid Nebula, M20, in Sagittarius, is composed of a reflection nebula (blue), an emission nebula (pink), and dark lanes of dust.
The body of Scorpius is at top, with the two stars in the Sting underneath, just above the centre of the picture. The red supergiant star Antares appears close to the top left corner. The stars in the lower half of the picture are in Sagittarius. Near the lower right margin is a graceful curve of fourth magnitude stars, Corona Australis, the Southern Crown.
Between Scorpius and Corona Borealis are two large but faint constellations, Serpens, the Serpent, and Ophiuchus, the Serpent Bearer. Though Ophiuchus is not as spectacular as Scorpius, this year it is favoured by the presence of the ringed planet Saturn, which is brighter than any of the stars in that part of the sky. Saturn will cross into Sagittarius on November 19, and in the week between Christmas and New Year it will skirt the Trifid Nebula (M20) and will pass across outlying stars of the open cluster, M21.
High in the south-south-west, Crux (Southern Cross) is at an altitude of 50 degrees. Crux was in a vertical position about two hours ago (6.00
pm on July 1), but now it is has tilted over to the west so that it leans at an angle of
30 degrees from the vertical. The two
Pointers, Alpha and Beta Centauri, lie to its left and form a horizontal line. The two pointers are 8 degrees apart. Alpha is the one further away from Crux. Whereas Alpha Centauri is the
nearest star system to our Sun, only 4.37 light-years distant, Beta is 390
light-years away. Alpha is composed of two Sun-like stars, but Beta Centauri is
a supergiant, which accounts for its appearing almost as bright despite being
nearly 90 times further away. If the night is dark and the skies are clear, a black dust cloud known as the
Coalsack can be seen just to the left of Acrux,
the bottom and brightest star of the Cross. Surrounding Crux on three sides is the large constellation
Centaurus, its two brightest stars being the brilliant
Alpha and Beta Centauri. The rest of the constellation of Centaurus arches over Crux to its right-hand side, where it adjoins
Carina and Vela.
At left - the two Pointers, Alpha and Beta Centauri. Centre - Crux (Southern Cross) with the dark cloud of dust known as the Coalsack at its lower left. Right - star clusters in the Milky Way and the Eta Carinae nebula.
Beta Crucis (left) and the Jewel Box cluster
Herschel's Jewel Box
Below Crux and to its left is a small, fainter quadrilateral of stars,
Musca, the Fly. Out of all the 88 constellations, it is the only insect.
Below and to the left of Alpha Centauri is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation
Triangulum Australe, the Southern
Triangle. Between Scorpius and Centaurus is an interesting constellation composed of mainly third magnitude stars,
Lupus,
the Wolf. Midway between Triangulum Australe and Scorpius is an asterism like a small, elongated triangle. This is
Ara, the Altar.
The constellations surrounding the Southern
Cross Very close to the south-south-western horizon, the star
Canopus may be glimpsed soon after darkness falls.
It will have set by 8:30 pm on July 1. You will need a flat horizon in this direction. Canopus is the second-brightest star in the sky after Sirius, the Dog Star. The path of the Milky Way between Aquila and Canopus is filled with clusters, dark clouds, glowing nebulae, multiple stars and other
interesting objects. Check it out with binoculars or a telescope. Halfway between Crux and the southern horizon is a white star of magnitude 1.7,
Miaplacidus. It is the second-brightest star in the constellation
Carina, after Canopus, so it has the alternative name of Beta Carinae. Half a handspan to the right of Miaplacidus is the
False Cross, larger and more
lopsided than the Southern Cross. The False Cross is two handspans below Crux, and is also tilted in the same way. It is about a handspan above the south-western
horizon, and will have completely set by midnight. Both of these Crosses are actually more like kites in shape, for, unlike
Cygnus (the Northern Cross, rising in
the north-north-east just before midnight on July 1), they have no star at the intersection of the two cross arms. Between the Southern Cross and the False Cross may be seen a glowing patch of light. This is
the famous Eta Carinae Nebula, which is a remarkable sight through binoculars or a small telescope working at low magnification. A photograph of this emission
nebula with dark lanes appears below. The brightest star in the nebula, Eta Carinae itself, is a peculiar unstable star which has been known to explode, becoming
very bright. It last did this in 1842, and is called a 'cataclysmic variable star', or 'recurrent nova'.
It is also an extremely large and massive star, and is a possible candidate for the next
supernova in our Galaxy. The central part of the Eta Carinae nebula, showing dark lanes, molecular clouds, and glowing clouds of fluorescing hydrogen.
