Starwatch: The July night sky

Graphic: Finbarr Sheehy

Britain’s night-long summer twilight begins to subside in July so that lucky starwatchers under a dark sky may be able to glimpse the Milky Way as it arches high across the E at our map times from the Scorpius-Sagittarius region in the S towards Cassiopeia and Auriga in the NNE. On the way, it flows through the familiar Summer Triangle formed by Deneb, Altair and (brightest of all) Vega.

Much brighter still are Venus and Jupiter which now form a spectacular close pairing low down at nightfall.

However, it is a conjunction of a different type that is July’s solar system highlight. Nine years after its launch, Nasa’s New Horizons probe is to sweep within 12,500km of Pluto at 12:50 BST on the 14th. It has already returned our sharpest views of the dwarf planet, itself 2,368km wide, and its system of moons. The craft will then speed onwards, hopefully to visit at least one more object in the Kuiper Belt of icy worlds beyond the orbit of Neptune.

During the encounter, Pluto lies 4,772 million km from us in Sagittarius and shines dimly at the 14th magnitude, so we need a very detailed star chart, a good-sized telescope and a degree of skill to glimpse it.

Venus and Jupiter stand at their closest, 0.5° or less than a Moon’s breadth, on the 1st and are an impressive sight low in our W evening twilight. Venus, at mag -4.4, outshines the mag -1.8 Jupiter by more than a factor of ten.

Over the following days, Venus draws to the left of Jupiter but both sink lower and may well be lost in Britain’s twilight by mid-month. As it swings around the Sun’s near side, Venus approaches from 76 million to 60 million km between the 1st and 16th, swelling in diameter from 33 to 41 arcsec as its dazzling crescent (visible through binoculars) changes in phase from 33% to 21% illuminated.

Mercury and Mars are hidden in the Sun’s glare, but Saturn continues as the brightest object low in our S sky at nightfall, moving to set in the SW less than two hours after our map times. It fades from mag 0.3 to 0.4 as it edges westwards in E Libra and is 16 arcsec wide, set within stunning 40 arcsec rings at midmonth. Catch it to the left of the Moon on the 25th and right of the Moon on the 26th.