Starwatch: The May night sky

Graphic: Finbarr Sheehy

The coming month sees night-long twilight return over northern Britain as the Sun climbs 7° northwards to within 1.5° or 3 solar diameters of its latitude at the summer solstice. Venus and Jupiter are also converging in our W evening sky, though their spectacular conjunction is not due until 1 July.

The Plough is overhead as Leo squats on the meridian at nightfall at present, but both topple westwards by our star chart times. Follow the curve of the Plough’s handle to reach the orange giant Arcturus in Bootes, the brightest star in the sky’s N hemisphere and one that is rushing past our solar system at 122 km per second at a range of only 37 light years.

Saturn reaches opposition on the 23rd and is the brightest object low in the S for much of the night. At mag 0.0, it rivals Arcturus in brightness though, unlike the latter, it does not twinkle. Currently just N of the star Graffias, or Beta Scorpii, Saturn is overtaken by the Moon on the 5th as it edges westwards into Libra. Its ring system, tipped 24° earthwards and spanning 42 arcsec at opposition, is wide open for admiration through any telescope.

Venus is now at its highest as a brilliant evening star in the W at nightfall. Brightening further from mag –4.1 to –4.3, it speeds eastwards from between the Horns of Taurus to pass 4° S of Pollux in Gemini as the month ends. Venus-set occurs in the NW at about 01:15 BST throughout the period, while a telescope shows its disc swell from 17 to 22 arcsec and its phase change from 67% to 53% sunlit. Find it 9° above-right of the earthlit Moon on the 21st.

Jupiter, mag –2.1 and brighter than any star, stands in the middle of our SW sky at nightfall and sets in the NW before dawn. Currently 6° to the left of the Praesepe star cluster in Cancer, it creeps 3° further away during May and appears 35 arcsec wide when it stands 7° above the Moon on the 23/24th.

Concluding its best evening spell of the year, Mercury is more than 9° high in the WNW 40 minutes after sunset until the 12th, setting itself more than 90 minutes later still. Binoculars show it 2° to the left of the Pleiades on the 1st but it becomes harder to spot as it dims from mag –0.3 to 1.2 on the 12th.