The Evening Sky in September 2015
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The Evening Sky in September 2015
Mercury and Saturn are bright planets in the evening sky. At the beginning of the month Mercury is making its best evening sky appearance of the year, low in the west. Cream-coloured Saturn is northwest of the zenith at dusk and midway down the western sky by late evening. Mercury and Saturn are similar in brightness to orange Arcturus on the northwest skyline. Above Saturn is orange Antares. Crux, the Southern Cross, and the Pointers are midway down the southwest sky. Canopus, the brightest star in the sky, skims along the southern skyline. It is matched on the northern skyline by Vega, one of the brightest northern stars. The Milky Way spans the sky from northeast to southwest.
Mercury makes its best evening sky appearance of the year at the beginning of the month, setting more than two hours after the sun. It slips down the sky night to night, setting earlier, then disappears as it passes between us and the sun at the end of the month. The crescent moon will be below and right of Mercury on the 15th. Above Mercury is Spica, the brightest star in Virgo. Mercury is small in a telescope. It looks like a tiny first-quarter moon at the beginning of September, waning to a taller crescent as it comes closer. Mercury is one-third of Earth's diameter. It is 116 million km away mid-month.
Saturn, being distant, moves little against the background stars. It stays by the Scorpion's claws through the month as the region sinks lower in the west. Saturn is worth a look in any telescope. Even binoculars will show it as an oval, the planet and rings blended together. It is 1550 million km away mid-month.
The Milky Way spans the sky from north to south. Many of the brightest stars are scattered along it or near it. Two exceptions are Canopus, near the south skyline, and Arcturus, setting early in the northwest. Both stars are shining through a lot of air which makes them twinkle colourfully. Canopus, being white, shows all colours like a diamond. Orange Arcturus twinkles red and green. Canopus is a truly bright star: 13 000 times the sun's brightness and 300 light years* away. On the opposite horizon is Vega, the second-brightest northern star after Arcturus. It is due north at dusk and sets in the late evening. Vega is 52 times brighter than the sun and 25 light years away.
Midway down the southwest sky are 'The Pointers ', Beta and Alpha Centauri. They point down to Crux the Southern Cross. Alpha Centauri is the third brightest star. It is also the closest of the naked eye stars, 4.3 light years away. Beta Centauri, along with most of the stars in Crux, is a blue-giant star hundreds of light years away.
West of overhead, above Saturn, is orange Antares. It marks the heart of the Scorpion. The Scorpion's tail hooks toward the zenith like a back-to-front question mark, the 'fish-hook of Maui' in Maori star lore. Antares is a red giant star: 600 light years away and 19 000 times brighter than the sun. Above Scorpius is 'the teapot' made by the brightest stars of Sagittarius. It is upside down in our southern hemisphere view.
The Milky Way is brightest and broadest overhead in Scorpius and Sagittarius. In a dark sky it can be traced down past the Pointers and Crux into the south. To the north it passes Altair, meeting the skyline right of Vega. The Milky Way is our edgewise view of the galaxy, the pancake of billions of stars of which the sun is just one. The thick hub of the galaxy, 30 000 light years away, is in Sagittarius. The actual centre is hidden by dust clouds in space. The nearer clouds appear as gaps and slots in the Milky Way.
The Large and Small Clouds of Magellan, LMC and SMC, look like two misty patches of light in the south sky. They are easily seen by eye on a dark moonless night. They are galaxies like our Milky Way but much smaller. The LMC is about 160 000 light years away; the SMC about 200 000 light years away.
Brilliant Venus (not shown) is the 'morning star'. It rises in the east two hours before the sun. A telescope shows it as a thin crescent. Earth-sized Venus is 60 million km away mid-month.
*A light year (l.y.) is the distance that light travels in one year: nearly 10 million million km. Sunlight takes eight minutes to get here; moonlight about one second. Sunlight reaches Neptune, the outermost major planet, in four hours. It takes sunlight four years to reach the nearest star, Alpha Centauri.