If December’s best natural sky show seems to get better every year, it isn’t your imagination. It’s science, and it’s spectacular!
Every year, the Earth travels some 940 million kilometers through space in its orbit around the Sun. Over the past years, decades, and centuries, comets and asteroids have traveled through the same region of our Solar System, leaving a trail of debris in orbit around the Sun. If the alignment is right, then once a year, the Earth will pass through that debris stream, creating a meteor shower when it does. The most spectacular ones of all occur in August (the Perseids), in December (the Geminids), and sometimes in November (when the Leonids are favorable). What you see varies from year-to-year, but this year’s Geminids just might be the most spectacular treat you’ve ever seen. If you have clear skies and a little bit of time on the night of the 13th/morning of the 14th, the Geminids will be at their peak. Here’s the story.
It all starts with either a comet or asteroid that gets hurled into the inner Solar System, close enough to the Sun to sprout a tail. Don’t be fooled by a common misconception: the tails themselves aren’t what give rise to meteor showers at all. Because the Sun blows the tail particles directly away from where the comet/asteroid is located, they’re not coherent enough to cause a “shower” if and when they do ever collide with Earth again. These tiny dust grains wind up as part of the micrometeroids populating interplanetary space, but play no other special role in our cosmic neighborhood. However, due to the tidal forces from the Sun and other massive bodies in the Solar System, the nucleus of the comet/asteroid gets stressed, causing tiny pieces of it to break apart. Thanks to the infrared imaging capabilities of the Spitzer Space Telescope, we’ve actually seen this in action!
The little dust grains — the particles between the major fragments — wind up getting stretched out over the entirety of the comet’s (or asteroid’s) elliptical orbit over time. In the rare occasions where the orbital path of such a comet or asteroid actually crosses the orbit of Earth, these particles will collide with our upper atmosphere. Those of you who remember your introductory physics class might recall a formula for the kinetic energy of a moving body: KE = ½mv². Even though the masses of these individual dust grains are tiny, from about the mass of a grain of sand up to a small pebble, they’re hurtling through space at tens of thousands or even hundreds of thousands of miles-per-hour (or meters-per-second) when they strike our atmosphere. And when it comes to energy, that “squared” on the velocity makes a big difference!
When a strike does occur, we see a streak of light lasting a fraction of a second (or more, if it’s a particularly large fragment), known as a shooting star or a meteor. There are three things that make a shower spectacular from your point of view:
- How frequent the meteors are, which has everything to do with the density of the particle stream that the Earth passes through.
- How bright the meteors are, which depends a little bit on the size of the fragments, but which depends much more on the speed of the fragments.
- And finally, how visible the meteors are, which depends on how dark your sky is.
The first one is something we can pretty much fully predict; we understand the physics of most of the particle streams, and so we have an excellent ability to predict the big meteor showers of the year. In general, the Perseids (in August) and the Geminids (in December, peaking this year on the nights of the 13th/14th) are the most reliably frequent. (Every 33 years or so, the Leonids become spectacular, but the next great storm won’t come until around 2030.)
The second one — how bright the meteors are — is something we can partially predict. Because we know the orbits of the comets and asteroids that give rise to the showers, we can fully predict how fast they’ll be moving when they strike Earth, and hence we know their velocity. For the Geminids, they’re caused by asteroid Phaethon 3200, a massive chunk of rock in an Earth-crossing orbit for the past 150+ years. Each orbit leads to more and more Geminid meteors, and the December show has continued to get more spectacular over time.
Predicting the masses of the particles that create these meteor showers are a bit of a different story. The physics there is complex, and the difference between a 0.1 ounce chunk of rock and a 1.0 ounce chunk of rock is a factor of 10 in energy. That’s why, above, you see a range of brightness in meteors. Sometimes, the brightness or faintness of a shower can surprise us, solely because of the size of the particles!
But the third one, how visible the meteors are, is subject to the amount of light pollution, both natural and artificial, in the sky.
The difference between a pristine, dark sky and an urban, light polluted sky is absolutely tremendous. The brightest, most infrequent meteors can still be seen from a badly polluted sky, but they won’t appear very spectacular. On the other hand, a very dark sky can result in you seeing ten times as many meteors, with the brighter meteors appearing much more spectacular! (If you’re wondering, a full Moon can turn a dark sky from a “1” into an “8” on the Bortle Dark-Sky Scale, above.) To find a dark-sky location near you, you can either download an overlay for Google Earth or (if you’re in North America) use this free online tool. In my experience, green or better (where blue or grey is best) is where you’ll want to be for meteor watching.
This year, at the peak of the Geminids, the Moon will be a waning crescent, not even rising until well after midnight. Even when it does, it will be thin enough and far enough away from the origin of the Geminids that you’ll still have a spectacular show. If you have dark, cloudless skies, you should be able to see up to two or three meteors per minute once the sky reaches full darkness this year. While the cold snaps affecting much of the country might make it a little unpleasant to be outside for too long, it also provides the best viewing conditions for the night sky. This year, you won’t want to pass up the opportunity.
However, don’t expect too much. The most spectacular images you often see (like the one below) are time-lapse images, often over the course of multiple nights, where a slew of meteors are composited together. But seeing multiple meteors at the same time should never be what you expect; when it happens, it’s all the more beautiful because of its rarity!
Seeing a bright light zip rapidly across the heavens might not seem like something so special, but when you consider the tremendous cosmic story it takes to bring us such a sight, it’s worth appreciating. Even if you don’t see a one, spending time with a dark sky gives you an appreciation unlike any other. If you’ve got the time next Sunday or Monday nights, wait until the crescent moon sets and head out to a dark sky location. Find the constellation of Orion and trace the bright blue star (Rigel) to the bright red star (Betelgeuse) and keep going until you’re just above the bright “twin” stars, Castor and Pollux.
This is the radiant, or the point from which all the meteors will emerge. Every meteor shower has one, and this one’s named the Geminids because the radiant occurs in the constellation of Gemini, the twins, which in this case is named for the twin stars: Castor and Pollux. Although you can look anywhere in the sky for meteors, you’re likely to see more if you look a little bit away from the radiant, watching for meteors emanating from that point in the sky. The Geminids should peak at about 140 meteors-per-hour right after midnight in the pre-dawn hours of the 14th. The Geminids should be more numerous but slightly less bright than the Perseids, since the debris stream is denser but the particles move a little slower. If you’ve got clear, dark skies, get to know them this December. It’s a reward and a natural wonder unlike any other.
Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.