The moon has been awesome these past several nights in the western twilight sky! It’s been moving past the planet Venus, the star Spica in the constellation Virgo, and the ringed planet Saturn. But of course the moon never stops moving. As darkness falls this evening (Tuesday, August 13) in North America, the rather wide waxing crescent moon appears in front of the constellation Libra, quite close to Libra’s alpha star Zubenelgenubi.
You may need to blot out the moon with your finger or use binoculars to see Zubenelgenubi tonight. Libra’s other main star, Zubeneschamali, may be easier to see since it’s not as close to tonight’s moon.
In the Eastern Hemisphere – Europe, Africa, Asia, Australia and New Zealand – the moon on the evennig of August 13 will be somewhat farther away from the star Zubenelgenubi and a bit closer to Saturn than it is in the Western Hemisphere. As darkness falls around the world, however, the moon shines in the vicinity of the star Zubenelgenubi and the planet Saturn. We highlight the ecliptic – the center stripe of the zodiacal highway – on our sky chart because the moon and planets are always found in front of the constellations of the Zodiac.
Zubenelgenubi has the star designation “alpha” in the constellation Libra, even though Zubeneschamali is the slightly brighter star. Zubenelgenubi sits almost exactly on the ecliptic. If you could see the stars during the daytime, you’d see the sun and Zubenelgenubi in conjunction on or near November 7 every year.
The sun goes by Zubenelgenubi once a year, but the moon sweeps by Zubenelgenubi every month. Unlike the sun, the moon can swing anywhere from five degrees (10 moon diameters) north to five degrees south of this star. During the period of time after July 26, 2004, and before January 7, 2013, the moon has been swinging south of Zubenelgenubi every month. But the moon is presently having an occultation series with Zubenelgenubi, whereby the moon occults – momentarily covers over – Zubenelgenubi every month from January 7, 2013, until May 14, 2014. (However, you have to be at the right place on Earth to witness any occultation.) After that, the moon will swing north of (above) Zubenelgenubi each month, not to occult this star again until October 8, 2021.
As soon as darkness falls, look for the moon close to the star Zubenelgenubi and to the east of the planets Saturn and Venus.