Get Ready to Greet the Geminid Meteor Shower!

By: Bob King December 13, 2023 5

The year’s most spectacular meteor shower is upon us. Prepare to enjoy the Geminids under a dark, moonless sky, when you might see more than 100 meteors per hour.

Geminids 2020 — Jeff Sullivan, who lives at Topaz Lake on the California-Nevada border, created this composite of the 2020 Geminid shower over Death Valley. Most of the meteors are Geminids. They are not aligned on one radiant point because this is a very long exposure, during which the sky rotated and the radiant moved. The starfield is a short exposure. Sullivan and his wife lead night-photography workshops in wilderness, dark-sky locations.
*Copyright Jeff Sullivan Photography*

August's Perseids may have the nicer weather but the Geminids have the numbers. No annual shower is richer. Up to 120 meteors per hour might be visible under ideal circumstances giving you plenty of opportunities to make a wish upon a (shooting) star. No Moon will trouble the view so this year's Gems should be a visual feast. If possible, try to view the shower from a rural location to make the most of the opportunity. For help in finding a dark sky check out the interactive map at lighpollution.info.

The annual flurry of meteors peaks at 19h UT (2 p.m. EST, 11 a.m. PST) on December 14th. Since this is the middle of the day for U.S. observers, the night of December 13–14 when the radiant stands highest in the pre-dawn hours will likely be best time to catch the shower's peak. On the other side of maximum, the evening of December 14th could be equally bounteous. Although there are generally fewer meteors visible before about 10 p.m. local time more observers may prefer the evening hours because of somewhat more moderate temperatures and the chance to get a good sleep.

Geminid radiant — The Geminids will appear to stream from a radiant near the star Castor in the constellation Gemini. Meteors headed toward the horizon will be mostly out of view, but those aiming upward will be well visible. The radiant reaches its peak altitude around 2 a.m. local time on December 14th. Although seeing more than 100 meteors per hour is possible, half that number is a more realistic expectation under typical suburban and semi-rural skies.
*Sky & Telescope diagram*

The split-the-difference maximum this year is good news for another reason. If one or the other night is overcast you've got a backup. And if you're socked in both nights, astronomy popularizer Gianluca Masi will livestream the shower on both December 13th and 14th starting at 23:00 UT (6 p.m. EST). Pop some popcorn and watch it from your easy chair.

It may sound counterintuitive but shower-watchers can begin their Geminid odyssey as early as late twilight on both evenings. While the radiant will be only a few degrees above the horizon at that time, viewing geometry favors the appearance of earthgrazers. These are slow-burn meteors that skim the upper part of the atmosphere and travel large distances before fading out. Although their numbers are few, if you catch sight of one you'll never forget the experience.

Click for a clear summary of what to expect in the upcoming shower.
Sky & Telescope

By 10 p.m. local time the radiant climbs to around 40° altitude in the northeastern sky, high enough to begin shower-watching in earnest. Radiant height is one of the key factors in determining how many meteors show. When it hovers low, most of the downward-traveling shower members are cut off by the horizon. When it stands highest around 2 a.m. local, time the Geminids have the entire sky as their stage and many more meteors are visible.

Meteor geometry — In most cases, a meteor shower will appear its strongest in the hours before dawn (as at left) — when Earth's orbital velocity combines with the particles' velocity to increase arrival speeds. These same particles, hitting Earth's "trailing" side after sunset (right), will strike at relatively slower speed.
*Sky & Telescope*

Meteor numbers also increase in the morning hours because the rotating Earth faces in the direction of its orbital motion. Cosmic dust hits the planet's atmosphere head-on at increased speed, which also makes them brighter. For the same reason otherwise-invisible meteors rise to the visual threshold and increase the tally.

Meteor-watching requires no equipment unless you count warm clothing, insulated boots, a reclining lawn chair, a wool blanket pulled to your chin, and a hot drink nearby. Well, no optics anyway, except for the "binoculars" you were born with. Since the Geminids will flash anywhere in the sky, the best direction to face is the one where the sky is darkest at your location. If you're watching from the countryside, tilt your chair back about halfway and look 45° to 90° degrees to either side of the radiant. From this corner-of-the-eye perspective the meteors will appear longer and more dramatic.

