Planets line up across our western evening sky

THE Sky in June By Alan Pickup.

WITH twilight lasting all night and obliterating the dimmer stars, June can be a challenging month for astronomy from our northern latitudes.

Indeed, from northern Scotland only the brighter stars and planets may be glimpsed during the short nights. Travel southwards into Europe, though, and the nights lengthen and darken and the rich starscapes of the southernmost Milky Way, in Scorpius and Sagittarius, climb clear of the southern horizon.

Pride of place this month must go to Venus. Brilliant at magnitude -4.0, the evening star is unmistakable in our western sky after sunset and sinks to set in the north-west about three hours later. Its east-south-easterly motion against the stars carries it from central Gemini, below Castor and Pollux, tonight to the western border of Leo, 11 to the right of Regulus, by the 30th. On the way, it sweeps only 0.7 north of the Praesepe star cluster in Cancer on the 20th.

Next in a line of three bright evening planets, and forming a wide naked-eye double with the star Regulus in Leo, 2 away to its left tonight, is Mars. The pair lie more than 40 to Venus's left and rather higher in the sky and draw closer together until Mars passes 0.9 north of Regulus next Sunday. At magnitude 1.1, Mars is a little brighter than the 1.4 of Regulus. More noticeable, and particularly through binoculars, is the contrast between the reddish hue of Mars and the blue-white of the star. By the month's end, Mars is magnitude 1.3 and 13 to the left of Regulus.

The third and highest of our planetary trio is Saturn which is slow-moving in western Virgo, 28 to the left of Regulus and slightly higher. Don't confuse it with Spica which lies a similar distance to Saturn's left – Spica rivals Saturn in brightness but is more blue-white and twinkling, while Saturn has a steady creamy cast. At midmonth, Saturn is magnitude 1.0 and the rings about its 18 arcsecond disk reach across 40 arcseconds but are tipped at less than 2 when viewed through a telescope.

Mercury remains hidden in our morning twilight but Jupiter, magnitude -2.3 to -2.5, is conspicuous in the east to east-south-east before dawn.

Tracking eastwards in Pisces, it lies below the Moon as it passes only 0.5 south of Uranus on the 6th. Uranus is magnitude 5.9 and would be an easy binocular object were the sky properly dark. One of Jupiter's major dark cloud bands, its South Equatorial Belt, has disappeared, albeit temporarily. Perhaps it has been obscured by higher and lighter cirrus clouds of ammonia.

Our northern chart plots the path of Comet McNaught (formally C/2009 R1 McNaught) which reaches perihelion just outside Mercury's orbit on July 2. Numbers along the track indicate its position at the beginning of that date in June. Discovered from Australia last September by the exiled Scot Robert McNaught, it was a seventh magnitude binocular smudge last week but may be near the sixth magnitude tonight when it lies 5 east of Mirach in Andromeda. It is 1.9 south of Almach on the night of the 6th, only a few arcminutes south of Delta Persei on the 14th, 2 above-right Capella on the 20th and 0.6 above-left of Beta Aurigae on the 24th. By then it may be as bright as the third magnitude with an upwards-pointing tail. On the other hand, it may be a damp squib and barely visible against our bright twilight.

The Sun reaches its northernmost point at the summer solstice at 12:28 BST on the 21st. Sunrise/sunset times for Edinburgh change from 04:36/21:47 BST on the 1st, to 04:27/22:03 on the 21st and 04:31/22:02 on the 30th. The Moon is at last quarter on the 4th, new on the 12th, first quarter on the 19th and full on the 26th when a partial lunar eclipse is visible across the Pacific. Look for the young Moon to the left of Venus on the 15th, below-left of Mars on the 17th, below-right of Saturn on the 18th and near Spica on the 20th.

Ophiuchus the Serpent Bearer sprawls in the south at our map times, above the red supergiant Antares which is near the Moon on the 23rd and 24th. The name of its leading star, Rasalhague, means "Head of the Snake Charmer" and it lies only 5 from Rasalgethi, "The Head of the Kneeler", in Hercules. The former is a white star at 47 light years (ly) while Rasalgethi consists of a trio of stars some 380 ly distant. The leader of the three is a red giant so large that, were to replace our Sun, Earth would lie midway between its surface and its core. It also pulsates somewhat erratically in brightness so that Rasalgethi fluctuates between magnitudes three and four.

In northern Hercules, and almost overhead at our map times, are two globular star clusters that appear as fuzzy balls of light through binoculars. The brightest, M13, shines at magnitude 5.8 and lies on the western edge of the Keystone, a quadrilateral of third magnitude stars. It contains hundreds of thousands of stars at a distance of 25,100 ly, yet may be difficult to spot until the twilight subsides later in July. Its apparently sma