Take a look at the modest constellation of Libra, the Scales. It isn’t particularly outstanding in terms of bright stars, but it does have a few of the best star names in the entire starry realm.
The leading star of Libra is no exception to these rules. It’s not bright, but its name is fantastic: Zubenelgenubi.
With 13 letters and six syllables, it’s a bit of a tongue-twister. And on 26 January 2022, observers will be able to see the star pass behind the Moon.
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Zubenelgenubi is actually a double star containing the components α¹ Librae and α² Librae, which are mag. +5.1 and mag. +2.8, respectively.
It’s a binary star, with the two stars in orbit around each other. They lie at the same distance of 77 lightyears away from Earth and both have similar velocity and direction in space.
As a consequence, they are very likely to be a physical pair.
According to a 2016 decision by the International Astronomical Union, these days the name ‘Zubenelgenubi’ refers specifically to the brighter component, Alpha-2.
What does Zubenelgenubi mean?
Like many stars, the name Zubenelgenubi was created during the major Arabic influences on astronomy, around 1,000 years ago (for more on this, read our guide to Arabian astronomy).
Translated, Zubenelgenubi means ‘the southern claw of the Scorpion’, which harks back to the time when Libra was not a separate constellation but the front part of the constellation of Scorpius, the Scorpion.
As seen from Earth, Zubenelgenubi sits just above the ecliptic in the sky so can often be found near the Moon in a wonderful conjunction, or even being occulted by the Moon. And one such occultation is coming up on 26 January.
See a lunar occultation of Zubenelgenubi on 26 January 2022
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The 42%-lit waning crescent Moon will pass in front of the star Zubenelgenubi on the morning of 26 January, in an event known as a lunar occultation.
Bright star occultations aren’t rare but are they are uncommon enough to make them worth putting in a bit of effort to see.
In this case there’s a hidden bonus because Zubenelgenubi is a double star with both components visible to the naked eye.
When will the occultation occur?
From the centre of the UK, the Moon’s bright limb will hide Alpha-1 at 05:25 UT, Alpha-2 following suit nine minutes later at 05:34 UT.
Alpha-1 reappears from behind the Moon’s dark limb at 06:35 UT, Alpha-2 reappearing at 06:43 UT.
Dawn will be well underway as the reappearance occurs but both stars are bright enough to remain visible for the event.
Times will vary slightly depending on your location so it pays to observe starting about 15 minutes before the quoted disappearance and reappearance times.
It can be seen with a small/medium scope, i.e. a reflector/Schmidt-Cassegrain under 6 inches or a refractor under 4 inches.
This article originally appeared in the June 2009 issue of BBC Sky at Night Magazine.