Perseids Meteor Shower 2016

August sees the return of the most reliable meteor shower of the year; the Perseids, with rates of shooting stars possibly increasing to over 100 per hour under perfect conditions.

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 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. On the night of 12/13 August we’ll pass through the densest part of a dust cloud left behind by Comet 109P/Swift-Tuttle, and will see the rate of meteors 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 2016, 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, once the radiant, the point from where the meteors appear to originate, rises above the horizon. However the moon will be in the sky until after midnight, and will interfere slightly for observers in dark sites. 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.

This year the Perseid meteor shower has an expected zenith hourly rate (ZHR) of around 150. 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 2016.

ZHR = 150 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

(150 x sin(30))/((1/(1-0) x 2^(6.5-6.5) = 75 meteors per hour at 2200
(150 x sin(40))/((1/(1-0) x 2^(6.5-6.5) = 96 meteors per hour at 0000
(150 x sin(50))/((1/(1-0) x 2^(6.5-6.5) = 115 meteors per hour at 0200
(150 x sin(65))/((1/(1-0) x 2^(6.5-6.5) = 135 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 135 meteors per hour, but “only” 75 per hour if the peak occurs at 2200 on 12 August.

Remember that these rates are for perfectly dark skies. If you live in suburbia 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.

Live in or near Glasgow? Come and join me for the Perseids #Meteorwatch at Whitelee Wind Farm!

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Orionids Meteor Shower 2014

October sees a fine meteor shower, the Orionids, grace our skies. While not reaching the maximum rates of one of the Big Three Meteor Showers (Perseids, Geminids, and Quadrantids), this month’s shower is still impressive, coinciding as it does this year with the new moon, meaning dark skies and lots of meteors for those who can get out of towns and cities.

Orionids

See: Meteor Showers: The What, How, Where, When, Why 

All meteor showers happen as the Earth passes through a cloud of dust left behind by comets (or in rare cases asteroids). The Orionids is associated with the most famous of all comets, Halley’s Comet, which orbits the Sun every 75.3 years.

The best time of night to watch the Orionids meteor shower is from local midnight onwards, once the radiant (the point from where the meteors appear to originate) rises well above the horizon. The maximum rate for this shower is due to occur on 21 October 2014, but unlike the big three meteor showers the Orionids peak is very broad, lasting several days rather than several hours, and so it’s worth watching on any night for a few days either side of the peak.

The ZHR of the Orionids 2012, from imo.net

Compare this chart with that for the Perseids meteor shower peak, which is much narrower (although because the Perseids is a much more active shower the ZHR stays high – above 20 – for roughly the same amount of time!):

The ZHR of the Perseids 2013, from imo.net

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 Orionids meteor shower has a maximum zenith hourly rate (ZHR) of  around 25 (sometimes the ZHR can be higher, up to 40, sometimes even 70, but that isn’t expected this year). 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 perfect. In the UK, for example, the radiant of the shower will not be at the zenith; it will be around 15° above the horizon at midnight, 30° above the horizon at 2am, 45° at 4am, and reaching a maximum height of 50° due south before dawn

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 Orionids 2014.

ZHR = 25 (maximum)

h = 15° at 0001, 30° at 0200, 45° at 0400

k = 0 (let’s hope!)

m = 6.5 (if you get somewhere really dark!)

So your actual hourly rate under clear dark skies is

(25 x sin(15))/((1/(1-0) x 2^(6.5-6.5) = 6.5 meteors per hour at 0001
(25 x sin(30))/((1/(1-0) x 2^(6.5-6.5) = 12.5 meteors per hour at 0200
(25 x sin(40))/((1/(1-0) x 2^(6.5-6.5) = 17.7 meteors per hour at 0400

If you’re observing in suburbia you need to divide these numbers by around 4, and in bright cities by 10!

Remember though that unlike other meteor showers where the peak lasts only a few hours these rates for the Orionids can last days, so any time you’re outside at night it’s worth looking up!

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

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.

Perseus at 0200 Monday 13 August 2012

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

Quadrantids 2012: Peak Activity Rate

This morning (Wednesday 04 January 2012) at  around 0530 the Quadrantids meteor shower reached its peak activity rate. According to the International Meteor Organisation the ZHR(max) was 78 +/- 7. As is usual with this meteor shower the peak was quite narrow, with activity starting to increase after 0000 on 04 January, and dropping off again by 1800 the same day.

Quadrantid Meteor Shower 2012, credit imo.net

As was predicted, observers who were out under clear skies between 0430 and 0630 would have got the best views, as within this two hour window the Moon had set and the Sun had yet to brighten the sky. The ZHR(max) of 78 was rather disappointingly low compared with the predicted maximum rate of ~120, but predicting these things isn’t an exact science. The IMO estimated that the rate usually falls somewhere between 60 and 200, so this year’s shower was certainly at the lower end of that, however that might be down to the fact that few people had clear skies (a common problem at this time of year) and so some meteors were missed.

A ZHR (max) occurring at 0530 on 04 January 2012, when the radiant was around 60° above the eastern horizon (in the UK), if seen from a cloudless dark sky site with no light pollution (i.e. a limiting magnitude of 6.5) would have meant that you’d have seen around 70 shooting stars an hour, still quite an impressive show

Perseids Meteor Shower 2011

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

Meteor Showers & the Zenith Hourly Rate

Today (6th May 2010) was the maximum of a meteor shower called the eta-Aquarids. This shower lasts many weeks, beginning on 19th April and ending around 28th May, with a peak of activity today. If you look up info for this shower you will find quoted a Zenith Hourly Rate (ZHR) of 85.

That means  if you were oberving this shower in perfect conditions – no light pollution, no cloud, no moon, and with the radiant of the shower (the point where the meteors appear to emerge from) directly overhead (at the zenith) you would expect to see around 85 shooting stars per hour.

It won't look like this

When people talk about meteor showers they sometimes quote the ZHR as an indication of how many you can expect to see, but this is often misleading, for reasons I’m about to discuss.

Let’s use the eta-Aquarids shower as an example.

If I wanted to view the shower at it’s most advantageous position I’d need to go out around 0200 tomorrow morning, after the radiant had risen in the east, and before the sky starts to brighten as we enter twilight.

The radiant at this point will be very low, about 5° above the horizon, well short of the 90° it needs to be for the radiant to appear directly overhead.

This isn’t the only problem; the Moon is up in last quarter phase, rising before the radiant, and so the Moon’s light will drown out the fainter meteors.

Then there’s light pollution. From my garden I can see stars (and meteors) down to around magnitude 4, and so the fainter ones won’t be visible through the orange city glow. Let’s assume the Moon’s glow won’t be any worse than the man-made light pollution.

And finally there’s the more mundane problem of cloud cover. If my sky isn’t entirely clear of clouds, then I may miss some of the shooting stars.

How do these factors effect the number of meteors I’ll see? We can put them into an equation to find out:

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

Tonight for the eta-Aquarids at 0200 ZHR = 85, h = 5°, m = 4 (optimistically), k = 0 (very optimistically)

Therefore the actual hourly rate of eta-Aquarids I can expect is

(85 x sin(5))/((1/(1-0)) x 2^(6.5-4)) = 1.3, or just over one per hour. Not great at all, and that’s assuming a cloudless sky!

But don’t let that put you off. You never know, if you pop out at 0200 (perhaps taking a break from the UK election coverage) look over to the east and maybe you’ll catch that one bright eta-Aquarid meteor…