Courtesy of EarthSky
A Clear Voice for Science
The waxing crescent moon and the constellation Libra’s two major stars – Zubenelgenubi and Zubeneschamali – appear rather low in your southwest sky at nightfall. The moon can help you find these stars. As evening deepens, the moon and these Libra stars descend westward, to sink beneath the southwest horizon by mid to late evening.
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 – the sun’s annual path in front of the background stars. If you could see the stars during the daytime, you would 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 5 degrees (10 moon diameters) north to 5 degrees south of this star. For the next few years, the moon will pass to the south of (below) Zubenelgenubi.
Starting on January 7, 2013, the moon will occult – swing in front of – Zubenelgenubi every month until May 14, 2014. After that, the moon will swing north of (above) Zubenelgenubi each month, not to occult this star again until October 8, 2021.
Written by Bruce McClure