The Keyhole, a dark cloud obscuring part of the Eta Carinae Nebula.
The Homunculus, a tiny planetary nebula ejected by the eruptive variable star, Eta Carinae.
Low in the south-south-west, about 10 degrees above the horizon, the
Large Magellanic Cloud (LMC)
is faintly visible as a diffuse glowing patch. It is about a handspan to the left (south) of Canopus. About a handspan to the left of the LMC is the
Small Magellanic
Cloud (SMC), a smaller glowing patch, also close to the southern horizon. From Nambour's latitude, these two clouds never set. Each day they circle the
South Celestial
Pole, which is a point in our sky 26.6 degrees above the horizon's due south point. Objects in the sky that never set are called 'circumpolar'. The LMC and SMC are
described below. Between Arcturus and Denebola, and 30 degrees above the north-western horizon, is a faint Y-shaped cluster of stars called
Coma Berenices, or
Berenice's Hair. Most of the stars in this group have a visual magnitude of about 4.5. The area of sky between Spica and Coma Berenices is called a 'galactic window'. Being well away from the plane of the Milky Way (which we can see
passing from the west-south-western horizon through the star clusters and nebulae of Carina to Crux and Centaurus high in the south, and then through Scorpius and
Sagittarius high in the east to Aquila on the east-north-eastern horizon), there are fewer stars and dust clouds to obscure our view, and we can see right out of our galaxy
into the depths of inter-galactic space. A 20 cm (eight inch) telescope can see numerous galaxies in this region, nearly thirty being brighter than twelfth magnitude. A
larger amateur telescope can detect hundreds more. Large telescopes equipped with sensitive cameras can detect millions of galaxies in this part of the sky. The line of the ecliptic along which the Sun, Moon and planets travel, passes through the following constellations this month: Leo, Virgo, Libra,
Scorpius, Sagittarius and Capricornus. The aborigines had a large constellation which is visible tonight, the
Emu. The Coalsack forms its head, with the faint sixth magnitude star in
the Coalsack, its eye. The Emu's neck is a dark lane of dust running east through the two Pointers, to Scorpius. The whole constellation
of Scorpius forms the Emu's
body. The Emu is sitting, waiting for its eggs to hatch. The eggs are the large
and bright star clouds of Sagittarius. If you would like to become familiar with the constellations, we suggest that you access one of the world's best collections of constellation
pictures by clicking
The Milky Way
A glowing band of light crossing the sky is especially noticeable during the winter months. This glow is the light of millions of faint stars combined with that coming from
glowing gas clouds called nebulae. It is concentrated along the plane of our galaxy, and this month it is seen crossing the sky, starting from the south-west in the
constellation Carina, and passing through Crux, Centaurus,
Lupus, Scorpius, Sagittarius and Scutum to Aquila and
Lyra in the north-east.
The plane of our galaxy from Scutum (at left) through Sagittarius and Scorpius (centre) to Centaurus and Crux
(right). The Eta Carinae nebula is at the right margin, below centre. The Coalsack is clearly visible, and the dark dust lanes can be seen. Taken with an
ultra-wide-angle lens. It is rewarding to scan along this band with a pair of binoculars, looking for star clusters and
emission nebulae. Dust lanes along the plane of the Milky Way appear to split it in two in some parts of the sky. One of these lanes can be easily seen, starting
near Alpha Centauri and heading towards Antares. At 10.00 pm in mid-July, the Milky Way crosses the zenith, almost dividing the sky in two. It runs from
south-west to north-east, and the very centre of our galaxy passes directly overhead.
Between Crux and the south-western horizon is a very large area of sky filled with interesting objects. This was once the constellation
Argo, named by ancient
Greeks for Jason’s famous ship used by the Argonauts in their quest for the Golden Fleece. The constellation Argo was found to be too large, so modern star atlases divide
it into three sections - Carina (the Keel), Vela (the Sails) and
Puppis (the Stern).
The centre of our galaxy. The constellations partly visible here are Sagittarius (left), Ophiuchus (above centre) and Scorpius (at right). The planet Jupiter is the bright object below centre left. This is a normal unaided-eye view, except that it reaches fainter stars than the eye can see.
The Season of the Scorpion
The spectacular constellation of Scorpius is about 45 degrees above the eastern horizon at sundown in July. Three bright stars in a gentle curve mark his head, and another three mark his body. Of this second group of three, the centre one is a bright, red supergiant, Antares. It marks the red heart of the scorpion. This star is so large that, if it swapped places with our Sun, it would engulf the Earth and extend to the orbit of Mars. It is 604 light years away and shines at magnitude 1.06. Antares, an M type star, has a faint companion which can be seen in a good amateur telescope.
