Perseids Meteor Shower 2015

This month sees the return of the most reliable meteor shower of the year; the Perseids. And with a New Moon occuring at the same time as the peak of this shower this is the perfect opportunity to see hundreds of shooting stars.

Read my previous blog post: Meteor Showers: The What, How, Where, When, Why for general advice on how best to observe meteor showers.

A shooting star – otherwise known as a meteor – is a tiny piece of space dust that burns up in our atmosphere, forming a bright, brief streak of light in the sky. Many people have never seen a shooting star, and think they’re rare events, but given clear dark skies you can expect to see a few every hour on a clear night. From cities, under light polluted skies, you can’t see most of the faint ones, and so only the rarer bright ones are visible.

However at regular times each year the Earth moves through thick clouds of space dust – left behind by comets – and we get a dramatically increased rate of meteors. We’re already within the diffuse outer reaches of the dust cloud that forms the Perseids, and on the night of 12/13 August we’ll be in the densest part of that cloud, and will see the rate increase by a factor of 20!

You can begin watching for Perseid meteors now, and the shower will last until late-August, but the peak of the shower occurs overnight on 12/13 August 2015, which means that the nights on either side of this will be best for meteorwatching.

Location of the Perseids Radiant at 0001 on 13 August

The best time of night to watch the meteor shower is from around 2200 onwards on 12 August 2014, once the radiant, the point from where the meteors appear to originate, rises above the horizon. The later you observe the higher the radiant will be, and the more meteors you’ll see.

The number of meteors that you will observe every hour depends on a number of factors:
•the density of the dust cloud 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 darkness of your sky, measured using naked-eye limiting magnitude, a measure of the faintest object you can see.

The Perseid meteor shower has a zenith hourly rate (ZHR) of around 100. 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 2200, 40° high at midnight, and 50° high at 0200.

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 2015.

ZHR = 100 at the peak, say.

h = 30° at 2200, 40° at 0000, 50° at 0200, 65° at 0400

k = 0 (let’s hope!)

m = 6.5 (if you’re observing under skies free from light pollution)

So your actual hourly rate under clear dark skies is

(100 x sin(30))/((1/(1-0) x 2^(6.5-6.5) = 50 meteors per hour at 2200
(100 x sin(40))/((1/(1-0) x 2^(6.5-6.5) = 64 meteors per hour at 0000
(100 x sin(50))/((1/(1-0) x 2^(6.5-6.5) = 77 meteors per hour at 0200
(100 x sin(65))/((1/(1-0) x 2^(6.5-6.5) = 90 meteors per hour at 0400

Remember though that these numbers apply only to the peak of the Perseid occuring at these times. If the peak happens at 0400 on 13 August (and we’re not sure exactly when it’ll occur) then you might see 90 meteors per hour, but “only” 20-25 per hour if the peak occurs at 2200 on 12 August.

Remember that these rates are for perfectly dark skies. If you live in surbribia then divide these numbers by 4 or 5; if you live in a bright city divide these numbers by 10. Take home message: get somewhere dark!

It is worthwhile having a look on the days leading up to the peak, when the numbers of meteors will be gradually increasing towards this rate.