Mars gets close to Neptune on December 31
Following opposition on 2016 May 22, Mars is now saying goodbye as it heads into the sunset. Now only a shadow of its opposition glory of magnitude -2.1 and diameter 18 arc seconds, it shines at magnitude +0.9 with a diameter just under 6 arc seconds. Nevertheless, as a first magnitude “star” it is still quite obvious in the south-western sky and on December 31 acts as a guide to outer planet Neptune.
On December 31 the Sun sets (from mid UK latitudes) at around 15:54 with the end of astronomical twilight some 2 hours later at 17:59. At this time Mars will lie at an altitude of 24 degrees in the south-west (azimuth 213 degrees). The crescent Moon – new on the 29th at 07:00 – will be just a couple of degrees above the horizon and brilliant Venus, at magnitude -4.3, will lie approximately between the Moon and Mars, but closer to Mars.
Neptune, at magnitude +7.9, sports a disc diameter of just over 2 arc seconds but should be an easy binocular object less than one Moon diameter north-east of Mars. And being so close to Mars (23 arc minutes away), will comfortably fit into the field of a medium power eyepiece.
If the sky is clear (we can always hope) this should be a fascinating event on which to end 2016.
A very Happy Christmas to all and very best wishes for 2017.