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The Evening Sky in April 2023
Venus is the brilliant ‘evening star’, setting in northwest 1½ hours after the Sun. The planet will be left of the Pleiades/Matariki star cluster around April 10th. Though bright, Venus is of little interest in a telescope, looking like a tiny gibbous moon, a moon between first quarter and full. It is 160 million km away from us mid-month. The thin crescent Moon will be near Venus on the 23rd.
After Venus sets, Sirius is the brightest star in the sky. It is northwest of overhead at dusk and sets in the southwest after midnight. Sirius is the brightest true star. The second brightest is Canopus, southwest of the zenith at dusk. Below Sirius are bluish Rigel and orange Betelgeuse, the brightest stars in Orion. Between them is a line of three stars: Orion's belt. To southern hemisphere star watchers, the line of three makes the bottom of 'The Pot', now tipped on its side. In the Pot's handle, or Orion's Sword, is the Orion Nebula, a glowing cloud easily seen in binoculars.
Below and right of Sirius is Procyon marking the head of Canis Minor one, of the two dogs following Orion the hunter across the sky. Sirius marks the head of Canis Major, the big dog. The big dog's hindquarters are made by the bright stars above Sirius.
Low in the northwest below Procyon are three stars of similar brightness. The vertical pair are Pollux and Castor, the heads of Gemini the Twins. Left of them is Mars, looking like an orange star. We passed Mars late last year. It is now 240 million km away and just a tiny disk in a telescope. The Moon will be between Mars and the Twins on the 26th. Though related in myth, the twins are quite different from each other. Pollux is an orange star 31 times brighter than the Sun and 34 light-years (l.y.)* from us. Castor is a hot white star about 47 times the Sun’s brightness and 51 l.y. away.
Above and right of the Twins is the Praesepe star cluster, looking like a hazy spot to the eye. It marks the shell of Cancer the Crab. Praesepe is also called the Beehive cluster, the reason obvious when it is viewed in binoculars. Praesepe is 600 light-years away. Its age is around 600 million years, so its biggest and brightest stars have long ago burnt out.
Right of Praesepe, almost due north, is the medium-bright star Regulus. It is the brightest star in Leo the Lion. The curve of stars below Regulus outlines Leo's mane, upside down in our southern hemisphere view. A crooked vertical line of stars right of Regulus makes Leo's hind quarters with the brighter star further right being his tail. The Moon will be below Regulus on the 29th.
The lone bright star due east is Spica, the brightest star in Virgo. Above Spica is the roughly kite-shaped constellation of Corvus the Crow. In the later evening the bright orange star Arcturus rises toward the northeast, often twinkling red and green. (It isn’t on the chart.)
Crux, the Southern Cross is high in the southeast. Below it, and brighter, are Beta and Alpha Centauri, often called 'The Pointers'. Alpha Centauri, the brighter Pointer, is the closest naked-eye star, 4.3 light-years away. Beta Centauri, like most of the stars in Crux, is a blue-giant star hundreds of l.y. away. Canopus is also a very luminous and distant star; 13,000 times brighter than the Sun and 300 l.y away.
The Milky Way is brightest in the southeast above Crux. It can be traced to nearly overhead where it fades and becomes very faint in the northwest, right of Orion. The Milky Way is our edgewise view of the Galaxy, the pancake of billions of stars of which the sun is just one.
The Clouds of Magellan, LMC and SMC, are midway down the southwest sky, easily seen by eye on a dark moonless night. They are small galaxies about 160,000 and 200,000 light years away.
Saturn is the only naked-eye planet in the dawn sky, rising due east after 2 a.m. It looks like a medium-bright star with a cream tint in an empty region of sky. By dawn it is well up the eastern sky. The Moon will be near Saturn on the mornings of the 16th and 17th.
*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.
Notes by Alan Gilmore, University of Canterbury's Mt John Observatory, P.O. Box 56, Lake Tekapo 7945, New Zealand. www.canterbury.ac.nz