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The Evening Sky in September 2023
Bright stars shine around the skyline. Arcturus is on the northwest skyline. Canopus, the brightest star in the sky, skims along the southern skyline. 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 matched on the northern skyline by Vega, the second-brightest northern star after Arcturus. From northern Aotearoa the star Deneb can be seen near the north skyline in the Milky Way. It is the brightest star in Cygnus the Swan.
Orange Antares, northwest of the zenith, marks the body of the Scorpion. The Scorpion's tail hooks toward the zenith like a back-to-front question mark. It is the 'fish-hook of Maui' in Maori star lore. Below or right of the Scorpion's tail is 'the teapot' made by the brightest stars of Sagittarius. It is upside down in our southern hemisphere view.
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.
Saturn is the only bright planet in the evening sky. It is due east and midway up the sky at dusk. It looks like a cream-coloured star, shining with a steady light. By midnight it is north of the zenith. Binoculars show Saturn as an oval, the planet and rings blended. Almost any telescope will separate the planet and the ring. The ring is ‘closing’ now, getting more edge-on to us. Saturn is 1320 million km away mid-month. The Moon will be near Saturn on the 27th.
Right of Saturn is the medium brightness star Fomalhaut, marking the Southern Fish. Further right, in the southeast, is Achernar. It is the same brightness as Saturn and the third brightest of the stars in the south after Canopus and Alpha Centauri.
Mars is the only other planet in the evening sky, low in the west and setting early. It looks like a reddish star straight below the medium bright white star Spica. It is 380 million km away, on the far side of the Sun. The thin crescent Moon will be between Mars and Spica on the 17th.
The Milky Way spans the sky from north to south. It 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 southwest. To the northeast 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, 27 000 light years away, is in Sagittarius. Dust clouds near us appear as gaps and slots in the Milky Way. Binoculars show many clusters of stars and some glowing gas clouds 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.
On moonless evenings in a dark sky the Zodiacal Light is visible in the west. It appears as a faint broad column of light extending up past Mars and Spica. It is sunlight reflecting off meteoric dust in the plane of the solar system. The dust may have come from a big comet, many centuries ago.
Jupiter is in the late night and morning sky (so not on the chart.) It rises at 11:30 at the beginning of the month, and 10:30 NZDT at the end. By dawn it is in the northwest sky. It is the brightest ‘star’ in the morning sky till Venus rises. The Moon will be next to Jupiter on the morning of the 5th. Venus is the brilliant ‘morning star’. It rises at 5 a.m. at the beginning of the month and an hour earlier, at 5 a.m. NZDT, at the end.
*A light year (l.y.) is the distance that light travels in one year: nearly 10 million million km or 10^13 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.
Notes by Alan Gilmore, University of Canterbury's Mt John Observatory, P.O. Box 56, Lake Tekapo 7945, New Zealand. www.canterbury.ac.nz