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

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Orionids Meteor Shower 2011

Some time in the small hours of Friday or Saturday morning (21-22 October 2011) the Orionids meteor shower will reach its peak activity rate.  The peak occurs some time around 21 October each year, but this year it’s uncertain which day it will fall on.

The Orionid's parent Comet P/Halley as taken March 8, 1986 by W. Liller, Easter Island, part of the International Halley Watch (IHW) Large Scale Phenomena Network.

Meteor showers result from the Earth passing through the trail of dust and debris left behind by a comet. In the case of the Orionids the parent object is the most famous of all the comets – Halley’s Comet.

The peak meteor rate for the Orionids is lower than some of the more spectacular showers (the Perseids in August, the Geminids in December, and the Quadrantids in January all regularly outperform the Orionids) but it is still worth looking out for.

The meteors will appear to radiate from the constellation of Orion (hence the name) but they will streak across the sky in all directions, and so you shouldn’t confine yourself to only looking towards this one constellation.

On Thursday and Friday evenings the radiant rises in the east around 2200 BST (2100 UT)  and continues to rise to its highest in the south just before the sky starts to brighten at 0600 BST (0500 UT). The higher the radiant above the horizon the more meteors you will see. However a crescent Moon will rises in the east on both mornings, the light from which will drown out some of the fainter meteors.

This shouldn’t matter much to you if you’re observing from an urban or suburban area, as the man-made light pollution in the sky will do a far better job of obscuring the meteor shower than the Moon will, but for lucky observers in dark sites (and I’ll be one of them, as I’m spending the weekend on Sark, the world’s first Dark Sky Island) the Moon may interfere.

Here’s a table with estimated hourly rates based on dark skies / suburban / urban areas, at hourly intervals throughout the night, assuming a ZHR =40 throughout this period (It may be that the peak will fall outwith this period, e.g. in daylight hours, so these are best-case-scenario numbers).

Time (BST)	Radiant Altitude	Radiant Direction		Hourly Rate Urban Site	Hourly Rate Suburban Site	Hourly Rate Dark Sky Site (if Moon not present)
2200	rises	ENE		<1	<1	<1
2300	8°	ENE		1	2	4
0000	16°	E		1	4	8
0100	24°	ESE		2	6	16
0200	33°	ESE		2	8	22
0300	40°	SE		2	9	26*
0400	46°	SSE		3	10	29*
0500	50°	S		3	11	31*
0600	50°	S		3	11	31*

* the true rates, given that the Moon is causing natural light pollution, are probably half these values.

All of these timings and altitudes are based on an observer in central Scotland. For other UK observers the values in columns 2-4 may be slightly off, but not noticeably so.

Observing Advice: wrap up warm, head out before midnight, sit youself in a reclining lawn chair, and enjoy the spectacle. The rates may pick up around 0200 BST on Thursday or Friday and may stay high until dawn.

Meteor Showers for 2011-2012: Ones to watch

With the Quadrantids meteor shower that has just past yielding around 100 meteors per hour in near-perfect New Moon conditions, which showers of the next two years will give us as good a display?

Meteor Shower

There are a few regular, dependable showers that can be relied on to put on a good show year after year, given a good Moon phases, so let’s concentrate on those:

Lyrids 2011
The Lyrids peak this year on April 21/22, only three days after the Full Moon, making conditions far from ideal. The ZHR is around 20, but under bright Moon conditions this will be much reduced, so that from the UK you might only see a few Lyrids per hour.

Persieds 2011
The Perseids peak on 12/13 August 2011 coincides exactly with a Full Moon, making this shower pretty much a write-off in 2011.

Orionids 2011
The Orionids peak occurs on 21/22 October 2011 just after the last quarter Moon, with the Moon rising a little after midnight, just as the meteor shower radiant is gaining height. Again, far from ideal.

Leonids 2011
The Leonids peak on 17/18 November occurs during a last quarter Moon, which unfortunately is smack bang in the direction of Leo, and so will obscure many of the Leonids in 2011

Geminids 2011
The Geminids peak on 13/14 December 2011 will likewise be completely obscured by an almost-full Moon in Gemini.

Quadrantids 2012
The Quadrantids peak on 3/4 January 2012 will feature a waxing gibbous Moon which won’t set until 0400.

Lyrids 2012
The Lyrids peak on 21/22 April 2012 is the first major shower peak in 15 months where the Moon is absent, meaning that you should get good views of this shower which has a ZHR of only around 20.

Persieds 2012
The Perseids peak of 12/13 August 2012 will feature a thin waning crescent moon that’s visible in the sky from midnight, obscuring some of the Perseids. Here’s my up-to-date guide to the Perseids 2012.

Orionids 2012
The Orionids peak on 21/22 October 2012 is pretty much Moon-free from around 2330, as the Moon sets.

Leonids 2012
The Leonids peak on 17/18 November 2012 will also be Moon free from early evening, and so presents an opportunity to see a few Leonids.

Geminids 2012
Rounding off this two year run of poor Moon conditions for meteor showers, we end with the Geminids on 13/14 December, coinciding wonderfully with a New Moon on 13 December, meaning conditions will be near perfect.