Geminid Meteor Shower 13-14 December 2010

Despite the quarter-moon, the totally clear sky looked very promising on the afternoon of the Geminid meteor shower. However, after sunset, it wasn't very long before clouds moved in. Fortunately this turned out to be just partial cloud cover, and for only part of the night. Despite being partially obscured by clouds and bright moonlight, this seemed to be a very active shower as captured on a DSLR. The time-lapse video which resulted shows the Geminid meteor shower activity over the predicted peak evening (13-14 Dec.). It covers ~12-1/2 hours, starting at about 17:35 PST, just after sunset, to shortly before sunrise. Coverage is continuous except for a few minutes in the middle while memory cards were changed on the camera, and for 5-second gaps between frames. Each frame is a 30-second exposure. Geminid meteors can be seen in bursts throughout the night.

The wide angle view, which approximates what one can see visually, is looking slightly south of east. Polaris can be seen prominently in the upper left corner of the frame. Watch the rotation in the video to identify it. As the sky darkens after sunset, the Milky Way becomes visible, oriented vertically in the left half of the frame, running from Cassiopeia at the top through Taurus near the bottom. The half-illuminated, first-quarter moon is visible in the frame until about 19:30, but sets much later in the evening. As the moon leaves the frame, the meteor shower radiant, near the star Castor, rises over the wall at about the 8 o'clock position in the frame.

The moon set after midnight, changing the nice blue sky into the ugly orange-brown of city lights scattering off a thin layer of clouds. The radiant leaves the camera frame (top, center) at about 02:15 PST, but Geminid meteors continue to be visible till morning. Just before sunrise the zodiacal light and Venus are visible. The last meteor is visible at 05:20.

99 frames out of over 1250 frames show one or more Geminid meteors, which is pretty good, considering that moonlight and clouds must have obscured many meteors. A graph of the meteor count (in 15-minute bins) can be seen above. Note that the count above reflects what I could detect on each frame while looking at each frame nearly full-screen for a few seconds. I may have missed more faint meteors. In addition, though the camera sensitivity seemed to be close to what I could see visually, many meteors were lost in the seemingly short interval between frames (Murphy's law!). I visually watched a few hours of the shower, but gave up counting the meteors lost to the camera after 15 instances. In some cases two meteors were missed in a single frame gap.

On the video many of the meteors are very faint or very short (near the radiant). As a help when viewing the video, when a meteor is visible in the frame, a round yellow indicator appears in the lower left corner of the frame, accompanied by an audible click in the sound track. Each camera frame actually spans 3 video frames to make it easier to see the meteor as the video plays, but the meteor still may not be visible due to the low resolution and compression of the video. When stepping through the video slowly, use the yellow indicator to help locate the frames with meteors in them.

There are many things happening in the video. At first glance it may be very confusing, so here are some notes to make sense of what you're seeing:

Geminid meteors look like they are "radiating" from near the star Castor, in the head of the western twin. These are almost always fully on a single frame, lasting typically for less than a second in real time, though they could be actually so slow that they can be seen for 5-10 seconds. Near the radiant point, the trails are very short in length.

Sporadic meteors (i.e. not associated with the Geminid shower), are almost always fully on a single frame, though in real time, like Geminid meteors, they could actually be so slow that they can be seen for 5-10 seconds in real time. They are distinguished from true Geminid meteors by the direction of travel. These are not "tagged" in the video.

Satellites will be visible and often look like a meteor in these long exposures. They often appear to "flare" or suddenly brighten as the sun glints off a polished flat surface such as a solar panel. But these are distinguished from meteors by their slow speed, typically appearing on several frames. Sometimes a satellite trail will also look like a dashed line, indicating perhaps a rotating part catching the sunlight periodically.

Aircraft can be seen as streaks in these frames, sometimes with a dotted trail caused by the aircraft's beacon. Aircraft are visible in multiple frames, taking minutes to cross the sky and often in curving paths.

Contrails (and clouds) were common on this evening. Some of these are obvious as they can be seen to form as an aircraft crosses the sky. Most of the time meteors do not leave any aircraft-type contrails due to the higher altitude of the meteor flash, but occasionally very bright meteors leave a trail of some sort (possibly debris or ionized air) which glows and may be visible for several minutes. No meteor trails are obvious in this video, but some of the meteors were seen to leave small trails when the frames were examined very closely.

Lens flares are visible early in the evening, caused by scattering and reflections between the lens elements when a bright object like the moon is nearby.