Last month, I think I got more questions about the supermoon than any recent column topic I can remember. A recurring question was how I was going to watch it and what might be the best way to enjoy it. And since December’s full moon on the 14th is also at perigee, I thought this would be a good time to give a stargazer’s take on supermoons.
The technical term is perigee-syzygy. In astronomy, syzygy is when three or more celestial bodies happen to line up. A full moon happens when the sun, Earth and moon are all in a line. The moon’s orbit is elliptical, so every month there is a closest point, called perigee, and a farthest point, called apogee. If perigee happens to coincide with a full moon, you get perigee-syzygy. It looks larger because it is closer, so in that respect it certainly is a super moon.
The full moon in December is the third in a row to count as a supermoon. They happen about every 14th full moon, but there are many variables affecting the frequency, and there is not another one until 2018.
The term supermoon is a recent term more associated with astrology (think horoscopes) than astronomy. It is yet another product of social media and the information age. Yes, the moon is bigger and brighter, but that is exactly the problem for astronomers. Any full moon is so bright that its glare will wash out all but the brightest stars and clusters.
Instead of gearing up for a long night outside with the full moon, I suggest finding a good book. Or maybe a movie. Then wait several days. By then it should be dark enough in the evening to get a telescope out to find things in the night sky.
A full moon is definitely fun to look at through binoculars, but that is about the worst time to look at the moon through a telescope. When the sun is hitting the moon face on, you can’t see any shadows on the surface. Shadows are how you can see the relief of mountains and craters. The most detail is always found along the terminator, or the line between night and day on the surface of the moon. By watching how the shadows on the moon change from one day to the next starting with a thin crescent moon, you can appreciate the terrain. Along the terminator there are always unexpected places that are brightly lit or in shadow, depending on the time of the lunar day.
You should also note that without a dark gray neutral-density filter, the moon can be uncomfortably bright through a telescope eyepiece.
This monthVenus is still the evening star and is the brightest thing in the southwestern sky at dusk.
Although Mars has moved quite a bit against the background stars since last month, it is still roughly 30 degrees above the horizon to the south-southwest after sunset. It will remain there for the next few months, getting dimmer and dimmer as the Earth steadily pulls away.
The Geminid meteor shower peaks on the evening of the 13th. Since Gemini rises soon after sunset, this is one of the showers you don’t need to wait until after midnight to see. It is often one of the better showers of the year, if you can find a nice dark spot to view from. Unfortunately, there is that moon to deal with. Good luck.
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.