Moon in Gemini before dawn September 29 and 30

Star chart: 2 positions of crescent moon in front of the constellation Gemini, with slanted green line of ecliptic.
Look for the moon in Gemini – near the “twin” stars Castor and Pollux – September 29 and 30.

Last quarter moon in Gemini

On the mornings of September 29 and 30, 2021, you can see the moon near the two brightest stars of the constellation Gemini the Twins. These stars are Castor and Pollux. The moon will reach its last quarter phase in front of Gemini on September 29 at 01:57 UTC. By clocks in North America, that translates to the evening of September 28. A last quarter moon is a waning moon. So the September 28-29 moon will look half-illuminated, like half a pie. And the next night’s moon will look slimmer, like a fat crescent.

See how bright Castor and Pollux are, and how close together? Even without the moon, they’re noticeable on the sky’s dome.

A last quarter moon rises in the middle of the night, maybe after your bedtime. Find out the moon’s rising time in your sky via the Old Farmer’s Almanac (U.S. and Canada) or (worldwide). Find out the moon’s position relative to the constellations of the zodiac via Heavens-Above.

Antique colored etching of 2 young boys sitting close together, with scattered stars in black.
This depiction of the Gemini twins is from Urania’s Mirror, a set of cards containing astronomical star charts, first published in 1824. Image via Wikipedia.

Gemini, constellation of the Twins

Gemini is the constellation of the Twins, and Castor and Pollux are often called “twin” stars. But these two stars aren’t twins; they’re really very different. Although Castor and Pollux appear close, they’re not physically related or close together in space. Pollux, the closer star, lies about 34 light-years away, while Castor resides at a distance of about 52 light-years.

If you scrutinize Castor and Pollux, you might discern their difference in color. Castor appears white in contrast to the orange glow of Pollux. A white star is a relatively hot star in the heyday of youth. An orange star is a cool star in the autumn of its years.

Pollux is a giant star

Moreover, Pollux’s orange color reveals that it’s a giant star. According to the star expert Jim Kaler, any star with the mass of 0.8 to 5 times the sun’s swells up and becomes a giant star in old age.

Pollux, the nearest giant star to Earth, has a diameter of about 10 of our suns. It’s also one of the very few giant stars known to harbor a planet.

Castor is six stars in one

The other “twin,” Castor, is remarkable in its own right. Castor is actually six stars in one, consisting of 3 pairs of binary stars, all revolving around a common center of mass.

By the way, the moon will move out of the constellation Gemini and into the faint constellation Cancer the Crab on or near October 1, 2021.

Sun tiny like a BB, Pollux like a baseball, Arcturus like a basketball, each labeled.
The sizes of the sun and Pollux compared, along with a couple of other stars. Image via slideplayer.

Use Orion’s Belt to locate Castor and Pollux

Even with no moon to guide you, however, you can notice Castor and Pollux for their brightness and nearness to each other on the sky’s dome. Not sure you have the right stars? Try using Orion’s Belt to star-hop to Castor and Pollux (see the sky chart below):

Star chart with stars in black on white, with purple arrow through Orion pointing to the twins.
Sky chart of the constellation Gemini via International Astronomical Union. Draw an imaginary line from the easternmost star of Orion’s Belt and through the bright ruddy star Betelgeuse to locate the Gemini stars Castor and Pollux. Click here to learn more about Orion.

Bottom line: On September 29 and 30, 2021, use the waning crescent moon to find the Gemini stars Castor and Pollux.

September 28, 2021

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Bruce McClure

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