Moon and Gemini stars on April 18 and 19

On April 18 and, 19, 2021, you’ll find the moon displaying a wide waxing crescent phase in the vicinity of Castor and Pollux, the two brightest stars in the constellation Gemini the Twins.

And there are two other objects to watch for near the moon these next few nights. One is Procyon, the brightest star in the constellation Canis Minor the Lesser Dog. The other is the planet Mars, which isn’t as bright as the brightest stars now, but perhaps still noticeable for its reddish color. Both Procyon and Mars might hard to pick out without the help of tonight’s moon, or a good constellation chart. On the other hand, Castor and Pollux are extremely noticeable in the night sky, moon or no moon. No other two such bright stars appear so close together.

Regardless of the seeming connection between these two stars, Castor and Pollux aren’t close together in space. They just happen to reside along the same line of sight from Earth.

Both Castor and Pollux are bright stars, and they’ve been known as Twins for centuries at least. But they don’t really look alike. Pollux is golden in color, and Castor is pure white. If you have binoculars, they’ll help you to more easily distinguish the color contrast between Castor and Pollux.

Also, Castor and Pollux are different kinds of stars. Castor is a hot, white-colored star that is well known for being a multiple star system. It consists of three pairs of binary stars, that is, six stars bound together in an intricate gravitational dance. Pollux is a cool and bloated orange-colored star, said to be the closest giant star to Earth. A star with the mass of our sun swells up into a giant in its old age. But astronomers assure us that our sun won’t become a giant for another 5 billion years or so.

On April 16, 2021, the moon crossed the ecliptic – Earth’s orbital plane and the annual pathway of the sun across our sky – at its ascending node. That happened at 05:53 UTC. Then about four days later, the moon will reach its first quarter phase on April 20 at 06:59 UTC.

The moon will remain north of the ecliptic (Earth’s orbital plane) until the moon crosses its descending node on April 29, 2021, at 09:18 UTC. Then the moon will stay south of the ecliptic until it crosses the ecliptic at its ascending node on May 13, 2021, at 10:29 UTC.

Sun tiny like a BB, Pollux like a baseball, Arcturus like a basketball, each labeled.
You can see the comparative size of the star Pollux and our sun in this image, as well as some other stars. Pollux is a giant star by virtue of its age. Many stars – including our sun – will swell to the giant stage as they age.

In many cultures, Castor and Pollux were seen as twin stars, usually as heroes. Many old sky myths invoke the idea of twins to explain their proximity on the sky’s dome.

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Diagram with two bright stars, lines of stars, and radial arrows.
If you have a dark sky, notice that 2 nearly parallel streams of stars extend from Castor and Pollux. These stars likely reinforced the idea of twins in various cultures around the world. Every December, the Geminid meteor shower radiates from near star Castor in Gemini.

In Greek and Roman mythology, Castor and Pollux were the twin sons of Jupiter and Leda and brothers of Helen of Troy. They sailed with Jason as two of his Argonauts.

Pollux, represented by the brighter star, was immortal, but his brother Castor was not. When Castor was killed in a fight, Jupiter wanted the two to remain together, so he decreed that they each should spend some time in the underworld and some time in the heavens. This is a fanciful way of explaining why the constellation is above the horizon for part of each day and below the horizon for the rest. Castor and Pollux are sometimes said to represent brotherly love.

Meanwhile, in China, these two stars were associated with water, as part of constellations representing rivers. They were sometimes also seen as the complementary elements of yin and yang.

Antique color etching of twin boys with lyre, club and bow in a star field.
Castor and Pollux, the Gemini twins, via Wikipedia.

Because Gemini is a constellation of the zodiac, the sun passes in front of this constellation for about a month each year, from about June 21 to July 20.

Bottom line: The moon is near the stars Castor and Pollux in the constellation Gemini the Twins on April 18 and 19, 2021. These stars represent twins in many cultures.

Read more: EarthSky’s guide to the bright planets

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April 18, 2021

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