Archive

Posts Tagged ‘meteor’

Perseids Meteor Shower 2015

August 5, 2015 2 comments

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

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.

Lyrids Meteor Shower 2015

April 18, 2015 3 comments

UPDATE 24/04/15 Now that we’re past the peak it looks like the Lyrids meteor shower performed as expected. Reports from the Society for Popular Astronomy suggest that plenty of meteors were seen over the UK.lyr2015overview

A wider survey made by volunteers submitting data to the International Meteor Organisation shows that a peak with ZHW=18 occurred more or less on cue around midnight on 22/23 April, with a possible second several hours later around 0700UT where the rate if anything was a little higher, with ZHR=22.

 

Over the next week one of spring’s best meteor showers will start to put on a show. The Lyrids meteor shower peaks overnight on the night of 22/23 April 2015, and should be best around midnight.

lyrid-meteor-shower-592x309

It’s quite hard to predict when exactly the peak will occur, and indeed you’ll still see some Lyrid meteors on the nights either side of the peak, so whenever you’ve got clear dark skies between now and 25 April it’s worth gazing skywards (isn’t it always?) in the hope that you’ll see a shooting star.

Why is the Lyrids Meteor Shower Happening This Week?

Meteor showers like the Lyrids happen when the Earth passes through a cloud of dust in space, These clouds are left behind by comets as they orbit the Sun, and the cometary cast-offs burn up in our atmosphere causing lots of bright streaks of light which we call meteors, or shooting stars. On any clear dark night you should see a few shooting stars, as random bits of space dust burn up overhead, but on the nights around the peak of a meteor shower, when the Earth is passing through a dense cloud of comet-dust, the rates can dramatically increase.

How Many Lyrids Will I See? There are a few ways you can maximise your chances of seeing some Lyrids (see The What, How, Where, When and Why of Meteor Showers) but the best way is to get somewhere dark, like one of the UK’s International Dark Sky Places. On the peak of the Lyrids meteor shower, under ideal conditions, you might see around 18 meteors per hour.

The peak of this particular shower doesn’t last very long, and so the rate on either side of the peak might be quite a bit less. Nonetheless it’ll still be well above the background rate of meteors. However the Lyrids occasionally surprises us and puts on a much better show. Back in 1982 there was a short-lived burst of Lyrid activity that saw the rate increase from 18 to 90. The same thing could happen this year: you never know until you look!

Ideal Conditions It’s the “ideal conditions” clause above that’ll reduce the rate from this maximum of 18. Ideal conditions are: perfectly clear skies; perfectly dark skies, free of light pollution; and the meteor shower radiant (the point where they all appear to emanate from) sitting directly overhead. The Lyrids’ radiant will be around 30° above the horizon at midnight, when the peak is meant to occur, but you can begin your meteorwatch as soon as it gets dark enough. You’ll then have until the sky brightens again pre-dawn. . 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.

Crunching the Numbers The Lyrids meteor shower has a maximum zenith hourly rate (ZHR) of  around 18. 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 20° above the horizon at 2200, 30° above the horizon at 0000, 50° at 0200, to a maximum height of 70° pre-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 Lyrids 2015.

ZHR = 18 (maximum) h = 30° at 0000 (assuming the maximum occurs at midnight; it might not) k = 0 (let’s hope!) m = 6.5 (if you get somewhere really dark)

So your actual hourly rate under clear dark skies is (18 x sin(30))/((1/(1-0) x 2^(6.5-6.5) = 9 meteors per hour If you’re observing in suburbia you need to divide this by around 4, and in bright cities by 10! Nonetheless, even in a city you’ll see a few Lyrids over the course of the night.

Geminids Meteor Shower 2014

December 8, 2014 2 comments

One of the most active and reliable meteor showers, the Geminids, happens every year in mid-December. This year’s display promises to be a good one for those meteorwatchers with clear skies.

The maximum rate of Geminids is predicted to occur between sunset on 13 Dec and sunset on 14 Dec, so the night of 13 Dec is the best bet, although nights on either side will still show plenty of shooting stars.

UPDATE: The excellent International Meteor Organisation (imo.net) have issued a live graph of Geminid activity. Last night the peak rate was around 46/hr +/- 21/hr. That rate will only increase overnight tonight, to a peak of around 120/hr.

When Gemini Sends Stars to Paranal  Image Credit & Copyright: Stéphane Guisard (Los Cielos de America), TWAN

When Gemini Sends Stars to Paranal
Image Credit & Copyright: Stéphane Guisard (Los Cielos de America), TWAN

There are a few ways you can maximise your chances of seeing some Geminids (see The What, How, Where, When and Why) but the best way is to get somewhere dark, like one of the UK’s International Dark Sky Places. I’ll be heading down to Galloway Forest in SW Scotland.

The Geminids’ radiant (the point in the sky where all the meteors appear to emerge from) rises at sunset, so you can begin your meteorwatch as soon as it gets dark enough. You’ll have until near midnight under dark skies, at which point the last quarter moon will rise to brighten the sky a little and drown out some of the fainter meteors.

