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The Evening Sky in May 2023
Venus is the 'evening star' setting in the northwest 2½ hours after the Sun. It is swinging out from the Sun as it catches up on Earth on the inside lane, so sets later. It is 130 million km from us mid-month. In a telescope it looks like a first-quarter Moon. When the sky is dark, Mars appears above and right of Venus, looking like a medium brightness orange-red star. At mid-month Mars will be at the top of an equally spaced line with Castor and Pollux, the heads of Gemini the Twins. Mars holds its position night-to-night while the Twins slip slowly down the sky. The crescent Moon will be below Venus on the 23rd, between Venus and Mars on the 24th, and to the right of Mars on the 25th.
As the sky darkens Sirius appears midway down the western sky. It is the brightest of all the stars but much fainter than Venus. It twinkles with all colours when setting in the southwest around 11 pm. Sirius, 'the Dog Star', marks the head of Canis Major the big dog, now head down, tail up. Canopus, second brightest star, is southwest of overhead.
Below Sirius are bluish Rigel and reddish 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.
Crux, the Southern Cross, is southeast of the zenith, to the right of '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 light years away. Canopus is also very luminous and distant: 13 000 times brighter than the sun and 300 light years away.
Following the Milky Way down into the southeast finds Scorpius. Orange Antares marks the Scorpion's body. Its upside-down tail curves to the right of Antares. Antares is a red-giant star like Betelgeuse: around 12 times the mass of the sun but wider than Earth's orbit. It is 600 light years away and 19 000 times brighter than the sun.
Orange Arcturus is the brightest star in the northern sky. It often twinkles red and green when low. Arcturus is the brightest red star but, at 37 light years, is much closer than Antares. It is about 120 times brighter than the sun.
The Milky Way is brightest in the southeast toward Scorpius and Sagittarius. In a dark sky it can be traced up past the Pointers and Crux and fading toward Sirius. 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 nearby outer edge is by Orion where the Milky Way is faintest. A scan along the Milky Way with binoculars shows many clusters of stars and some glowing gas clouds, particularly in Carina and Scorpius.
The Clouds of Magellan, LMC and SMC, are midway down the southern sky, easily seen by eye on a dark moonless night. They are small galaxies compared to our Milky Way Galaxy. The Large Magellanic Cloud is 160 000 light years away and the Small Cloud is 200 000 light years away.
Bright planets are in the morning sky. Saturn rises due east around 1 a.m., looking like a medium brightness cream-tinted star. It is north of the zenith at dawn. Golden Jupiter rises around 5 a.m. and is the brightest ‘star’ in the dawn sky. Mercury first appears as a faintish (3rd magnitude) star below and right of Jupiter around the 10th. It slowly climbs toward Jupiter and brightens. By the 20th it is a medium-bright ‘star’ 7° below and right of Jupiter. It continues to brighten but starts slipping lower after that. The Moon will be by Saturn on the 14th, and near Jupiter and Mercury on the 18th.
The full moon will look a little odd in the late morning hours on May 6th as it grazes the fuzzy edge of Earth’s shadow, the penumbra. The Moon’s bottom-right edge will be darkest around 5:30 a.m.
*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