The May night sky

Unless the focus is on our local star, the Sun, this month offers a contracting window for star watching. Effective darkness at the latitude of Manchester lasts for about 6 hours at present, but halves to barely 3 hours at the month’s end, by which time night-long twilight bathes most of Scotland.

The S sky at nightfall is dominated by Jupiter and, 30° higher and to its left, Arcturus in Bootes. The latter is the brightest star in the sky’s N hemisphere, being only just brighter than Vega in Lyra, climbing in the E, and Capella in Auriga, sinking in the NW.

Outshining every star, Jupiter remains our most conspicuous planet as it tracks from about 30° high at nightfall to set in the W shortly before dawn. Above and to the right of Spica, and moving slowly to the WNW against the stars of Virgo, it fades a little from mag –2.4 to –2.2 as it recedes from 678 million to 724 million km.

Catch Jupiter just below the Moon on the 7th when a telescope shows its disc to be 43 arcsec wide and, since it is polar-flattened by its rapid rotation, 40 arcsec high. Its four main moons are easy through any telescope and may be glimpsed through binoculars.

Venus is brighter at mag –4.5 to –4.3 but hangs low above our E horizon in the dawn twilight. It rises less than 90 minutes before the Sun, and shows a dazzling crescent through a telescope, shrinking from 38 to 25 arcsec in diameter as its phase grows from 27% to 48% sunlit. It stands left of the waning Moon on the 22nd. Mercury is too low in the dawn twilight to be seen from Britain though both it and Venus are well placed as morning stars for watchers in the S hemisphere.

Saturn now rises in the SE in the middle of the night and is bright (mag 0.3 to 0.1) but low in our S sky before dawn. It lies above-right of the Teapot asterism as it edges westwards from Sagittarius to Ophiuchus. When Saturn is close to the Moon before dawn on the 14th, a telescope shows its globe to be 18 arcsec wide while its glorious rings span 41 arcsec and have their N face inclined 26° towards the Earth.

Mars, currently mag 1.6 and above-right of Aldebaran, is destined to be swallowed by our NW evening twilight this month as it tracks between the horns of Taurus.

May diary