Primary Moon Phases — December 2021!

Hey, Sky Fans!

Welcome back! Well, if you can believe it. It’s the holidays again, and another year is just about behind us. If you’re celebrating, too, a happy Hanukkah to you.

Thanks for reading again this year, no matter how spotty my posting has been. I appreciate you and that you take the time to check in.

So, away we go… as we round third and head for home, here are the Moon’s primary phases for December 2021!

Primary Moon Phases — December 2021
  • New Moon: December 4 — Solar eclipse… if you’re in Antarctica!
  • First Quarter: December 10 (US Time Zones) / December 11 (UTC)
  • Full Moon: December 18 (US Time Zones) / December 19 (UTC)
  • Third / Last Quarter: December 26 (US Time Zones) / December 27 (UTC)

I’ve really been looking forward to this month for a while. December’s usually a pretty great time to watch the skies, but the nights’ twists and turns have left us in a really cool spot to close out the year.

As I write this, the Moon’s in waning away in the last few hours of this lunar month. Tomorrow, December 4, its, and then the new lunation starts. We won’t really be able to see it for the first couple days after new. There’s nothing unusual there, but when it does come out from behind the curtain of dusk, it’ll pretty much immediately join the planets’ evening broadcasts, already in progress.

As night falls, we’ll see a line of three bright lights stretching across a couple dozen degrees of the southwest sky, angled upward into the night. Those three are sizzling Venus toward our left, then mellow Saturn in the middle, and then beautiful Jupiter at the right. Make a point of trying to find these three planets so when the Moon joins them, you’ll already know where to look. Maybe, just maaaaaaybe, you can spot a couple of stars calling out to you from the dusk, low above the horizon. These are a couple of stragglers from Saggitt… Saggi… Sagittarius (huzzah!), the Archer; still with us even though we usually think of them as summer stars. If you remember, we talked about how the direction of the center of our Milky Way galaxy is in the Archer’s general direction.

Now, it’s cut across a huge chunk of the sky. So when we look toward the Sun, and the new Moon, they’re in the direction of the galactic center, too. The Moon orbits us, we orbit the Sun, the Sun orbits the galactic center. This is the way.

Here’s a little fun with math.

It takes the Moon about 29 days to go through a full lunation. That’s a lunar month measured from one phase back to that same phase again. Usually, we go from new to new, but it doesn’t really matter. The Moon’s orbit isn’t exactly a circle, but it’s close enough. As we all learned in junior high or middle school, there are 360 degrees in a circle. Three-hundred sixty degrees divided by 29 days means the Moon moves across about 12 degrees of sky from one night to the same time the next, on average.

What this means is we can use the Moon and planets to help us measure the sky.

On Monday, December 6, the very thin waxing crescent Moon glides past Venus. Then it’s near Saturn on Tuesday the 7th, and finally it passes Jupiter on the 8th & 9th.

Six to seven (one night), seven to eight (two nights), eight to nine (three nights). Three nights, 12 degrees per night, that’s 36 degrees. In other words, that string of planets is about a tenth of the Moon’s orbit. And it adds up, too…. three nights is a tenth of a month…. you know, again, roughly. Cool stuff.

Three planets and the Moon’s not even at first quarter yet.

First quarter is on the 10th US time. The brightest star nearby is is Fomalhaut, the brightest star in Piscis Austrinus, the Southern Fish, but it’s not even that close. Fomalhaut never gets very night off the horizon while the Moon rides a high arc this time of year. This dim constellation isn’t to be confused with the more famous Pisces, the directionally agnostic fishes, which the Moon catches up with a few days later, on the 13th. Don’t be disappointed if you don’t see it, though. Pisces is tough with the light pollution and everything.

We cross between the Hyades and Pleiades clusters sometime between the 16th and 17th, and then we’re on our way through the Winter Hexagon. The Hexagon’s enormous, so it won’t be until the 21st until the Moon crosses back out near Pollux (in Gemini).

In the meantime, though full Moon is on the 18th, near the bright star Elnath. Remember how we talked about the new Moon being near Sagittarius and the galactic center? Well, now we’ve gone from new Moon to full. We’ve turned so we’re facing away from where the Moon was when it was new. Full Moon is half way around. Since the Moon was near the galactic center at new, what do we have here? Elnath happens to be near the galactic anti-center; the spot directly opposite the center of the galaxy.

As we look at the sky around the Moon when it’s full this month, we’re looking clear out of our galaxy and off into the deep emptiness of the universe. Wow.

The December solstice is on the 21st. That’s the start of winter in the north, and the star of summer if you’re tuning in from the southern hemisphere.

Third quarter is on the 26th near the bright star Arcturus (also Comet C/2021 A1 (Leonard)), Happy Kwanzaa and Merry Christmas, and then from there, there’s less than a week left in 2021.

The pleasure’s been mine, everyone. Happy everything, no matter what you’re celebrating.

Clear skies, everyone.


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