Although it’s summer in the Northern Hemisphere, a major sign of winter now looms large in the August predawn/dawn sky. These next few mornings – August 25 and 26, 2019 – the waning crescent moon shines right in front of the great big lasso of stars known as the Winter Circle. The Winter Circle (sometimes called the Winter Hexagon) is an asterism – a star pattern that is not a recognized constellation. This humongous star formation consists of six 1st-magnitude stars in six different constellations.
Capella of the constellation Auriga the charioteer
Pollux of the constellation Gemini the Twins
Procyon of the constellation Canis Minor the Smaller Dog
Sirius of the constellation Canis Major the Big Dog
Rigel of the constellation Orion the Hunter
Aldebaran in the constellation Taurus the Bull
The star Castor in the constellation Gemini, although not a 1st-magnitude star, counts as the sky’s brightest 2nd-magnitude star.
By the way, are you familiar with the constellation Orion the Hunter? The Winter Circle even dwarfs the Mighty Hunter, which only makes up the southwest (lower right) portion of the Winter Circle. If you’ve never seen the Winter Circle, but are acquainted with Orion, this constellation presents a great jumping off place for circumnavigating this brilliant circle of stars. Best of all, these stars are so bright that the’re even visible in the morning twilight.
The Winter Circle is so named because we in the Northern Hemisphere see this star formation on winter evenings. The Winter Circle is also visible from the Southern Hemisphere, though on their summer evenings.
The green line on the feature sky chart at top depicts the ecliptic – the Earth’s orbital plane projected onto the sky. The ecliptic can also be regarded as the sun’s apparent yearly pathway in front of the constellations of the zodiac. If you could see the stars in the daytime, you’d see the sun directly north of the star Aldebaran around June 1. Thereafter, the sun stays in front of the Winter Circle for about 1 1/2 months. Then the sun passes to the south (below) the Gemini stars, Castor and Pollux, in mid-July.
We don’t see the Winter Circle in June and July because it’s lost in the glare of the sun. In late August, the Winter Circle returns to the morning sky. But we still won’t see the Winter Circle in the evening sky for months to come.
Aldebaran is out all night long, from dusk till dawn, around December 1; and the Gemini stars, Castor and Pollux, are out all night long, from dusk till dawn, around mid-January. Hence, we in the Northern Hemisphere associate the Winter Circle with short days and long nights, when these brilliant stars light up the dark months of the year.
These next few days, before daybreak, let the waning crescent moon show you the majestic Winter Circle.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.