Starwatch: moon draws close to Jupiter in retrograde


The moon draws close to the shining planet of Jupiter this week, making a pretty pairing in the evening sky.

The chart shows the view looking south-southeast from London at 2300 BST on 8 October, although the conjunction should be visible from sunset onwards. The moon will be almost full, with 98.7% of its visible surface illuminated. Officially the full moon takes place on 9 October, but to the eye it is going to look almost full until 12 October, when the illuminated percentage falls below 95.

The planet Jupiter shines brightly and is currently travelling westward against the backdrop of stars in the constellation of Pisces, the fish. Known as retrograde motion, it means that on successive nights, the planet will appear to have moved a little west of its previous position. This is opposite to its normal progress of eastward motion, and is caused by the geometrical effect of the Earth “undertaking” the more distant planet.

Jupiter began this backward motion on 28 July. Earth slipped between Jupiter and the Sun on 26 September, effectively undertaking it. Jupiter will return to its normal eastward progress (prograde motion) from 23 November. To see the pairing from Sydney, Australia, look west-northwest.