Starwatch: The July night sky

Our notes for June concluded with advice to watch for noctilucent clouds low in the N sky between the NW after dusk and the NE before dawn. This proved timely, for the first decent display of these “night shining” electric-blue clouds of ice were sighted widely from Britain within a couple of days and there have been others since. Expect more until mid-August.

Although the Sun has turned southwards, Britain’s summer twilight is slow to subside and we must wait until late in July to savour true darkness with no interfering moonlight.

Our maps plot Capella in Auriga low in the N as the Plough sinks in the NW and the “W” of Cassiopeia climbs in the NE. As it arches high across the E, the Milky Way flows through Auriga and Cassiopeia and close to Deneb in Cygnus, the top-left of the three corner stars of the Summer Triangle.

The latter stands high in the S by our map times but is squashed by our map projection. Its other corners are marked by Vega in Lyra and Altair in Aquila, and the Milky Way continues through it on its way to Scorpius and Sagittarius, low in the S, where the centre of our galaxy lies.

Mars and Saturn remain the brightest objects low in the S at nightfall, shifting lower still into the SW by our map times. As it moves eastwards in Libra, Mars almost halves in brightness (mag -1.4 to -0.8) and recedes from 86 million to 108 million km. Catch it below the Moon on 14 July, when its disc appears 15 arcsec wide if viewed telescopically.

Saturn, fainter at mag 0.2 to 0.4, lies in S Ophiuchus some 6° above Antares in Scorpius. When it stands below-left of the Moon on the 15th, it appears 18 arcsec across and its marvellous ring system is tipped 26° in our favour.

Jupiter remains conspicuous at mag -1.9 to -1.7 but its altitude in the W one hour after sunset sinks from 14° on the 1st to only 3° on the 31st. Look for it above-left of the young Moon on the 8th and to the right of the Moon on the 9th.

Venus, brilliant at mag -3.9 and pulling slowly away from the Sun, lies still deeper in the evening twilight. Even by the 31st, it stands only 5° high in the WNW at sunset and sets 40 minutes later. Mercury is probably too dim in the same area of twilit sky to be seen from Britain.

July diary

2nd 05h Moon 0.4° N of Aldebaran

4th 12h New moon; 17h Earth farthest from Sun (152,103,776 km)

5th 05h Juno probe enters orbit about Jupiter

7th 03h Mercury in superior conjunction

8th 01h Moon 1.8° S of Regulus

9th 11h Moon 0.9° S of Jupiter

12th 02h First quarter

14th 19h Moon 8° N of Mars

16th 06h Moon 3° N of Saturn

20th 00h Full moon

27th 00h Last quarter

29th 12h Moon 0.3° N of Aldebaran

* Times are BST