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 Geminids meteor shower has a maximum zenith hourly rate (ZHR) of  around 120 (the highest of any meteor shower). 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 10° above the horizon at 1800h, 25° above the horizon at 2000h, 40° at 2200h,, 60° at 0000h just as the Moon rises to spoil the view a little.

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 Geminids 2014.

ZHR = 120 (maximum)

h = 10° at 1800, 25° at 2000, 40° at 2200, 60° at 0000

k = 0 (let’s hope!)

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

So your actual hourly rate under clear dark skies is

(120 x sin(10))/((1/(1-0) x 2^(6.5-6.5) = 21 meteors per hour at 1800
(120 x sin(25))/((1/(1-0) x 2^(6.5-6.5) = 50 meteors per hour at 2000
(120 x sin(40))/((1/(1-0) x 2^(6.5-6.5) = 77 meteors per hour at 2200
(120 x sin(60))/((1/(1-0) x 2^(6.5-6.5) = 104 meteors per hour at 0000

If you’re observing in suburbia you need to divide these numbers by around 4, and in bright cities by 10! Nonetheless, even in a city if you’re out at midnight during peak activity you’ll see around 10 meteors.

Remember though that these numbers are assuming perfectly clear skies under perfectly dark conditions, and are assuming a peak rate of 120 at each of these times. It probably won’t be nearly this good, but the bottom line is: there’s never a better night to see meteors!

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

Lyrids Meteor Shower 2014

April 16, 2014 3 comments

UPDATE: the peak of this year’s Lyrids is expected at 1700 UT on 22 April, so for UK meteorwatchers the best time is pre-dawn on 22 April or after midnight on 23 April. Via @popastro

Starting tonight and peaking next week on 22/23 April 2014 is our spring season’s best meteor shower, the Lyrids. It’s not nearly as dramatic as the Big Three meteor showers – the Perseids in mid-August, the Geminids in mid-December, and the Quadrantids in early January –  but dedicated meteorwatchers will catch glimpses of plenty of shooting stars overnight around or after midnight on 21/22 or 22/23 April.

Meteors

Meteors

The peak rate (more correctly the Zenith Hourly Rate) of the Lyrids is around 20 meteors per hour, but that’s under ideal conditions: 100% clear skies, zero light pollution, and the radiant (the point at which the meteors appear to emerge from) at the zenith (directly overhead). In the nights leading up to the peak you can still expect to see a few but the ZHR is much lower, around 5 meteors per hour.

So how many Lyrids can we expect to see from the UK next week at the peak? To work this out we’ll have to make some assumptions, and then crunch some numbers.

Let’s assume clear skies at least. Then we’ll assume that the peak will fall either some time between dusk on 21 April and dawn on 22 April or some time between dusk on 22 April and dawn on 23 April (currently the suggestion is that it’ll be pre-dawn on 22 April, but it’s worth watching out on both nights). Finally we’ll assume that the ZHR at the peak will be around 20.

The only limiting factors then are (a) the height of the radiant above the horizon, which changes as Lyra rises in the east, climbing high in the south by dawn; and (b) the light pollution

Time Height of Radiant* Hourly rate if peak occurs at this time
2200 21 or 22 April 18° 6
2300 21 or 22 April 25° 8
0000 21 or 22 April 32° 11
0100 22 or 23 April 40° 13
0200 22 or 23 April 49° 15
0300 22 or 23 April 57° 17
0400 22 or 23 April 65° 18**
0500 23 April 71° 19**

* This is based on my observing location in Glasgow, but it’ll only be a few degrees out .
** The last quarter Moon rises around 0330 and so will create enough light pollution to significantly reduce these numbers.

Location Limiting Magnitude Divide above hourly rates by…
Bright Urban 3.5 8
Urban 4 6
Bright Suburban 4.5 4
Suburban 5 3
Rural 6 1.5
Truly Dark 6.5 1

These graphs of previous years show the how the Lyrids activity rate increases and decreases with time centred round a peak on 22 or 23 April:

Lyrids Activity 2011

Lyrids Activity 2011

lyr2012overview

Lyrids Activity 2012

lyr2013overview

Lyrids Activity 2013

My Perseids Meteorwatch

August 9, 2013 3 comments

On Monday night one of the year’s most spectacular meteor showers is set to peak.

You’ll get a great view of it if you’re somewhere dark with clear skies. But those of us stuck in the city can see plenty too.

I’ll be heading to the grounds of Glasgow Science Centre to view it, and you’re welcome to join me. A few things first:

If it’s cloudy or raining DON’T COME! I won’t be there.

I’ll be there from around 10pm, probably till around midnight.

If you want a great view then head somewhere dark, but if you’re in Glasgow and don’t want to head out of the city this is a decent site.

This ISN’T a formal event; I’ll be hanging out there and you’re welcome to come under your own steam. You’ll be responsible for your own safety and comfort; bring extra clothing, a torch, and a deck chair if you have one.

You can park in Glasgow Science Centre car park for £3. Public transport at that time of night is pretty sparse.

Glasgow Science Centre will NOT be open, so there’s no access to refreshments or toilets etc.

Thanks to Glasgow Science Centre for letting us use their outside space.

%d bloggers like this: