Dark Sky Diary

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 mid-August, but the peak of the shower occurs in the small hours of Saturday 13 August 2011.

Perseus under dark skies

Perseus under moonlit skies

Unfortunately this year’s shower will be obscured by the full Moon which occurs on the same day, and so it won’t present its usual excellent display.

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. Observing from Glasgow, as I will be, the radiant will be around 60° above the eastern horizon at 2am. (In the far south of the UK it will be a few degrees lower, and in the far north a couple of degrees higher).

Assuming a clear night, the other factor is the limiting magnitude of the sky, a measure of the faintest object you can see. Even if the Moon were not in the sky, man-made light pollution would be an issue for most people. From my garden the limiting magnitude of the sky is ~4.5 (around 500 stars visible), so I 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.

However a full Moon introduces natural light pollution that can be as bad the man-made glare in a city centre, with a limiting magnitude of ~3 (it is hard to estimate what the limiting magnitude will be exactly, but this is a decent estimate).

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 2011.

ZHR = 100, say (might be as low as 50 or as high as 200, so our final answer might be out by a factor of two in either direction)

h = 60°

k = 0 (let’s hope!)

m = 3 (for the full Moon)

So your actual hourly rate under clear skies is

(100 x sin(60))/((1/(1-0) x 2^(6.5-3) = 7.7, or 8 meteors per hour. This might be out by a factor of two, so you might see as few as 4 per hour, or as many as 16 per hour.

If the full Moon wasn’t present we might expect somewhere around 80 meteors per hour.

So, this might be a poor show compared to moonless Perseids displays, but you will still still see plenty of shooting stars if you’re out for a few hours around the peak time (between 2200 UT* on 12 August and 0300 UT* on 13 August 2011).

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