See the Moon hide Aldebaran – yet again

Twelve months ago I drew attention to a forthcoming Moon / Aldebaran occultation. Well it’s happening again, but unfortunately not at quite such a user friendly hour. This time you will either have to get up very early or go to bed extremely late.

 On 2016 December 13 (a Tuesday morning) at around 05:20 the Moon occults Aldebaran, the magnitude 0.9 red giant star in the constellation of Taurus and the 13th brightest star in the sky. Because of its brightness Aldebaran is commonly known as the Eye of the Bull and to the casual observer appears to be part of the nearby Hyades star cluster. It is in fact an unrelated foreground star lying about 68 light years away – around half the distance of the cluster itself.

 The Moon will be just under 1 day from full so there will be no discernible dark limb on view (it’s always fun to watch a star blink-out or reappear from the dark limb). From mid-UK latitudes the Moon will lie at an altitude of about 13 degrees just north of west, so you will need a reasonably clear western horizon.

From Greenwich Aldebaran will disappear at 05h 23.8m and from Edinburgh at 05h 25.8m. Reappearance will be at 05h 53.5m and 05h 41.1m respectively. Times are from the 2016 BAA Handbook (page 31). Watching a star disappear is relatively easy as both the star and Moon can be seen. The fun comes in catching its reappearance when you don’t know exactly where to look on the lunar limb.  Well, the BAA Handbook can help you here, as it gives the position angle on the Moon at which the star pops back into view. From Greenwich this is at 321 degrees (disappearance is at 31 degrees).

 Position Angle (PA) is measured from north (the top of the Moon) anticlockwise round the lunar surface. So, if you imagine the Moon as a 12 hour clock face, disappearance will take place from Greenwich at close to the 11 o’clock position and reappearance just after the 1 o’clock position.

 Other stars in Taurus are also occulted that morning – see the BAA Handbook for details.

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