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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!

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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.

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