Perseids Meteor Shower 2012
UPDATE: Here’s my easy guide to the what, how, where, when and why of the Perseid Meteor Shower.
This month sees the most reliable meteor shower of the year; the Perseids. You can begin watching for Perseid meteors now, and the shower will last until late-August, but the peak of the shower occurs around mid-day on Sunday 12 August 2012, which means that the nights on either side of this will be good for meteorwatching.
The best time of night to watch the meteor shower is from around 2200 onwards on both 11 and 12 August 2012, once the radiant, the point from where the meteors appear to originate, rises above the horizon.
The number of meteors that you will observe every hour depends on a number of factors:
- the density of the cloud of dust that the Earth is moving through, that is causing the shower in the first place;
- the height above the horizon of the radiant of the shower, the point from which the meteors appear to radiate;
- the fraction of your sky that is obscured by cloud;
- the naked-eye limiting magnitude of the sky, that is a measure of the faintest object you can see.
The Perseid meteor shower has a zenith hourly rate (ZHR) of between 50 and 200. This is the number of meteors that you can expect to see if the radiant is directly overhead (the point in the sky called the zenith), and you are observing under a cloudless sky with no trace of light pollution.
However conditions are rarely that perfect. In the UK, for example, the radiant of the shower will not be at the zenith; it will be around 30° above the horizon at midnight, and 45° above the eastern horizon at 2am.
Assuming a clear night, the other factor is the limiting magnitude of the sky, a measure of the faintest object you can see. Man-made light pollution will be an issue for most people. From suburbia the limiting magnitude of the sky is ~4.5 (around 500 stars visible), so you will only be able to see meteors that are at least this bright; the fainter ones wouldn’t be visible through the orange glow. In a big city centre your limiting magnitude might be ~3 (only around 50 stars visible); in a very dark site like Galloway Forest Dark Sky Park the limiting magnitude is ~6.5 (many thousands of stars visible), limited only by the sensitivity of your eye. So in most cases it’s best to try and get somewhere nice and dark, away from man-made light pollution.
The calculation that you need to make in order to determine your actual hourly rate is:
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
Let’s plug the numbers in for the Persieds 2012.
ZHR = 100 at 1200 on Sunday 12 August 2012, but by 0001 on Monday 13 August, ZHR might be down to 50 say, maybe less.
h = 30° at 0001, 45° at 0200, 60° at 0400
k = 0 (let’s hope!)
m = 6.5 (assuming you can get somewhere dark).
So your actual hourly rate at 0200 under clear dark skies is
(50 x sin(30))/((1/(1-0) x 2^(6.5-6.5) = 25 meteors per hour at 0001 Monday 13 August
(50 x sin(45))/((1/(1-0) x 2^(6.5-6.5) = 35 meteors per hour at 0200 Monday 13 August
(50 x sin(60))/((1/(1-0) x 2^(6.5-6.5) = 45 meteors per hour at 0400 Monday 13 August
Remember though that these numbers might be lower if the ZHR drops off after the daytime peak.
It is of course worthwhile having a look on the days leading up to the peak, when the numbers of meteors will be gradually increasing towards this rate.
You can keep track of the increasing ZHR at the International Meteor Organisation website.
*UT = Universal Time = GMT, so for UK times (BST) add one hour to these