A long-exposure photo of a meteor shower. Credit: Juraj Tóth

Tomorrow night, December 13-14, will be one of the most impressive meteor showers of the year: the Geminids. Meteor showers are caused when Earth crosses the orbit of a comet. Even if the comet is nowhere near Earth at the time, there will still be tiny pieces of rock from the comet strung out along its orbit. These pieces collide with Earth’s atmosphere and burn up as meteors. Most of these pieces are mere pebbles, but there will be a few larger chunks that cause bright fireballs that light up the sky and occasionally hit the ground.

The Geminids are so called because they appear to come from the constellation Gemini. Their true source is the “asteroid” 3200 Phaethon, an old, dead comet that has lost all of its ices, leaving only a loose pile of rock. To see them, look out toward Gemini (near the more familiar Orion and Jupiter). A few hours after sunset (in the Northern Hemisphere), it should look like the picture below, but the best views will be after midnight.

The eastern sky in December, rendered by Celestia.

It’s best to be in a dark area with a fairly wide view of the sky to see the shower. If you have good viewing conditions, you should be able to see over a 100 meteors per hour after midnight, which is when your part of the world is most face-on to the stream of meteors.

The Geminids are one of the three big meteor showers that are easily visible in the Northern Hemisphere. The others are the Quadrantids in January and the Perseids in August. In the Southern Hemisphere, you can see the Eta Aquariids in April. All of these showers regularly have close to or over 100 meteors per hour, but the Geminids hold the most promise for the future. Unlike the Perseids, which have been observed for over 1,000 years, the Geminids were first seen in 1862 and have steadily increased in intensity over the years.

The neat thing about meteors is that even though they are mostly pebbles and may look rather faint, they burn bright enough to be seen for hundreds of kilometers. If you see a meteor low on the horizon, it may be as far as 800 km (500 miles) away! So if you count 100 meteors per hour, you’re counting all of the tiny pebbles hitting a huge area of Earth. Multiply that by 100, and you’ve got the whole planet covered. That makes sense because the bits of rock scattered along the comet’s orbit have to be spread out enough that we don’t see them as a tail.Meteors can be seen from so far than astronomers can use reports of the same fireball from different places to triangulate where any pieces might have hit the ground. If you see a fireball, you can report it here. So check it out, count a few meteors, and you may get lucky and see something bigger.