We may shake a fist at the moon when the northern lights are out, but on most nights it's a huge help. And not just because it helps us see better in darkness. The moon travels in a ring around the entire sky that passes through the zodiac constellations. Learning the dozen figures that lie along its path helps us find the planets because they also pass this way. To the savvy eye, the five classical planets are interlopers not permanent residents.

We won't have to wait long before one of their ilk arrives in Gemini. Mars enters the constellation on April 24 and stays through June 7 before moving on to Cancer the Crab.

On Sunday evening, April 18, the thick crescent shines about a fist below Castor and Pollux, the brightest stars in the constellation Gemini the Twins. Once you've found them, connect the dots to find the fainter stars that outline two stick figures, one for each brother. (Stellarium)
On Sunday evening, April 18, the thick crescent shines about a fist below Castor and Pollux, the brightest stars in the constellation Gemini the Twins. Once you've found them, connect the dots to find the fainter stars that outline two stick figures, one for each brother. (Stellarium)

On Sunday night, April 18, the waxing crescent moon shines squarely in Gemini the Twins directly below its two brightest stars, Pollux and Castor. Find the moon (easy enough), and look a fist above it to claim them. Pollux is the brightest star in Gemini and also known as Beta Geminorum. Like the alpha wolf in a pack, a constellation's brightest star normally gets the exalted alpha designation, so Pollux should be Alpha Geminorum. Instead, its brother Castor wears that crown.

Why? Because celestial cartographer Johann Beyer didn't carefully distinguish between the stars' brightness when he assigned their names in 1603 in his influential star atlas, Uranometria. Had he paid closer attention Pollux would be the rightful alpha. It makes you wonder how many other historical mistakes have become accepted practice.

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Pollux is an orange giant star about nine times the size of the sun and twice as massive. As this artist's view shows, it may have starspots similar to the sun's sunspots. (Public domain)
Pollux is an orange giant star about nine times the size of the sun and twice as massive. As this artist's view shows, it may have starspots similar to the sun's sunspots. (Public domain)

Both Pollux and Castor are each amazing in their own way. Pollux is one of the brightest stars in the night sky that's orbited by another planet, dubbed Thestias. Discovered in 2006, Thestias is about three times as massive as Jupiter and orbits the star every 1.6 years at nearly the same distance Mars circles the sun. Pollux is bright because it's close to the Earth — just 34 light-years away — and an orange giant star nine times larger than the sun and 33 times as luminous. From Thestias, it would span 3° or 5.7 times bigger than the sun viewed from Earth.

Castor is a six-star system made of three orbiting pairs: Castor Aa + Ab; Castor Ba + Bb; and Castor Ca + cb.  The diagram also shows the relative sizes of each star compared to the sun. Orbits not to scale. (NASA)
Castor is a six-star system made of three orbiting pairs: Castor Aa + Ab; Castor Ba + Bb; and Castor Ca + cb. The diagram also shows the relative sizes of each star compared to the sun. Orbits not to scale. (NASA)

When you're out Sunday (April 18), compare Pollux to its brother Castor. The contrast in their colors — orange Pollux and white Castor — should be apparent. Castor lies 51 light-years away and belongs to the same class of stars as brilliant Vega in the Summer Triangle. Like many stars, Castor looks like a single, solitary point of light with the naked eye, but a small telescope reveals it's actually three suns, with a close, similarly bright companion, Castor B, and a fainter red dwarf called Castor C.

A spectroscope, a device that breaks down a star's light into a "bar code" of colors, reveals that each is double again, making Castor a sextuple star with three pairs of twins! It couldn't have happened to a better constellation.

Use this mythological portrait of Gemini to help you visualize how the stars outline the Gemini twins. (Stellarium)
Use this mythological portrait of Gemini to help you visualize how the stars outline the Gemini twins. (Stellarium)

While the moon may make a few of the fainter stars in Gemini a little tricky to see, its location is just perfect for finding the constellation right now. As we approach summer, Pollux and Castor will remain together, arm in arm and nearly parallel to the horizon, as they slowly sink westward.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.