1st Feb: see the Moon occult λ Geminorum

Observers in the British Isles with clear skies around 6:23 pm GMT on Sunday night will see naked-eye star lambda (λ) Geminorum, otherwise known as Kebash or Alkibash (α=7h18.1m δ=+16°32′ J2000), slip behind the dark limb of a 96% illuminated waxing gibbous Moon. The occultation will take place some 28° above the eastern horizon in deep twilight for the centre of the UK.

Although λ Geminorum is normally an easy naked-eye star, the proximity of a nearly Full Moon will mean that powerful binoculars or a small telescope will be required. For telescope users, what’s noteworthy about an occultation of Alkibash is that it’s a 9.6-arcsecond double star; the westernmost component will disappear first, closely followed by the companion star.

As a rule of thumb, the further west you live, the earlier the event will occur, and vice versa to the east (the predicted disappearance at London is 6:23:24 pm and 6:26:24 pm in Edinburgh). So, be prepared with your telescope and sitting at the eyepiece about 5 minutes before the event to watch the approaching limb of the Moon.

The reappearance of a star from occultation is somewhat harder to prepare for as you don’t know precisely where it will happen! This will occur close to 7:32 pm for the mid-British Isles (7:31:12 pm London; 7:35:36 pm Edinburgh). As with the disappearance, the further west you live, the earlier reappearance will occur, and vice versa to the east.

The star’s reappearance occurs at the Moon’s bright limb in line with prominent crater Langrenus on the edge of the Mare Fecunditatis. (Note: users of refractors or catadioptrics with a star diagonal will need to mentally flip the map left-right, whereas Newtonian users should rotate the view 180°.)

Fortunately, we can take advantage of the fact that Alkibash is a double star, extending what would normally be a virtually instantaneous reapparance into one lasting a second or two for both stars. Clear skies and good luck!