It’s time for one of the most reliable meteor showers of the year, the Geminids – and unlike last year’s show (and the Perseids earlier this year), the Moon won’t be around to spoil the party. In fact, the Moon is giving somewhat of an encore to early morning viewers, rising as a waning crescent around 4am with a lovely conjunction with Jupiter in the pre-dawn east.
The Geminids are a bright, prolific shower with as many as 120 meteors per hour in dark skies (you can reliably anticipate about half that number), so if weather graces you with clear skies, it ought to be spectacular. Wednesday night into early Thursday morning (Dec 13-14) will be the peak, but several vanguard meteors ought to be visible now.
So, what’s going on here?
I’ve written a previous post on the predictability and geometry of meteor showers – the short description is that comets produce trails of dust from those dramatic tails, and then the Earth passes through that dust, and millimeter-sized grains burn up in our atmosphere, producing meteor streaks. The peak for these showers coincides with Earth’s passage through the middle of the dust stream, and no matter where you are on Earth, your best viewing tends to be in the early morning (2am or so), because at that point you can look directly along the path of Earth’s travel, straight at the shower’s “radiant” (apparent point of origin – in this case, Gemini, hence the name), and into the Star Wars hyperdrive effect of all these fireballs coming right at you.
Only, the Geminids are a little different. First off, there’s no comet involved. The Geminid parent is a rocky asteroid named Phaethon 3200. Coincidentally, that asteroid happens to be nearby Earth during this shower, cruising along at small-telescope-viewable brightness (Magnitude ~11) through Perseus. Well, how does an asteroid leave a dust trail, you ask? That’s a very good question – and one where we don’t have a solid 100% answer. The best guess, since Phaethon has an orbit that takes it fairly close to the Sun, is that acceleration and tidal forces tend to pull bits of gravel off the asteroid as it slings by – not enough to completely tear apart the rock, as is the fate of many sun-grazers, just enough to grab whatever’s loose.
The result, for us, is that instead of tiny little dust grains, Geminid meteors derive from tiny little pebbles instead – hard, rocky, even metallic particles that burn very brightly, for a long period of time. That combo generates some dramatic streaks and the occasional fireball – like this one, seen this week over the northeast US and captured by a police cruiser dash-cam.
In fact, this feature favors early-evening viewers. At 9pm, Gemini (and the direction of Earth’s travel) is comfortably above the eastern horizon. Picture a small chunk of rock entering the atmosphere from that direction at a shallow, grazing angle (as opposed to plunging straight toward the ground). What this produces is a long, streaking meteor that arcs across the sky. With their rocky composition, the Geminids can keep burning for a good, long portion of that journey. Of course this is a special case trajectory, but it happens enough that it’s worth trying to catch a glimpse.
What better way to spend your time do you have planned on on a cold December Wednesday night?
Get Out There!