The rest of the stars run around the scorpion's tail, ending with two blue-white B type stars, Shaula (the brighter of the two) and Lesath, at the root of the scorpion's sting. These two stars are closest to the eastern horizon tonight, and are near the bottom of the picture below. Above Lesath in the body of the scorpion is an optical double star, which can be seen as two with the unaided eye.
Scorpius, with its head at top left and tail (with sting) at lower right.
Probably the two constellations most easily recognisable
(apart from Crux, the Southern Cross) are Orion the Hunter and Scorpius the Scorpion. Both are large constellations containing numerous bright stars, and are very obvious
'pictures in the sky'. Both also contain a very bright red supergiant star, Betelgeuse in Orion and
Antares in Scorpius.
The red supergiant star Antares.
The star which we call Antares is a binary system. It is dominated by the great red supergiant Antares A which, if it swapped places with our Sun, would enclose all the planets out to Jupiter inside itself. Antares A is accompanied by the much smaller Antares B at a distance of between 224 and 529 AU - the estimates vary. (One AU or Astronomical Unit is the distance of the Earth from the Sun, or about 150 million kilometres.) Antares B is a bluish-white companion, which, although it is dwarfed by its huge primary, is actually a main sequence star of type B2.5V, itself substantially larger and hotter than our Sun. Antares B is difficult to observe as it is less than three arcseconds from Antares A and is swamped in the glare of its brilliant neighbour. It can be seen in the picture above, at position angle 277 degrees (almost due west or to the left) of Antares A. Seeing at the time was about IV on the Antoniadi Scale, or in other words below fair. Image acquired at Starfield Observatory in Nambour on July 1, 2017.
Some fainter constellations
Between Regulus and Alphard is the inconspicuous constellation of Sextans, the Sextant. Between Sextans and the quadrilateral of Corvus, the Crow is another faint star group, Crater, the Cup.
Between the Milky Way and the southern horizon may be found the lesser-known constellations of Apus the Bird of Paradise, Chamaeleon, Pavo the Peacock, Octans the Octant, Mensa the Table Mountain, Dorado the Goldfish, Indus the Indian, Hydrus the Southern Water Snake, Pictor the Painter's Easel and Telescopium the Telescope.
Why are some constellations bright, while others are faint ?
If we look at ninety degrees to the plane, either straight up and out of the galaxy or straight down, we are looking through comparatively few stars and gas clouds and so can see out into deep space. These are the directions of the north and south galactic poles, and because we have a clear view in these directions to distant galaxies, these parts of the sky are called the intergalactic windows. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus.
This window is 50 degrees above the north-western horizon early in the evenings this month, so it's a good time for observing galaxies in the Virgo cluster. The southern window is in the constellation Sculptor, not far from the star Fomalhaut. This window will be rising well before midnight. Some of the fainter and apparently insignificant constellations are found around these windows, and their lack of bright stars, clusters and gas clouds presents us with the opportunity to look across the millions of light years of space to thousands of distant galaxies.
Finding the South Celestial Pole
The South Celestial Pole is that point in the southern sky around which the stars rotate in a clockwise direction. The Earth's axis is aimed exactly at this point. For an equatorially-mounted telescope, the polar axis of the mounting also needs to be aligned exactly to this point in the sky for accurate tracking to take place.
To find this point, first locate the Southern Cross. Project a line from the top of the Cross down through its base and continue straight on towards the southern horizon for another 4 Cross lengths. This will locate the approximate spot. There is no bright star to mark the Pole, whereas in the northern hemisphere they have Polaris (the Pole Star) to mark fairly closely the North Celestial Pole.
Another way to find the South Celestial Pole from southern Queensland is to choose a time when the Southern Cross is vertical (6.00 pm on July 1), and simply locate that spot in the sky which is midway between the bottom star of the Cross (Alpha Crucis), and the theoretical southern horizon.
Interesting photographs of this area can be taken by using a camera on time exposure. Set the camera on a tripod pointing due south, and open
the shutter for thirty minutes or more. The Earth's rotation will cause the stars to appear to move during the exposure, being recorded on the film as short
arcs of a circle. The arcs will be different colours, like the stars are. All the arcs will have a common centre of curvature, which is the south celestial pole.
A wide-angle view of trails around the South Celestial Pole, with Scorpius and Sagittarius at left, Crux and Centaurus at top, and Carina and False Cross at right.
