The Night Sky in September
Planets grab the headlines this month for a number of reasons. Neptune, the dimmest seen from Earth, reaches opposition – opposite the Sun in the sky – on Sept 11. For such a distant world opposition makes hardly any difference to its appearance and you’ll still need optical assistance to spot it. Binoculars show it east of the faint star, Phi Aquarii, a telescope revealing the planet’s tiny blue disc.
Uranus, reaches opposition at the very end of October and, as with Neptune, any change in appearance will be minimal. You’ll need a good, dark sky to spot it unaided.
As darkness falls, look low above the southern horizon for bright Jupiter with dimmer Saturn to its east (left from the UK). Both planets appear 8 degrees apart midseptember but this gap is shrinking and will continue to do so now, until Dec 21. On that date Jupiter and
Saturn will be separated by just 6 arcminutes; one fifth the apparent diameter of the Moon. The last time these planets appeared so close was on July 16 1623! Watch out for a bright Moon passing below both Jupiter and Saturn on Sept 24 and 25.
But Mars is the show-stopper. The Red Planet reaches opposition mid-october and this makes a huge difference to its appearance. Mars reaches opposition every 2.1 years but its physical circumstances and positioning vary a lot. When Mars is nearest the Sun in its orbit, as now, opposition brings it closer to Earth, making it appear bright and, through a telescope, quite large. Oppositions won’t be as good again until 2033!
This year, Mars is closest to Earth on Oct 6 and this is when it’ll look largest through a telescope. As it approaches this position, its brightness grows to overtake Jupiter and the rusty nature of the planet’s surface gives it a distinctly reddish hue. Make no mistake, Mars will look spectacular as we head into October.
Mars is tricky to observe through a telescope and it’s important to let your telescope cool outside for an hour or two to remove localised thermal currents. Currently, Mars has its southern pole tilted towards Earth, it being summer in the Martian southern hemisphere.
When this happens, the planet’s bright southern polar cap shrinks as energy from the Sun causes it to sublimate. Patches of light and dark material appear, the most famous of which are the so-called Mountains of Mitchel, named after astronomer Ormsby Macknight Mitchel who first identified them back in 1846.
Despite their name, the Mountains of Mitchel are a Martian plain!
The morning sky is dominated by beautiful Venus above the eastern horizon before sunrise, Venus is spectacular but a difficult world to observe due to its surface being covered in thick, reflective clouds.
A full night’s watch will potentially net you six of the Solar System’s eight planets, seven if you look around at your surroundings! The only absentee is tiny Mercury, poorly positioned after sunset.
This month the stars of autumn begin to take centre-stage, the large area to the south being known as the sea due to the number of watery constellations that it contains. These include Capricornus the Seagoat, Aquarius the Water Bearer, Pisces the Fish, Cetus the Whale, Piscis Austrinus the Southern Fish and Eridanus the River in the early morning sky to the east.
The autumn sky is dominated by Pegasus the Flying Horse. This upside-down mythical creature is best identified from the large Great Square of Pegasus asterism. Identify this and you have a useful navigational tool to find the stars.