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The Evening Sky in August 2023
Venus is the brilliant ‘evening star’ at the beginning of the month, setting around 7:30. It slips lower night-to-night as it moves between us and the Sun. By the 10th it is setting 40 minutes after the Sun. It disappears in the twilight soon after that (so isn’t on the chart.) After passing between us and the Sun on the 13th, Venus appears in the eastern dawn sky. By the 20th it is rising an hour before the Sun.
Mercury makes its best evening sky appearance of the year in August. At the beginning of the month it is above and right of Venus, and is the second brightest ‘star’ in the western sky after Venus. At mid-month Mercury is setting due west two hours after the Sun. Mars is then above and right of Mercury, looking like a reddish star. Mercury sinks lower in the twilight in the second half of August as it moves between us and Sun. It also fades as more of its sunny side is turned away from us. The Moon will be below Mercury and Mars on the 18th and above and right of Mars on the 19th.
Saturn rises due east before 8 pm at the beginning of the month. It looks like a medium bright star with a cream tint. By the end of the month it is up at dusk. Saturn's ring is visible in any telescope magnifying 20x or more. Its biggest moon, Titan, is four ring-diameters from the planet. Big telescopes show other moons looking like faint stars closer in than Titan. The Moon will be near Saturn on the night of August 2nd-3rd and again on the 30th-31st. At dawn Saturn is low in the western sky.
Bright stars are widely scattered over the sky. Vega on the north skyline is balanced by Canopus low in the south. Canopus twinkles with all colours as its white light is broken up by the air. So does Vega but, being fainter, it's not so obvious. Orange Arcturus is in the northwest, twinkling red and green as it sets. Canopus is the second brightest true star. (Sirius, the brightest star, is in the morning sky.) Arcturus is the fourth-brightest star in the sky and the brightest north of the celestial equator. Vega is the fifth brightest of all the stars and the second brightest north of the equator.
North of the zenith is orange Antares, marking the body of Scorpius. 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 Māori star lore. Antares is a red giant star: 600 light years̽ away and 19 000 times brighter than the sun. 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 'The Pointers ', Beta and Alpha Centauri, point down and rightward to Crux the Southern Cross. Alpha Centauri is the third brightest star in the sky (planets not counted) 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.
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, 27,000 light years away, is in Sagittarius. The actual centre is hidden by dust clouds in space. The nearer dust clouds 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.
Jupiter rises after 1 a.m. at the beginning of the month, and before midnight at the end. It is the brightest ‘star’ in the morning sky till Venus appears. It shines with a steady golden light. The Moon will be near Jupiter on the morning of the 9th.
*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 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