Star trails between the South Celestial Pole and the southern horizon. All stars that do not pass below the horizon are circumpolar.
Double and multiple stars Estimates vary that between 15% and 50% of stars are single bodies like our Sun, although the latest view is that less than 25% of stars are solitary. At least 30% of stars and possibly as much as 60% of stars are in double systems, where the two stars are gravitationally linked and orbit their mutual centre of gravity. Such double stars are called binaries. The remaining 20%+ of stars are in multiple systems of three stars or more. Binaries and multiple stars are formed when a condensing Bok globule or protostar splits into two or more parts.
Binary stars may have similar components (Alpha Centauri A and B are both stars like our Sun), or they may be completely dissimilar, as with Albireo (Beta Cygni, where a bright golden giant star is paired with a smaller bluish main sequence star).
The binary stars Rigil Kentaurus (Alpha Centauri) at left, and Albireo (Beta Cygni) at right.
Rigel (Beta Orionis, left) is a binary star which is the seventh brightest star in the night sky. Rigel A is a large white supergiant which is 500 times brighter than its small companion, Rigel B, Yet Rigel B is itself composed or a very close pair of Sun-type stars that orbit each other in less than 10 days. Each of the two stars comprising Rigel B is brighter in absolute terms than Sirius (see above). The Rigel B pair orbit Rigel A at the immense distance of 2200 Astronomical Units, equal to 12 light-days. (An Astronomical Unit or AU is the distance from the Earth to the Sun.) In the centre of the Great Nebula in Orion (M42) is a multiple star known as the Trapezium (right). This star system has four bright white stars, two of which are binary stars with fainter red companions, giving a total of six. The hazy background is caused by the cloud of fluorescing hydrogen comprising the nebula.
Acrux, the brightest star in the Southern Cross, is also known as Alpha Crucis. It is a close binary, circled by a third dwarf companion.
Alpha Centauri (also known as Rigil Kentaurus, Rigil Kent or Toliman) is a binary easily seen with the
smallest telescope. The components are both solar-type main sequence stars, one of type G and the other, slightly cooler and fainter, of type K. Through a small
telescope this star system looks like a pair of distant but bright car headlights. Alpha Centauri A and B take 80 years to complete an orbit, but a tiny third component,
the 11th magnitude red dwarf Proxima Centauri , takes about 1 million years to orbit the other two. It is about one tenth of a light year from the bright pair and a little
closer to us, hence its name. This makes it our nearest interstellar neighbour, with a distance of 4.3 light years. Red dwarfs are by far the most common type of star, but,
being so small and faint, none is visible to the unaided eye. Because they use up so little of their energy, they are also the longest-lived of stars. The bigger a
star is, the shorter its life.
Close-up of the star field around Proxima Centauri
Knowing the orbital period of the two brightest stars A and B, we can apply Kepler’s Third Law to find the distance they are apart. This tells us
that Alpha Centauri A and B are about 2700 million kilometres apart or about 2.5 light hours. This makes them a little less than the distance apart of the Sun and Uranus
(the orbital period of Uranus is 84 years, that of Alpha Centauri A and B is 80 years.)
Albireo (Beta Cygni) is sometimes described poetically as a large topaz with a small blue sapphire. It is one of the sky’s most beautiful objects. The stars are of classes G and B, making a wonderful colour contrast. It lies at a distance of 410 light years, 95 times further away than Alpha Centauri.
Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separated them (Acrux, Castor, Antares, Sirius). Even closer double stars cannot be split by the telescope, but the spectroscope can disclose their true nature by revealing clues in the absorption lines in their spectra. These examples are called spectroscopic binaries. In a binary system, closer stars will have shorter periods for the stars to complete an orbit. Eta Cassiopeiae takes 480 years for the stars to circle each other. The binary with the shortest period is AM Canum Venaticorum, which takes only 17½ minutes.
Sometimes one star in a binary system will pass in front of the other one, partially blocking off its light. The total light output of the pair will be seen to vary, as regular as clockwork. These are called eclipsing binaries, and are a type of variable star, although the stars themselves usually do not vary.
Star Clusters
The two clusters in Taurus, the Pleiades and the Hyades, are known as Open Clusters or Galactic Clusters. The name 'open cluster' refers to the fact that the stars in the cluster are grouped together, but not as tightly as in globular clusters (see below). The stars appear to be loosely arranged, and this is partly due to the fact that the cluster is relatively close to us, i.e. within our galaxy, hence the alternate name, 'galactic cluster'. These clusters are generally formed from the condensation of gas in a nebula into stars, and some are relatively young.
