The Evening Sky in August 2015
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The Evening Sky in August 2015
At the beginning of the month three bright planets appear low in the western evening sky soon after sunset.
Brilliant silvery Venus is brightest and highest. Golden Jupiter is below and right of Venus. Mercury is well below the two bright planets on August 1st. It moves quickly up the sky, night to night, as Venus and Jupiter sink lower. On the 7th Mercury is just a full-moon's width to the right of Jupiter. Venus is left of the close pair of planets. All three set about 70 minutes after the sun. Mercury continues its ascent of the evening sky through August while Venus and Jupiter disappear in the twilight. By the 31st Mercury is setting due west after 8 pm, making its best evening sky appearance of the year. At month's end the bright orange star Arcturus is setting in the northwest, well to the right of Mercury, often flashing red and green as it goes. Mercury is a small and unimpressive planet in a telescope. It is one-third Earth's diameter and 180 million km away mid-month.
Cream-coloured Saturn is the only bright planet in the late-evening sky. It is just north of overhead. Orange Antares is in the same area of sky but fainter than Saturn and closer to the zenith. Well down the northwest sky is orange Arcturus, mentioned above. Low in the north is white Vega, making a brief appearance in our sky. Exactly opposite Vega, low in the south, is Canopus twinkling colourfully. In the southwest are 'The Pointers', Beta and Alpha Centauri with Crux, the Southern Cross, below them.
A small telescope shows Saturn's ring system and biggest moon, Titan, looking like a star about four ring-diameters from the planet. The moon appears near Saturn on the 22nd.
Antares marks the heart of the Scorpion. The Scorpion's tail hooks around the zenith like a back-to-front question mark. Antares and the tail make 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 and rightward to Crux the Southern Cross. Alpha Centauri is the third brightest star and the closest of the naked eye stars, 4.3 light years away. Beta Centauri, like most of the stars in Crux, is a blue-giant star hundreds of light years away and thousands of times brighter than the sun.
Canopus, the second brightest star, is near the south skyline at dusk. It swings upward into the southeast sky through the morning hours. Canopus is truly bright: 13,000 times brighter than the sun and 310 light years away. On the opposite horizon is Vega, one of the brightest northern stars. It is due north in mid-evening and sets around midnight. Vega is 52 times brighter than the sun and 25 light years away.
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 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, 30 000 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 low in the south, 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.
After passing between us and the sun mid-month, Venus appears in the eastern dawn twilight. By the 20th it is rising in the east an hour before the sun. Venus remains the 'morning star' for the rest of the year.
*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 four years for sunlight to reach the nearest star, Alpha Centauri.