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Thursday, November 26, 2015

Moon passes close to galactic anticenter star

By Bruce McClure
EarthSky Communications

Thursday, Nov. 5, 2009
The bright waning gibbous moon rises an hour or more after nightfall tonight. If you look closely, you’ll see a star near the moon. It’s Elnath, the star representing Taurus the Bull’s northern horn. It’s a fairly bright star, though it’ll be obscured by the moonlit glare. After rising, the moon and Elnath stay out all night long tonight.

Elnath is a special star, because it pretty much aligns with the galactic anticenter. The galactic anticenter is the direction in the sky that’s about-face from the center of our Milky Way galaxy.

The moon always moves in front of the background stars, traveling its own diameter eastward every hour. By tomorrow night, the moon’s change of position will be obvious. It will have moved eastward into the constellation Gemini the Twins.

For a fixed reference to the galactic anticenter, use Elnath, Taurus the Bull’s second brightest star. The galactic anticenter lies some 3 degrees east of Elnath on the stellar sphere. By the way, the width of your thumb at an arm length away spans roughly 3 degrees of sky.

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Friday, Nov. 6, 2009
The waning gibbous moon rises several hours after sunset today, lighting up your eastern sky around 8:30 to 9 p.m. Tonight’s moon shines in front of the constellation Gemini, in the vicinity of Gemini’s two brightest stars: Castor and Pollux.

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.

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