Double the fun as magnitude-3.6 star λ Geminorum is hidden by the Moon

Early risers in the British Isles blessed with clear skies will see naked-eye star lambda (λ) Geminorum, otherwise known as Kebash or Alkibash (α=7h18.1m δ=+16°32′ J2000), slip behind the 92% illuminated waning gibbous Moon close to 6 am on Tuesday, 9th December. The occultation will take place some 33° above the west-southwest horizon in a dark sky for the centre of the country.

Although λ Geminorum is normally an easy naked-eye star, the proximity of a dazzling Moon will mean that powerful binoculars or a small telescope will be required, especially since the star’s disappearance will occur at the Moon’s bright limb. 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.

For an observer in the heart of the British Isles, the star is occulted by the Moon close to 6:01 am in a dark sky. As a rule of thumb, the further west you live, the earlier the event will occur, and vice versa to the east. So, be prepared with your telescope and sitting at the eyepiece about 10 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 when it will happen! For the mid-British Isles this will occur close to 6:56 am. As before, the further west you live, the earlier reappearance will occur, and vice versa to the east. Be prepared at least ten minutes beforehand and study the graphic to familiarise yourself with the lunar features and orientation. (Note: users of refractors or catadioptrics with a star diagonal will need to mentally flip the chart 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. This occurs at the Moon’s dark limb just north of the prominent Mare Crisium. Bear in mind that twilight will be lightening the sky and the Moon will be about 25° above the western horizon. Clear skies and good luck!