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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 Meteor Shower

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 during 2012, from imo.net

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!):

Perseids 2013 Rate, from imo.net

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

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