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After the Peak: How Long Does a Meteor Shower Last?

August 13, 2014 Leave a comment

I had a question on Twitter asking whether it was worth while looking out for any Perseid meteors tonight after the shower peaked yesterday.

While individual meteors are short blink-and-you’ll-miss-them events, meteor showers themselves last many days, sometimes weeks. The rate of meteor activity builds up in the days before the peak, and tails off afterwards. The peak itself lasts a few hours, maybe a day or so for some broad-peaked showers.

Take a look at this graph of activity for the Perseids meteor shower 2013:

Perseids 2013 Rate (from imo.net)

Perseids 2013 Rate (from IMO.net)

The number of meteors per hour under ideal conditions (known as the ZHR, the Zenith Hourly Rate) built up to around 20 meteors//hour during the month before the peak. The rate started to increase around 9 Aug, doubling to 40 meteors//hour around 11 Aug, before tripling again to 120 meteors//hour around 13 Aug at the peak. The drop off was quicker, but even for the two days after the peak the rate was still above 20 meteors//hour.

So even in the days after the peak of the Perseids rates of meteors stat well above the background rate, at least 4 or 5 times that for a couple of nights.

Perseids Meteor Shower 2014

August 10, 2014 Leave a comment

This month sees the most reliable meteor shower of the year; the Perseids. However this year the near-full Moon will be in the sky too, brightening the sky so much that only the brightest Perseid meteors will be visible. It’s still worth looking up over the next week if you have clear skies, in case you might spot a bright Perseid.

You can begin watching for Perseid meteors now, and the shower will last until late-August, but the peak of the shower occurs on Tuesday 12 August 2014, which means that the nights on either side of this will be best for meteorwatching, albeit with interference from the moon.

Perseus at 0200 13 August 2014

The best time of night to watch the meteor shower is from around 2200 onwards on both 11 and 12 August 2014, 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.

This year though the full Moon is a great leveller, giving everyone a limiting magnitude of around 3.

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 2014.

ZHR = 100 at the peak, say.

h = 30° at 0001, 45° at 0200, 60° at 0400

k = 0 (let’s hope!)

m = 3 (pesky moon!)

So your actual hourly rate at 0200 under clear dark skies is

(100 x sin(30))/((1/(1-0) x 2^(6.5-3) = 4.4 meteors per hour at 0001
(100 x sin(45))/((1/(1-0) x 2^(6.5-3) = 6.2 meteors per hour at 0200
(100 x sin(60))/((1/(1-0) x 2^(6.5-6.5) = 7.6 meteors per hour at 0400

Remember though that these numbers might be lower if the ZHR drops off after the 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.

*UT = Universal Time = GMT, so for UK times (BST) add one hour to these

Perseids 2013: The What, How, Where, When, and Why

August 8, 2012 Leave a comment

Here’s a simple guide for observing the Perseids 2012 meteor shower this year, covering five basic questions:
What is the Perseids meteor shower?

The Perseids meteor shower is the most reliable of the active regular meteor showers that happen throughout the year. A meteor shower is a display of meteors (or shooting stars) where you see lots of them in the space of just a few hours. The Perseids occurs around the same time each year, in mid-August, and during the peak of the shower meteor rates increase from just a few an hour (the background rate that you’ll see on any clear, dark night) up to maybe 100 or 200 meteors every hour for observers in the perfect location. Meteorwatchers in the UK will probably see dozens per hour from dark sites, dropping to a few an hour (still worth watching for) in towns and cities.

How can I observe the meteor shower?

You don’t need any special equipment to observe a meteor shower; just your eyes. Try and get as far from city lights as possible (out into the countryside if you can, or into a local park if not), and get comfortable. You might want to bring a reclining deck chair with you, as that makes meteorwatching much more civilised! Just lie back and take in as much of the sky as possible. If you’re lucky enough to see a good display of meteors, you might see as many as one a minute, maybe more!

Where should I look?

Meteors streak across the whole sky, so you don’t need to look in any specific direction, but of course if you’ve got a tall building or tree that’s blocking the view, or a streetlight nearby that’s a bit glare-y, then put these to your back. The Perseids meteors all appear to streak from a point in the sky (called the radiant) in the constellation of Perseus (hence the name) which rises in the east about 10pm local time, climbing to its highest in the sky towards dawn.

When is it happening?

The peak of the meteor shower will probably happen some time around 1815 and 2045 UT (1915 and 2145 BST) on Monday 12 August 2013, although there are uncertainties here. The peak could happen any time between 1415 BST 12 Aug and 0215BST 13 Aug. This means that observers in the UK might catch the peak of the shower, if it happens after the sky darkens on 12 August. Even on the nights on either side we’ll still see plenty. In fact the peak of the Perseids is several days wide, so you can start meteorwatching early, and carry on well after 12 August, so that even if this weekend is cloudy you’ll almost certainly have a chance to see some Perseids. Whatever night you’re out you’ll see more the later you’re up. Starting after dusk, the meteor rate will increase each night as Perseus climbs higher in the sky towards dawn.

Why do meteor showers happen?

Meteors are tiny bits of space dust streaking through our atmosphere. These motes of dust float about in space and as the Earth orbits the Sun it hoovers them up. Sometimes the Earth passes through a particularly dense clump of dust, and we get lots of meteors, in a meteor shower. These clumps of dust are left behind by comets as the orbit the Sun, their streaking tails leaving behind a trail of tiny rock particles. The comet that left behind the space-rocks that we’ll see in the Perseids meteor shower is called Swift-Tuttle, after the two astronomers that discovered it in 1862.

Perseids Meteor Shower 2011

August 2, 2011 Leave a comment

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

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