The waning gibbous moon rises several hours after sunset today, lighting up your eastern sky around 8:30 to 9:00 p.m. Tonight’s moon shines in front of the constellation Gemini, in the vicinity of Gemini’s two brightest stars: Castor and Pollux.

Yesterday, on Thursday evening, the moon was in front of the constellation Taurus the Bull and near Taurus’ second brightest star, Elnath. Elnath nearly aligns with the galactic anticenter – the direction that’s about-face of the galactic center. The galactic center and anticenter are directly opposite one another on the galactic equator – the galactic plane projected onto the sky’s dome or stellar sphere.

Detailed sky atlases highlight the galactic equator (galactic plane) as a great circle that divides the stellar sphere into two equal halves. Sky atlases also show the ecliptic – a projection of the Earth’s orbital plane – as another great circle bisecting the stellar sphere.

Once every month, the moon goes full circle in front of the background stars, more or less following the path of the ecliptic. As seen from Earth, the moon crossed the galactic equator last night. When the moon exits Taurus to enter Gemini, the moon appears to cross the galactic equator going from south to north.

Will Earth pass through the galactic plane in 2012?

Written by Bruce McClure

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