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Moon and Gemini stars on March 7

Tonight – March 7, 2017 – the waxing gibbous moon shines in front of the constellation Gemini the Twins. If you look carefully, you might be able to see the Gemini stars Castor and Pollux, despite the moonlit glare. Another bright star, Procyon, is also nearby.

The dark side of the waxing moon always points in its direction of travel – eastward – in front of the backdrop stars. As Earth spins beneath the sky, the stars, planets and moon go westward throughout the night. Even so, the moon actually travels eastward through the constellations of the zodiac. That eastward motion of the moon is a reflection of the moon’s true motion in orbit around Earth.

As a result of the moon’s eastward (orbital) motion, the moon will move eastward from Castor, Pollux and Procyon. It’ll go through the faint constellation Cancer the Crab over the next few days, and then pair up with the bright star Regulus on March 9 and 10. See the chart below.

Every month, the moon travels full circle through the constellations of the zodiac. The green line depicts the ecliptic – Earth’s orbital plane projected onto the stellar sphere.

As seen from the Southern Hemisphere, the moon passes between the Gemini stars and Procyon once a month, as well. People living south of the equator will see the moon, Gemini stars, Procyon in their northern evening sky. We in the Northern Hemisphere will see all of these objects more south to overhead. From the vantage point of either hemisphere, the other hemisphere see things “upside down.” From the Southern Hemisphere, Procyon shines above the moon on March 7, and Castor and Pollux below the moon.

Up or down is a matter of perspective. To avoid ambiguity, we can say that Castor and Pollux lie north of the moon (in the direction toward the North Star), and Procyon lies south of the moon (in the direction away from the North Star). Meanwhile, Regulus is east of the moon – in the direction of sunrise – as darkness falls on March 7.

Bottom line: Tonight – March 7, 2017 – you’ll find the moon, the Gemini stars, the star Procyon in the same part of the sky. See the chart at the top of this post, and, if you’re in the Southern Hemisphere, turn this chart upside-down!

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Bruce McClure

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