Sky & Telescope - December 2020 - 17

Shower Attributes

ZHR G R A PH: IM O

As the chart at right shows, Geminid activity is asymmetric
- the shower features a slow rise, followed by a rapid decline.
That means you can expect to see a few Geminids perhaps
as early as December 4th. By December 10th, the ZHR is 10,
and on the night before the peak (December 12-13) the rate
reaches a very respectable 60 meteors per hour. But already
24 hours after the peak (December 14-15), the ZHR has
fallen precipitously to around 30, and on the following night
(December 16-17) it's back down to about 10. The display is
all but over the very next night.
Geminid meteoroids strike Earth's upper atmosphere at
around 35 km (22 miles) per second.
That's about 40% slower than the average
Perseid. Visually, the difference is fairly
obvious. Where a Perseid flits across your
field of view in a second or less, a Geminid
might be visible for a couple of seconds or
more. For this reason, many astronomy
books refer to Geminid meteors as "slow
and graceful."
Another difference between the two
showers is that about a third of all Perseids leave persistent incandescent trains in
their wake, yet only about 3% of Geminids do. This may have something to do
with the fact that Geminid fragments
are several times denser than average
meteoroids.

175

Geminids 2018 ZHR Graph
150

Zenithal hourly rate

one-third the distance from the horizon to the zenith.
But it's after local midnight when the radiant soars high in
the sky and our side of the planet rotates to face the direction
of Earth's orbital motion. That's when the numbers really
begin to increase and approach the advertised two-per-minute rate that makes the Geminids so gratifying. And since the
onset of dawn doesn't occur until around 5:30 a.m., we have
5½ hours of exciting meteor watching ahead of us.
By 2 a.m. the radiant will be almost directly overhead so,
if you are blessed with a very dark sky, you can see meteors
at the oft-quoted zenithal hourly rate (ZHR) of 150. The ZHR
is the predicted number of meteors that an experienced
observer could see when the shower's radiant is directly overhead and the naked-eye limiting magnitude is 6.5. If your sky
isn't quite that dark, or if part of your view is obstructed by
trees or clouds (or if the shower is less active than expected),
your count will be lower than the ZHR.

125
100
75
50
25
Dec. 5

6

7

8

9

10

11

12

13

14

15

UT date and time
p A GOOD YEAR Bright moonlight seriously hampered the Geminids in
2019, but in 2018 amateurs around the world followed the International
Meteor Organization's standardized procedures and counted 10,674
meteors to produce this activity graph. The shower maintained a zenithal
hourly rate of around 100 or more for about 22 hours, then, as usual,
dropped off faster than it rose.

Curiously, prior to the shower's peak Geminids tend to
be somewhat faint, whereas following the peak, for a dramatic 24 hours or so, they're considerably brighter. This may
be due to solar radiation pressure separating lighter cosmic
dust from larger and heavier meteoric particles. So, from the
appearance of the first forerunners up until the peak, the
preponderance of Geminids seen are generally in the range
of 3rd or 4th magnitude. But at the shower's peak, Earth
interacts with much larger fragments, of which 50% produce

u SUMMER SPLENDOR August's mild temperatures and frequent clear skies mean the Perseids
are the best-known annual meteor shower, even
though December's Geminids actually put on a
better show. This composite photo taken by Petr
Horálek shows the 2018 Perseid display and
includes 407 meteors recorded over nine nights,
between August 6 and 14, 2018.
sk yandtelescope.org * DECEMBER 2 02 0

17


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Sky & Telescope - December 2020

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