2015 October 21

See the Moon hide Aldebaran

Stewart Moore

On the evening of October 29 (yes, you don’t have to get up early for this one!) the waning gibbous Moon (just under 92% phase) will occult Aldebaran, the magnitude 0.9 red giant star in the constellation of Taurus. 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 a unrelated foreground star lying about 68 light years away – around half the distance of the cluster itself.

Occultations are always fun to watch (or image / video), especially seeing the star reappear from the Moon’s dark limb. No optical aid is necessary to enjoy an occultation involving such a bright star, although small binoculars or a telescope can make it more fun as you watch the star blink out on the bright limb and observe the more difficult reappearance from the dark limb. It is easy to see the star disappear as you can see where it is in relation to the Moon, but unless you know exactly where it will reappear you can be caught out. In this case the star will reappear at position angle 280 degrees (from London). 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 clock face, just before the 3 O’clock position. A chart showing the sky at 21:15UT – around 35 minutes before disappearance is given below. Aldebaran is the bright atar, alpha.

Full occultation details are given in the 2015 BAA Handbook (page 29) including times from Edinburgh and other occultations in Taurus that evening. From London Aldebaran disappears at the Moon’s bright limb at 21:48UT and reappears just under 1 hour later from the dark limb at 22:46UT. At the start of play the Moon will lie in the east at an altitude of around 27 degrees. Let’s hope the weather will be as kind for this event as it was for the recent eclipse.