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The Night Sky in July 2023
Venus is the brilliant evening star, appearing in the northwest soon after sunset. It is beginning to fall lower in the twilight as it moves between us and the Sun. At the beginning of July it sets around 8:40. By the end of the month it is setting an hour earlier. Though bright, Venus isn’t of much interest in a telescope, looking like a tall thin white crescent. It is 60 million km away, mid-month.
At the beginning of the month Mars appears as a medium-brightness reddish star above and to the right of Venus. At that time the star Regulus will be on the top end of the Venus-Mars line and similar in brightness to Mars. Regulus moves down the sky, night to night, making a close pairing with Mars around the 10th. Mars holds its position as Venus and Regulus set earlier. On the 13th Venus, Regulus and Mars will be in a line. The Moon will be near the two planets and star on the 21st. Mars is 343 million km away mid-month, so is just a tiny disk in a telescope.
Saturn is up late in the evening. It rises after 10 pm at the beginning of the month; around 8 at the end. It looks like a medium-bright star, due east, all on its own. The near-full Moon will be above Saturn on the 6th and below it on the 7th. By dawn Saturn is northwest of the zenith. Saturn is 1350 million km away mid-month. It is worth a look in any telescope but might be fuzzy when low in the sky. The ring can be seen at 20x magnification. Saturn's largest moon, Titan, appears as a star four ring-diameters from the planet.
Mercury begins its best evening sky appearance of the year in the third week of July. On the 15th it will be well below and left of Venus, setting 50 minutes after the Sun (so it is not on the chart.) On the 29th it will be 5° (a binocular field width) right of Venus, making a close pair with Regulus. The crescent Moon will be below Mercury on the 19th.
Sirius, the brightest true star, sets in the southwest as twilight ends, twinkling like a diamond. Canopus, the second brightest star, is also in the southwest at dusk. It swings down to the southern skyline before midnight then moves into the southeast sky in the morning hours. It is a 'circumpolar star': seen from NZ it never sets. Canopus is a truly bright star: 13 000 times the sun's brightness and 300 light years* away.
South of the zenith are 'The Pointers', Beta and Alpha Centauri. They point to Crux the Southern Cross on their right. Alpha Centauri is the third brightest star in the sky. It is also the closest of the naked eye stars, 4.3 light years away. Beta Centauri, like most of the stars in Crux, is a hot blue-giant star hundreds of light years away. Crux and the Pointers are also circumpolar.
Midway down the north sky is orange Arcturus. It sets in the northwest around midnight, twinkling red and green as it goes. It is the fourth brightest star and the brightest in the northern hemisphere sky. It is 120 times the sun's brightness and 37 light years away. It has an orange colour because it is cooler than the sun; around 4000°C. Vega rises in the northeast around 9 pm. It is on the opposite side of the sky to Canopus: low in the north when Canopus is low in the south.
The Milky Way is brightest and broadest in the east toward Scorpius and Sagittarius. In a dark sky it can be traced up past the Pointers and Crux, 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 actual centre is hidden by dust clouds in space. A scan along the Milky Way with binoculars shows many clusters of stars and some glowing gas clouds.
Jupiter is the brightest ‘star’ in the morning sky. It rises after 3 a.m. at the beginning of the month and around 1:40 a.m. at the end. By dawn it is midway up the north sky. Jupiter shines with a steady golden light, rarely twinkling. Any telescope will show Jupiter as an oval disc with its four big 'Galilean' moons lined up on either side. The Moon will be near Jupiter on the morning of the 12th.
*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