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The final meteor shower of 2010 is the Geminids, the peak of which falls on the night of the 13/14 December 2010. The Geminids is described by the IMO as “one of the finest, and probably the most reliable, of the major annual showers presently observable”, and this year’s shower is set to put on a good show. (You can read the IMO’s rather technical summary of the 2010 Geminids here: http://www.imo.net/calendar/2010#gem)

It won't look like this

The predicted Zenith Hourly Rate (see my previous post about ZHR and what it actually means here) is around 120. Although the peak is predicted to occur around 1100 on 14 December, it should happen some time between 1840 on 13 December and 1600 on 14 December 2010. The best time for the peak to occur for stargazers in the UK would be between 0030 and 0600 on 14 December, after the Moon sets but before twilight begins.

The radiant for this shower is actually quite favourable, and if you wait till the Moon sets at around 0030 on 14 December then the only light pollution limiting your view will be man-made. If you observe before the Moon sets then you will lose a few of the fainter Geminids in its glow, but it’s only a first quarter moon, and so will only really have an impact if you’re observing from very dark skies.

Let’s use the equation relating ZHR to actual observations of meteors to work out how many you might see:

Actual Hourly Rate = (ZHR x sin(h))/((1/(1-k)) x 2^(6.5-m)) where

h = the height of the radiant above the horizon

k = fraction of the sky covered in cloud

m = limiting magnitude

In the case of the 2010 Geminids, if observed from the UK, h = 45 degrees. Let’s assume you have clear skies (haha) with k = 0.

The number of Geminids you can expect to see from a variety of observing sites is as follows:

For very light polluted sites, such as city centres m = 3, and therefore you can expect to see only around 8 meteors per hour.

In suburban skies near a city or town centre m = 4, and you’ll see around 15 meteors per hour.

In rural skies where m = 5, you’ll see 30 meteors per hour.

Under very dark skies, where m = 6.5 (i.e. where there is no or negligible effect of light pollution, like in Galloway Forest Dark Sky Park) you’ll see up to 85 meteors per hour, once the Moon sets. A first quarter moon will impose a limiting magnitude, even at a very dark site, of around 6, in which case you’ll see a slightly reduced 60 meteors per hour.

Remember, all of these numbers assume perfectly clear skies. If half your sky is cloudy, cut these numbers in half!

How many Geminid meteors will I see?

Where are you observing from? Limiting magnitude Number of Geminids per hour
A very light polluted city centre 3 7 or 8
Suburban Site 4 15
Rural Site 5 30
Dark Sky Site 6.5 85 (after the Moon sets at 0030)

If you fancy a good view of this spectacular meteor shower, then head to Galloway Forest Dark Sky Park, where we have an evening of talks and meteorwatching planned, weather permitting!

Originally posted by Steve Owens (@darkskyman) on his blog Dark Sky Diary Pursuing darkness in an increasingly bright world

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