On April 12, 2019, you’ll find the moon at or near its half-lit first quarter phase and in the vicinity of Castor and Pollux, the two brightest stars in the constellation Gemini the Twins. And there’s another bright star on the other side of the moon on April 12. It’s Procyon, the brightest in the constellation Canis Minor the Lesser Dog. Procyon would be 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. 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 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 swells up into a giant in its old age.
The moon passes in front of Gemini for a few days every month. On this particular evening – April 12, 2019 – the moon crosses the ecliptic (Earth’s orbital plane) at its ascending node about one hour before the moon reaches its first quarter phase.
The sun, on the other hand, passes in front of Gemini for one month each year, from about June 21 to July 20.
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.
Bottom line: The moon is near the stars Castor and Pollux in the constellation Gemini the Twins on April 12, 2019. These stars represent twins in many cultures.