★ Geminid Meteor Shower

The Geminids are the strongest meteor shower of the year and, for many observers, the most reliable. Under a dark sky you can see a meteor every minute or so around the peak, and many are bright. They also do not make you wait for the small hours: the radiant is well up by mid-evening.

Where the meteors come from

Most showers come from comets, but the Geminids come from an object that looks like an asteroid: (3200) Phaethon. It was discovered in 1983 by the Infrared Astronomical Satellite, and the astronomer Fred Whipple recognized it as the source of the Geminids. Phaethon circles the Sun every 1.4 years on a stretched orbit that takes it closer to the Sun than Mercury. It shows no real comet tail and its spectrum resembles a rocky asteroid's, so scientists still debate whether it is a "dead" comet or a "rock comet". Geminid meteoroids are several times denser than typical comet dust, and they enter the atmosphere at about 35 km/s.

When to watch

The shower is active from December 4 to 20 and peaks around December 14; the table above gives the expected peak time and the Moon. The maximum is broad, with a ZHR of 100 or more for roughly 10 to 12 hours.

From mid-northern latitudes the radiant rises around sunset and is already about 30 degrees up by 9 p.m. It is highest around 2 a.m. local time, when rates are best. From the Southern Hemisphere the radiant rises later and stays low, so rates there are lower and best around the middle of the night.

How to find the radiant

The radiant is in Gemini, a couple of degrees from Castor, the slightly fainter of the bright pair Castor and Pollux. In the evening, find Orion in the east and look to its left, toward the east-northeast: the pair rises there one above the other, with Castor on top, at about the same height as Orion's red star Betelgeuse. By 2 a.m. they are almost overhead from mid-northern latitudes. From the Southern Hemisphere, look low in the north-northeast around midnight.

What to expect

The ZHR of 150 assumes a perfect sky with the radiant overhead. From a dark rural site, a realistic count around the peak is about 60 to 100 an hour once the radiant is high; around 9 or 10 p.m., expect roughly half that. From the suburbs, 20 to 40 an hour is a good result.

Geminids are medium-speed meteors, slower than the Perseids, and often look bright and yellowish. The population index of 2.6 means a good share of bright meteors, and the shower is known for fireballs; IMO notes that the brightest tend to arrive toward the end of the main peak. Lingering trains are rare.

Viewing tips

• Start early. The Geminids are worth watching from 9 or 10 p.m., which suits children and anyone with work the next day.
• Dress for winter: layers, a hat, gloves and a sleeping bag on a reclining chair.
• Look about halfway up toward the darkest part of the sky. You do not need to stare at Gemini.
• If the Moon is bright (see the table above), sit in the shadow of a building and face away from it. The brightest Geminids still show.

History and notable displays

The Geminids are young in the record. They were first noticed in 1862, by Benjamin Marsh and Alexander Twining in the United States and, independently, by observers in England. Those early displays gave only 10 to 20 meteors an hour, and the shower has grown into the strongest of the year. In November 2018 the WISPR camera on NASA's Parker Solar Probe imaged a faint dust trail along Phaethon's orbit; Karl Battams and colleagues concluded that it is part of the Geminid stream rather than fresh dust from Phaethon. Because the stream is on a short orbit, it can change fairly quickly, and IMO asks observers to record every return.