See the Moon get close to Jupiter and a double star at dusk on 27 May

In an earlier post I wrote about Jupiter at opposition on 9 May and the Jovian phenomena visible in backyard telescopes from Western Europe this month. As May draws to a close, it’s still prime time for the solar system’s largest planet. As seen from the heart of the British Isles, Jupiter is currently best placed for observation and highest in the southern sky just before midnight.

Skywatchers in the UK looking at the rising Moon at dusk on Sunday, 27 May will see that Jupiter is situated close by. At around 10:30pm BST this night the 13-day-old gibbous Moon lies just 3¼ degrees to the upper left of the solar system’s largest planet, so the pair will be framed nicely in the field of view of most binoculars.

Seeing double near Jupiter
Look more closely in the vicinity of Jupiter and you’ll notice a wide double star slightly more than two Moon widths (68 arcminutes, actually) to the lower right of the planet. These stars are alpha¹ (α¹) and α² Librae, the pair commonly known by their Arabic proper name of الزُبَانَى الجَنُوبِي, or Zubenelgenubi (sometimes written Zuben Elgenubi).

While the name Zubenelgenubi actually refers to the brighter magnitude +2.8 α² Librae component, this star and its magntiude +5.2 companion (α¹) are separated by a wide 231 arcseconds, hence even the smallest binocular will resolve them. Both α¹ and α² are part of a multiple star system some 76 light-years away. For owners of equatorially-mounted telescopes or computerised GoTo mounts, the J2000 coordinates of Zubenelgenubi are α=14h 50.9m, δ=-16° 02’.

Since Jupiter’s opposition occurred earlier this month, the planet is still moving retrograde (i.e., east to west) against the background stars of the constellation Libra. Following its stationary point on 11 July, Jupiter’s motion turns prograde (west to east) and the planet makes a closer brush with Zubenelgenubi on 17 August 2018 when the pair lie 35 arcminutes (almost 0.6 degrees) apart.