Geminid over Tucson — A spectacular Geminid fireball flares over Tucson, Arizona, on December 14, 2017.
*Eliot Herman*

Before they arrive at Earth Geminids are small rock fragments called meteoroids that range in size from sand grains to chocolate chips. Striking the atmosphere at around 34 kilometers per second (more than 76,000 mph), heating from ram pressure transforms the grit into piercing, pin-like streaks of light or meteors. While the majority of shower members are relatively faint, Geminids are renowned for cranking out fireballs, meteors that blaze as bright or brighter than Venus. Data from NASA's All Sky Fireball Network shows the shower ranks second after the Perseids in their frequency.

Observers first recorded the Geminids in 1862, making them relative newcomers to the meteor scene. Since then, the shower's activity has risen and now overtops them all. Most meteor showers originate from comets. When a comet passes near the Sun heat vaporizes some of the dust-laden ice that comprises its nucleus. Over time this creates a trail of debris around its orbit. When Earth passes through the stream the fragments slam into our atmosphere and light up as meteors.

Phaethon dust — Astronomers hypothesize that solar heating causes cracks in the surface of the asteroid Phaethon, which release dust that produces the Geminid meteor shower. But it doesn't account fully for the amount of dust found in the Geminid stream.
*NASA*

But the Geminids aren't like most meteor showers. Their parent is 3200 Phaethon, which to the best of our knowledge is an asteroid, although some astronomers consider it a rock comet because it has shed so much ice it more resembles a rocky asteroid. In their 2023 paper Formation, Structure, and Detectability of the Geminids Meteoroid Stream, W.Z. Cukier and J.R. Szalay (Princeton University) suggest a surprising new origin for the Geminids based on observations from the Parker Solar Probe (PSP). When they tested several models of its formation, including the standard version of gradual comet dissolution, the one that fit the PSP data best was a "violent, catastrophic destruction of bodies" near perihelion that led to a rapid release of material.

Princeton University researchers hypothesize that the Geminid meteor shower has its origins in a violent, high-speed collision between two cosmic bodies.
Princeton University

Their violent origin model predicted that the Geminid stream would lie outside of Phaethon's orbit when at perihelion (what Parker observed) rather than inside, which is the case with comet-dribbled meteoroids. Interestingly, the reflectance spectra of Phaethon and the asteroid Pallas (along with the nine members of the Pallas family) are all similar enough to posit Pallas as Phaethon's parent body. Numerous large craters pockmark Pallas. Their size and number may be due to the asteroid's elongated and steeply tilted orbit (currently 35°), which would have led to higher impact velocities. Phaethon's orbit is likewise elongated and steeply slanted (22°). Could these characteristics have led to a similar high-speed impact resulting in the birth of the Geminids? Something to ponder when you're waiting for that next meteor. Clear skies!

Read more on the Geminid meteor shower and other upcoming celestial events in the December issue of Sky & Telescope magazine.

Comments

StanR

December 2, 2023 at 12:41 am

The increased meteor count with radiant elevation isn't due to "downward-traveling" meteors being cut off by the horizon; if so, one might think that 50% of the meteors remain at a zero-elevation radiant. It's easier to reset the discussion and think of flux. (Also, I'm going to use zenith angle, ZA, which is just (90 degrees - elevation angle).)

For example, from the sun we get a flux some number of photons per square meter per second when overhead; when the sun is at 60 degrees elevation or 30 degrees ZA, the ground is angled to the sun at 30 degrees so (relative to zenith) the flux arriving is cosine(30 deg) or about 87%; at ZA 45 degrees, about 71%; at ZA 30 degrees, about 50%, on down to zero at the horizon. Sometimes this is called the "cosine effect."

In the present case we have some number of meteors per square mile per second, and again the flux of meteors arriving in one square mile (relative to zenith) is proportional to cosine(radiant ZA).

We can "see" an invisible "surface" above us at the altitude at which meteors "ignite", and that consists of some number of square miles. The number of meteors crossing that area and therefore igniting there will be proportional to cosine(radiant ZA). (This will not be exact for a number of reasons including extinction and different "ignition altitudes" for meteors entering at different angles.)

Anyway, crossing fingers for good weather.