An Evening Meteor Shower

Regular readers of this column know that whenever we’ve covered the various annual meteor showers, it seems that they always reach peak activity in the pre-dawn hours of the morning—and that relatively few “shooting stars” are seen during evening hours. (The reasons for this are discussed below.) But this month we have the very “considerate” Geminids which display lots of evening action, to the delight of those skywatchers who must get to bed at a reasonable hour!

The annual Geminid Meteor Shower reaches maximum activity on the evening of December 13th. (It does continue into the early hours of the 14th for those who want to stay up later, but a bright last-quarter Moon will rise around midnight and light up the sky greatly reducing the number of meteors seen.) The best hours to view the event will be between about 9:00 p.m. until then. As one of the year’s richest displays, the Geminids are always well worth watching, with as many as 60 to 100 meteors per hour typically being seen during its peak under a dark sky. They appear to radiate from near Castor, one of the two bright “Twin Stars” (the other being Pollux) marking Gemini. To find them, set your Scientifics Star and Planet Locator for about 8:00 p.m. and face northeast, where you’ll see the pair just clearing the horizon. This early rising is what makes the Geminids so unique: the radiants of most showers lie well below the horizon during the evening hours. But in this case, the radiant is already up and you’ll see meteors immediately streaming outward with the number increasing as the night grows later and Gemini climbs ever-higher in the sky.

As to why meteor showers seem to increase in activity towards dawn, in the evening we’re on the side of our planet “facing away” from the direction the meteors are approaching so they have to “catch up” with us. But after midnight we’re turned into the direction of the radiant, causing them to slam into the atmosphere at much higher speeds. This results in many more meteors being seen right up until morning twilight interferes, which is when the impact-effect is at its maximum.

A word here about the radiant “rising.” It’s actually the horizon “setting” or dropping as our Spaceship Earth spins ever eastward rather than sky objects themselves rising. The same is true in the case of the Moon or the Sun appearing to come up. Once the eye-brain combination makes this connection, you may be in for an amazing illusion. While it can be experienced with the unaided eye alone, it’s best seen in binoculars with their “porthole” effect. Watch the Moon (preferably over a sea or other relatively smooth eastern horizon) seemingly lift and “heave” itself into the sky. As it does, you will feel the sensation of the Earth spinning headlong to meet it!

— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing. His latest, Celebrating the Universe!, is available from