Even rarer than a twice-in-a-month full moon, is a month with no full moon. This can happen only in February.
It turns out there are many ways to define a moon period, and our modern calendar month is just a convenient (and approximate) way to fit the lunar cycle in with the more useful solar cycle of a year. A sidereal month is the time it takes the moon to go 360 degrees around the Earth. However, it is a synodic month that marks the repetition of the lunar phases, or how long it takes to get from one full moon to the next. To do this, the moon must travel an extra 30 degrees beyond the 360 degrees in a circle because the Earth has moved 30 degrees farther in its orbit around the sun.
Because both the Earth’s orbit and the moon’s orbit are elliptical, they change speeds slightly along their paths. This results in the length of a synodic month varying from just under 29.2 days to just over 29.9 days. That is shorter than every month except February.
This monthIf you are an early morning person, you can catch the outer planets this month. Jupiter rises a little after 1 a.m., followed by Mars at about 2:30 a.m. and Saturn about 4:30 a.m. By the end of February, Venus should be re-appearing from behind the sun and be the evening star for the next few months.
This is a great month to see the asterism called the winter hexagon, and it has been more than five years since I pointed out this major part of the winter sky. As a reminder, an asterism is just a recognizable pattern of stars, and in this case, it spans six different constellations and includes eight bright stars. The way you get eight stars from a hexagon is that the center of this hexagon has a bright star, and one of the points has a pair of twin stars.
If you can find Orion, Betelgeuse is the bright red star above and slightly to the left of the three belt stars. This is the center of the hexagon, and a good starting place to tell you how to find the others.
To identify the stars at the six points around the edge, let’s start with the brightest, Sirius, and continue around counterclockwise. Sirius is the brightest star in the sky, but if you need help finding it, the belt stars of Orion point southeast to Sirius. Sirius is in the constellation Canis Major, the Big Dog (or more literally, greater dog), one of Orion’s two hunting dogs.
The second point of the hexagon is at the other end of Orion from Betelgeuse. Rigel, Orion’s foot, is approximately equal distance from the belt stars as Betelgeuse, but is much bluer, so it is a much hotter star. With the center and two points identified, you now have an idea of the size of the winter hexagon.
The third point around the edge is the red star Aldebaran in the constellation Taurus. You can find Aldebaran on a line between the belt stars in Orion and the Pleiades (Seven Sisters), the open cluster in Taurus.
Continuing counterclockwise, the fourth point is Capella, in the constellation Auriga. It is slightly east of straight north from Aldebaran and will be the point opposite of Sirius in the hexagon.
The fifth point is the one with two stars. Castor and Pollux are the twins of Gemini. They are roughly equal in magnitude, and Castor is a beautiful double star in a small telescope. Many non-stargazers know the names Castor and Pollux from mythology, but a common problem that many stargazers have is remembering which is which in the sky. I just remember that Castor, with the “C,” is on the same side as Capella, with the “C,” and Pollux, with the “P,” is on the same side as Procyon, with the “P.”
Procyon is the sixth point, and the eighth star in the hexagon. It is in the constellation Canis Minor, the Little Dog. Not only is it a lesser dog, it really is a lesser constellation, since there are only two stars in it above third magnitude.
So this month be sure to enjoy the dark moonless nights and find the winter hexagon.
Charles Hakes teaches in the Physics and Engineering Department at Fort Lewis College and is the director of the Fort Lewis Observatory. Email him at firstname.lastname@example.org.