The May night sky

Graphic: Finbarr Sheehy

Mars stands at its brightest and closest at opposition in May but sadly hovers low down in the S sky as Britain’s nights dwindle in length and become dogged by twilight. The astronomical highlight of the month, though, can only be observed in bright sunlight. When the innermost planet Mercury slides around the near side of its orbit on 9 May, its silhouette appears as a miniature inky dot against the dazzling face of the Sun.

Jupiter, meanwhile, is conspicuous in the middle of our S sky at nightfall and is sinking into the W by our map times. It shines at mag -2.3 to -2.0 as it edges westwards in S Leo and lies alongside the Moon on the night of 14-15 May.

Transits of Mercury across the Sun are not particularly rare, there being more than a dozen each century, but few are as well timed for UK observers as this one. Seen from Britain, the tiny disc of the planet, only 12 arcsec across, begins to encroach on the E (left-hand) edge of the Sun at 12:12 BST and takes almost seven and a half hours to crawl across the Sun’s S hemisphere before it exits the SW edge of the Sun at 19:41. Parallax means that the times for elsewhere in the world differ by up to a couple of minutes.

To prevent serious eye damage, never view the Sun directly through binoculars or a telescope. Mercury is too small in transit to be seen using so-called eclipse glasses or by projecting the Sun’s light through a pinhole. Instead, equip your telescope with an approved solar filter or focus the solar image on to a shaded white card. Check Starwatch on the day itself, 9 May, for further details.

Mars rises in the SE by 23:00 BST at the start of May, and close to sunset by the 22nd, when it stands directly opposite the Sun at a distance of 76m km. Its rusty disc then appears 18 arcsec in diameter, perhaps with a hint of its white northern polar cap which is inclined 10° towards us. Our chart plots it 5° above the red supergiant Antares in Scorpius, but it moves 9° westwards into Libra during the month, and improves from mag -1.5 to rival Jupiter at -2.0.

Catch the full Moon above Mars on the eve of Mars’ opposition and even closer to Saturn on the next night. The latter lies to the E (left) of Mars and improves between mag 0.2 and 0.0 as it creeps westwards in S Ophiuchus on its way to opposition in early June. Venus, meantime, stays hidden beyond the Sun.