Moon still moving through Winter Circle
The Gemini twins are triplets on February 13
In recent nights, the waxing moon has been moving inside a large circular star pattern on the sky’s dome, known as the Winter Circle (or Winter Hexagon). It’s not one of the 88 official constellations. It’s a very large and noticeable pattern of stars, an asterism, spreading across much of the evening sky at this time of year. On the night of February 13, as the moon edges out of the Circle, it lines up with two stars that are noticeable for being both bright and close together on our sky’s dome. These stars are Castor and Pollux of the constellation Gemini the Twins.
Read about Pollux: The brighter twin star
Read about Castor: 6 stars in 1
Gemini? Here’s your constellation
The Winter Circle
The Winter Circle consists of seven bright stars in six separate constellations. The ecliptic – path of the sun, moon and planets, marked in green on our chart – cuts through the Winter Circle. So, during the months the Circle is visible, the moon spends a few days each month sweeping through these stars. The Winter Circle stars are:
Capella – brightest star in the constellation Auriga the Charioteer
Pollux – brightest star in the constellation Gemini the Twins
Castor – second-brightest star in the constellation Gemini the Twins
Procyon – brightest star in the the constellation Canis Minor the Lesser Dog
Sirius – brightest star in the constellation Canis Major the Greater Dog
Rigel – brightest star in the constellation Orion the Hunter
Aldebaran – brightest star in the constellation Taurus the Bull
With the exception of the star Castor in Gemini, the Winter Circle stars are what we call 1st-magnitude. That is, they’re among the brightest stars in our sky. Technically speaking, Castor is slightly fainter at 2nd magnitude, though it’s the brightest of all the 2nd-magnitude stars in the sky.
Near the center of the Circle, you’ll find Orion’s bright red star Betelgeuse, which, in late 2019, began an unexpected dimming, causing some to suggest that it might be about to go supernova (explode). It didn’t explode, although its curious dimming continued into early 2020. The star then returned nearly to its original brightness, only to dim again in mid-2020. Now Betelgeuse is as bright as it ever was. Its strange dimming is thought to have been caused by a cloud of hot gas, released by the star, that temporarily blocked some of Betelgeuse’s light.
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A huge circle of bright stars
On February evenings, in the Northern Hemisphere, we see the Winter Circle fill up much of our southern sky during the evening hours. As viewed from the Southern Hemisphere – where it’s autumn now – the moon will also be in the midst of these same stars we view as the Winter Circle, but, as seen from temperate latitudes in the Southern Hemisphere, this lasso of stars appears upside-down relative to our northern view. That is, southern sky watchers will see the star Sirius at top and the star Capella at bottom.
No chart can adequately convey the Winter Circle’s humongous size! It dwarfs the constellation Orion the Hunter, which is a large constellation occupying the southwestern part of the Winter Circle pattern.
Bottom line: On February 13, 2022, the moon swings close to Castor and Pollux, the Twin stars, both part of the Winter Circle, a large asterism made of seven brilliant stars.
Here are constellation charts via the International Astronomical Union (IAU)
Read more: Winter Circle, or Hexagon, made of bright stars
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