The Evening Sky in July 2016
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The Evening Sky in July 2016
Bright planets and bright stars are scattered over the evening sky. Golden Jupiter appears in the north soon after sunset and orange Mars in the northeast. Cream-coloured Saturn appears below and right of Mars as the sky darkens. 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 south later. Midway down the north sky is orange Arcturus, similar in brightness to Saturn. South of the zenith are 'The Pointers', Beta and Alpha Centauri. They point to Crux the Southern Cross on their right. Vega rises in the northeast around 9 pm.
Brilliant Venus (not shown) sets in the west about 25 minutes after the Sun at the beginning of the month so might be seen from places with a low western skyline. Its setting time gets steadily later. By the end of the month it sets more than an hour after the sun. Mercury (not shown) joins Venus in the first half of July. On the 17th the two planets will appear close together. Mercury will be much fainter than Venus. The apparent pairing is just a line-of-sight effect. Mercury is 194 million km from us, coming around from the far side of the Sun. Venus is 253 million km away on the far side of the sun. For the rest of the month Mercury will be above and right of Venus. On the 30th-31st Mercury will be passing Regulus, the brightest star in Leo. So all five naked-eye planets will be in the early evening sky in the second half of July.
Jupiter and Saturn are always worth a look in any telescope. Jupiter's four 'Galilean' moons can be seen lined up on each side of the planet. Sometimes one or two may be missing as they pass in front of, or behind, Jupiter. The Moon will be near Jupiter on the 9th. A small telescope shows Saturn's ring system and biggest moon Titan looking like a star about four ring-diameters from the planet. Big telescopes show fainter moons closer in. Jupiter is 890 million km away mid-month; Saturn is 1390 million km away. Mercury, Venus and Mars, though bright, are small in a telescope. Mars is 95 million km away mid-month and fading as we leave it behind.
Alpha Centauri is the third brightest star. 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 blue-giant star hundreds of light years away. Canopus swings down to the southern skyline before midnight then moves into the southeast sky in the morning hours. It is a 'circumpolar star': it never sets. Crux and the Pointers are also circumpolar. Canopus is a truly bright star: 13 000 times the sun's brightness and 300 light years away.
Arcturus, in the north, 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 twinkles red and green when setting in the northwest around midnight. It is an orange colour because it is cooler than the sun; around 4000°C.
East of the zenith, above Saturn, is the orange star Antares, marking the heart of the Scorpion. The Scorpion's tail, upside down, is stretched out to the right of Antares making 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. Below Scorpius is 'the teapot' made by the brightest stars of Sagittarius. It is also upside down in our southern hemisphere view.
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.
The Large and Small Clouds of Magellan, LMC and SMC, look like two misty patches of light low in the southern sky. They are easily seen by eye on a dark moonless night. They are galaxies like our Milky Way, but much smaller. The Large Cloud is 160 000 light years away and 5% of the mass of the Milky Way. The Small Cloud is 200 000 light years and 3% of the Milky Way's mass.
*A light year (l.y.) is the distance that light travels in one year: nearly 10 million million km or 1013 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.