Welcome to December, everybody! It’s hard to believe it’s already December, and that 2017 is almost over… but here we are.
I’m posting my weekly backyard astronomy thoughts a day early this week, because I don’t want you to miss the gloriously big, last (and only) Supermoon of the year, tonight, Dec 2. Now, the full Moon, the “Cold Moon” doesn’t actually occur until 15:47 UTC (mid morning on the US East Coast) on Sunday, but that will be far less dramatic than watching moonrise on Saturday. Then, added to the 7% increase in apparent size, you’ll also get the horizon-induced illusion that the Moon is even bigger. So, yeah, pretty super.
The Moon isn’t any bigger, of course – but the Full Moon happens to coincide with the Moon’s perigee, it’s closest approach to Earth during the course of a normal orbit, so it looks bigger. By the way, when the full Moon happens at apogee, its farthest distance from Earth, it’s called a “Micromoon”. Funny how we never hear about those (though apparently we had one in June!)
If you’re not tired of watching the Moon, and you happen to live in the US Pacific Northwest, Western Canada, Alaska, or much of eastern Europe, Russia, or Northern China, early Sunday morning (about 5:30am in British Columbia) you can watch the Moon occult Aldebaran, the bright red supergiant star in the face of Taurus, in the V-shaped Hyades cluster. Specific times can be found here.
Later this week, on Thursday, the 7th, we will have the earliest sunset of the year. Notice, this is NOT the shortest day, just the earliest sunset. Sunset will get later and later from this point, but sunrise is happening later too, and it’s changing faster, so overall, days are still getting shorter. The shortest day of the year is, of course, the winter solstice, on Dec 21, this year. But at my latitude (just to pick one for example), sunset will be almost 4 minutes later on the solstice than it will be on the 7th. Meanwhile the latest sunrise won’t occur until January 4, one day after the Earth’s perihelion, or closest approach to the Sun. The fact we’re approaching perihelion, and speeding up in our orbit with respect to the Sun, adds some quirky geometry tweaks to what is normally just a function of axial tilt. In any case, for most of us, this Thursday will FEEL like the shortest day of the year, and it’s how we feel that matters, right?
Last but not least, this week you may start seeing some Geminids, meteors from one of the most showy showers of the year. The Geminids radiate from Gemini, of course, but radiate across much of the sky. At the shower’s peak on Dec 14th, we may be able to see 100-120 bright meteors per hour! AND the skies will be nice and dark, since the Moon will be a thin waning crescent rising only shortly before dawn. If you’ve tried and failed to see some of the fainter showers over the past couple months, you’ll want to give the Geminids a try. The strange thing about the Geminids is that… well… we’re not 100% sure why they exist. They are generated by an asteroid, NOT a comet, called 3200 Phaethon that has a short (524 days, or ~1.4 calendar years) orbital period. Because it is “rocky” rather than snow-dusty like a comet, its debris tends to be harder and “chunkier” – and therefore, makes brighter meteors! But… is this old dust from when Phaethon used to be a comet, and is now down to just a rocky core? Or does Phaethon get close enough to the sun that tidal forces tend to rip parts of it off on every orbit? These are the leading theories, but we’re not really sure – so while you’re watching beautiful meteors, recognize that you’re viewing an annual mystery!
These are the highlights this week, but as I’ve mentioned before – winter skies are cold and clear, and excellent for stargazing!
Get Out There