Both Jupiter and Saturn came to opposition last month: Saturn on the 2nd and Jupiter on the 20th and they will therefore be in prime position throughout September and into October for observation. You will locate them across in the SE as dusk falls. At magnitude -2.7 Jupiter in particular is very conspicuous, a brilliant amber beacon low above the horizon. The good news for UK observers is that, compared to last year, Jupiter will appear almost 9 degrees higher in the sky, which equates to ‘better seeing’ and a longer observing window. It will be at its highest above the south horizon (culmination) shortly after 11pm, around 23 degrees up. Jupiter is currently in retrograde motion and moves from Aquarius into Capricornus before moving back again next month.
Through the eyepiece Jupiter is a splendid sight and many amateur astronomers consider it the most interesting planet to observe. The noticeably oblate disk will be a whopping 47 arc seconds in diameter, so there will be a good deal to observe. Even small telescopes will reveal the dark belts and bright zones, as well as the Galilean moons, adjacent to the disk. Scopes of 6 inches (150mm) or larger will reveal finer detail in the Jovian atmosphere, including the Great Red Spot (GRS) feature, a vast storm system which has been present for over 400 years, but has in recent years diminished in size and colour intensity. It may though be starting to re-establish itself.
The ‘dance’ of the Galilean moons around Jupiter throws up a different configuration each night. The four major moons are visible in 10x50 binoculars as specks of light. Very small telescopes won’t actually reveal much more, but 4 inch (100mm) scopes and above will show shadow transits – the jet black shadow dots of a Galilean moon on Jupiter’s disk as it passes in front. The most favourable shadow transits this month are on the 6th, 23rd and 29th – around 9pm. Io and Europa are the moons involved.
Saturn resides ahead of Jupiter and lies due south around 10–11pm, just shy of 20 degrees above the horizon. With an apparent magnitude of +0.3 its pearly white lustre will seem conspicuous in comparison to the faint stars of Capricornus, the constellation in which it resides, but rather feeble when compared to the nearby brilliance of Jupiter.
In the eyepiece of a telescope Saturn remains a glorious sight, with the rings still well orientated with respect to Earth, the North Pole tilted towards us by nearly 17 degrees. Although the disk size of 18.5 arc seconds is only half that of Jupiter’s, when you factor in the rings the overall image compares favourably with Jupiter, bearing in mind that Saturn is twice as far away! A scope with an aperture of 80–100mm (3.5-4 inches) will reveal the two brightest rings; ring A (the outermost) and ring B. These are separated by the Cassini division. Ring C lies closest to the disk but is difficult to spot, especially when Saturn is rather low. Saturn has at least 60 moons, most of which are very small, but you should at least spot Titan, its largest moon, as a speck nearby. Titan is bright enough (mag+8.4) to be spied in binoculars. It takes just under 16 days to orbit round Saturn.
Venus remains stubbornly low in the W twilight sky, just 6 or 7 degrees above the horizon. At magnitude -3.7 it should be easily spotted, but this brilliance is often diminished by haze and the lighter sky conditions present, resulting in Venus being trickier to spot than would normally be expected. An unobstructed W aspect is required otherwise you won’t spot Venus at all! Look for a slither of a new moon upper right of Venus on the 9th and upper left the following evening. View at 7.40pm. As we head though September Venus edges back towards the horizon, almost as if it had second thoughts about really making an impact in the western twilight sky.
If you fancy more of a challenge, Neptune comes to opposition this month and is visible in the late evening sky low to the SE, located midway between the faint loop of stars in Pisces and Aquarius. At magnitude +7.7 Neptune is visible in binoculars, but in reality a telescope of 100mm aperture is required to make out the tiny blue/grey disk. If you require a location chart visit www.whitby-astronomers.com then navigate to the September Sky Notes page.
September Night Sky
With the resumption of astronomical twilight (true darkness) the vast majority of stars are now visible before 10pm, making it somewhat easier for a casual observer to identify constellations in the night sky.
As twilight falls look for Arcturus, the brightest star in our summer sky over in the west. Overhead, lovely Vega in Lyra catches the eye, whilst low to the north, Capella in Auriga scintillates above the horizon. These three stars are the brightest north of the celestial equator visible from Britain. Each has a distinct hue: Arcturus appears orange, Capella, white, and Vega a lovely steely blue colour. High in the NW, look for the familiar saucepan outline of the ‘Plough’ – part of Ursa Major the Great Bear. The three stars marking the curved handle point down in the direction of Arcturus, located at the foot of Bootes the herdsman. Note the delicate starry circlet of Corona Borealis, the Northern Crown situated to the upper left of Arcturus. Returning to the Plough, use the ‘pointer’ stars in the ‘bowl’ to locate the pole star, Polaris, in Ursa Minor. To do this, simply extend an imaginary line through the ‘pointers’ and continue on a couple of hand spans to a relatively conspicuous, but solitary star located just over half-way up in the north.
Much of the SW aspect of the sky is occupied by two large and sprawling constellations. Hercules is best identified by the ‘keystone’ asterism, which stands upper left of Corona, whilst the vast and rather faint outline of Ophuichus, the Serpent Bearer, extends down from Hercules towards the SW horizon. As soon as twilight deepens in this direction look for the bright orange hue of Antares in Scorpius just above the horizon.
Should sky conditions be transparent, the asterism known as the ‘Teapot’, the most distinctive part of Sagittarius the Archer, may be traced above the S horizon. There are rich pickings in this part of the sky, which abounds in star fields, deep sky objects and is also home to the galactic centre.
Following Sagittarius above the S horizon, Capricornus, the Sea Goat, heads an aquatically themed constellation cavalcade, which continues with Aquarius, the Water Bearer, low in the ESE and Pisces, the Fish, extending up from the E. If sky conditions are not ideal these groups can be hard to trace, in particular Pisces which requires very dark skies to fully identify. Currently, Jupiter dominates this part of the sky with Saturn acting as side kick to its right.
High overhead to the S and E, reside three bright stars. Vega, in the small geometric pattern of Lyra, is the most conspicuous, whilst to the east, Deneb sits at the tail end of Cygnus, the swan. Pearly white Altair resides midway up in the S in Aquila the Eagle. These three stars form the asterism referred to as the ‘summer triangle’. Within the confines of the ‘summer triangle’ note the small stellar arrow shape of Sagitta, together with the close-knit group of Delphinus situated to its left. Both these small constellations do, with a little imagination, resemble the things they represent. Bizarrely Delphinus is sometimes known as ‘Job’s coffin’.
Over in the NE the great hero Perseus begins to climb away from the horizon, whilst above, the familiar ‘W’ pattern of Queen Cassiopeia holds court. Her daughter, Princess Andromeda, located to the lower right, is marked by a chain of stars extending towards the winged horse Pegasus, which occupies the mid regions of the E sky. Pegasus is best identified by ‘the great square’ an asterism comprising four stars that enclose a sparsely populated stellar area of sky. The asterism serves as a useful signpost in locating numerous other stars, deep sky objects and constellations in the autumn sky, including the Andromeda galaxy, which is visible as a misty patch to the naked eye on dark moonless nights upper left of the square. Clear skies!
Whitby and District Astronomical Society
WDAS usually meets in room H1, Caedmon College, Normanby campus (Whitby College). New members most welcome.