The photograph below shows a typical open cluster, M7
*. It lies in the constellation Scorpius, just below the scorpion's sting. It lies in the direction of our galaxy's centre. The cluster itself is the group of white stars in the centre of the field. Its distance is about 380 parsecs or 1240 light years. M7 is visible tonight.
Galactic Cluster M7 in Scorpius
Outside the plane of our galaxy,
there is a halo of Globular Clusters. These are very old, dense clusters,
containing perhaps several hundred thousand stars. These stars are closer to
each other than is usual, and because of its great distance from us, a globular
cluster gives the impression of a solid mass of faint stars. Many other galaxies
also have a halo of globular clusters circling around
them.
The largest and brightest globular cluster in the sky is NGC 5139
**, also known as Omega Centauri. It has a slightly oval shape. It is an outstanding winter object, and this month it is observable all night. Shining at fourth magnitude, it is faintly visible to the unaided eye, but is easily seen with binoculars, like a light in a fog. A telescope of 20 cm aperture or better will reveal its true nature, with hundreds of faint stars giving the impression of diamond dust on a black satin background. It lies at a distance of 5 kiloparsecs, or 16 300 light years.
The globular cluster
The central core of Omega Centauri
* M42:This number means that the Great Nebula in Orion is No. 42 in a list of 103 astronomical objects compiled and published in 1784 by Charles Messier. Charles was interested in the discovery of new comets, and his aim was to provide a list for observers of fuzzy nebulae and clusters which could easily be reported as comets by mistake. Messier's search for comets is now just a footnote to history, but his list of 103 objects is well known to all astronomers today, and has even been extended to 110 objects.
** NGC 5139: This number means that Omega Centauri is No. 5139 in the New General Catalogue of Non-stellar Astronomical Objects. This catalogue was first published in 1888 by J. L. E. Dreyer under the auspices of the Royal Astronomical Society, as his New General Catalogue of Nebulae and Clusters of Stars. As larger telescopes built early in the 20th century discovered fainter objects in space, and also dark, obscuring nebulae and dust clouds, the NGC was supplemented with the addition of the Index Catalogue (IC). Many non-stellar objects in the sky have therefore NGC numbers or IC numbers. For example, the famous Horsehead Nebula in Orion is catalogued as IC 434. The NGC was revised in 1973, and lists 7840 objects.
The recent explosion of discovery in astronomy has meant that more and more catalogues are being produced, but they tend to specialise in particular types of objects, rather than being all-encompassing, as the NGC / IC try to be. Some examples are the Planetary Nebulae Catalogue (PK) which lists 1455 nebulae, the Washington Catalogue of Double Stars (WDS) which lists 12 000 binaries, the General Catalogue of Variable Stars (GCVS) which lists 28 000 variables, and the Principal Galaxy Catalogue (PGC) which lists 73 000 galaxies. The largest modern catalogue is the Hubble Guide Star Catalogue (GSC) which was assembled to support the Hubble Space Telescope's need for guide stars when photographing sky objects. The GSC contains nearly 19 million stars brighter than magnitude 15.
Two close naked-eye galaxies
Close to the southern horizon, two faint smudges of light may be seen. These are the two Clouds of Magellan, known to astronomers as the LMC (Large Magellanic Cloud) and the SMC (Small Magellanic Cloud). The LMC is to the right of the SMC, and is noticeably larger. They lie at distances of 190 000 light years for the LMC, and 200 000 light years for the SMC. They are about 60 000 light years apart. These dwarf galaxies circle our own much larger galaxy, the Milky Way. The LMC is slightly closer, but this does not account for its larger appearance. It really is larger than the SMC, and has developed as an under-sized barred spiral galaxy.
From our latitude both Magellanic Clouds are circumpolar. This means that they are closer to the South Celestial Pole than that Pole's altitude above the horizon, so they never dip below the horizon. They never rise nor set, but are always in our sky. Of course, they are not visible in daylight, but they are there, all the same.
The Large Magellanic Cloud - the bright knot of gas to left of centre is the famous Tarantula Nebula (below).
These two Clouds are the closest galaxies to our own, but lie too far south to be seen by the large telescopes in Hawaii, California and
Arizona. They are 15 times closer than the famous Andromeda and Triangulum galaxies, and so can be observed in much
clearer detail. Our great observatories in Australia, both radio and optical, have for many years been engaged in important research involving these, our
nearest inter-galactic neigh
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