The Evening Sky in September 2016
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The Evening Sky in September 2016
All five naked-eye planets are visible in the evening sky at the beginning of the month. Mercury, Venus and Jupiter are low in the west, setting 90 minutes after the Sun. Venus is the brightest, with golden Jupiter below it. Mercury is fainter and left of the bright pair. (Jupiter and Mercury are not shown on the chart.) Venus remains the 'evening star' while Jupiter and Mercury slip into the twilight over the following nights. We are leaving Jupiter behind on the far side of the Sun. Venus is catching us up. Mercury is passing between us and the Sun. The thin crescent Moon will be between Jupiter and Venus on the 3rd. At the beginning of the month Mercury is 104 million km from us, Venus 230 million km, Jupiter 960 million km.
Orange Mars and cream-coloured Saturn are northwest of the zenith at dusk. Orange Antares is on their left and fainter. Saturn stays near Antares as both drift lower through the month. Mars holds its elevation night-to-night so moves upward away from Saturn. Saturn is worth a look in any telescope. Good binoculars will show it as an oval, the planet and rings blended together. It is 1530 million km away mid-month. Mars is 146 million km away and tiny in a telescope. The first-quarter Moon will be below Saturn and Mars on the 9th.
Arcturus is on the northwest skyline. Canopus, the brightest true 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.
Canopus is a truly bright star: 13 000 times the sun's brightness and 300 light years* away. Vega is 52 times brighter than the sun and 25 light years away. From northern New Zealand the star Deneb can be seen near the north skyline in the Milky Way. It is the brightest star in Cygnus the Swan. Deneb is around 1400 light years away and 50 000 times brighter than the Sun.
Orange Antares, left of Saturn, 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. Antares is a red giant star: 600 light years away and 19 000 times brighter than the sun. It is a relatively cool 3000 C, hence its red-hot colour. 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.
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. The actual centre is hidden by dust clouds in space. At the very centre is a black hole four million times the sun's mass. 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 is a faint broad column of light surrounding Venus and extending upward toward Libra. 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.
*A light year (l.y.) is the distance that light travels in one year: nearly 10 million million km or 1013 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.