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.
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.
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 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
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.
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…|
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:
On the night of 03/04 January 2012 the first meteor shower of the year will take place, the Quadrantids. This shower ranks as one of the best performers of the year, assuming your skies aren’t clouded, as they so often are in winter. If the peak of this shower occurs under ideal conditions – i.e. perfectly clear skies, free from light pollution – then you can expect to see in excess of 100 meteors every hour. The peak for this shower is very brief though, so you’ll have to catch just the right conditions at just the right time to see a display this good. This year’s peak is estimated to occur just before dawn on 04 January 2012.
Not only do you have the weather to contend with, but this year the waxing gibbous Moon will be up for much of the night. However the Moon sets at around 0415, giving you a couple of hours before the sky starts to brighten before sunrise. Given that the peak of this shower will probably occur within this short window, things are looking pretty good for this year’s display.
Last year’s graph of meteor activity shows how sharp the peak is, so you probably won’t see many Quadrantids on the nights either side of the peak, but it’s worth a look if you have clear dark skies. ZHR for this year may be anywhere between 60 and 200.
How best to view the Quadrantids 2012
- Get somewhere as far from street lights and city glow as possible, preferably somewhere really dark, like your nearest national park or one of the UK’s dark sky places: Galloway Forest Park, Sark or Exmoor.
- Go out at the right time, which for this year’s shower is between around 0400 and 0700 GMT.
- You don’t need binoculars or a telescope, your eyes are best for viewing meteors.
- Wrap up warm, as if you have clear skies (which you’ll be hoping for) it will be very cold in these early morning hours.
- Bring a reclining deck chair so you don’t have to stand all night, and a blanket to wrap yourself in!
- Although the radiant of the meteor shower (the point where the meteors will appear to stream from) is high in the E around 0400 you don’t need to worry about facing in any particular direction, just position yourself so that you can see as much sky as possible, and enjoy the view!
If you want to make more serious observations of this shower you can submit them to either the International Meteor Organisation, the British Astronomical Association, or the Society for Popular Astronomy.
On the night of 17/18 November 2011 the Leonids meteor shower reaches its peak. This annual performer is associated with Comet Temple-Tuttle, which orbits the Sun once every 33 years leaving a trail of debris as it goes. Once a year the Earth passes through this trail, and we see a meteor shower.
This year’s Leonids shower is hampered by the last quarter Moon which sits just to the right of the radiant of the Leonids, in Leo. Despite this there is good reason to observe the shower this year, as the International Meteor Organisation suggest there might be as many as three peaks of activity.
Throughout November the rate of Leonids will increase, with the main peak occurring at 0340 GMT on 18 November, at which time the Zenith Hourly Rate may be 20+. For observers in the UK, observing under cloudless skies, away from light pollution, this translates as an hourly rate of ~14, but the Moon will interfere and reduce this value somewhat. Two other peaks may also occur, at ~2100 on November 17, and at ~2300 on 18 November, with similar rates. This means that both the nights of 17/18 and 18/19 November may offer good opportunites to observe this shower.
The Leonids has the distinction of being the most dramatic meteor shower that I’ve ever seen, as I observed the Leonid meteor storms every year from 1998 to 2002, when we saw hundreds of meteors each night at the peak of the shower. These storm peaks are predictable, and occur every 33 years, associated with the pass of comet Temple Tuttle, as it refreshes the trail of debris that cause the meteors. The next pass of Temple Tuttle is due 2031, so we’ve a long wait for the next storm.
Interestingly, the Leonid storm of 1833 was truly stunning, with rates estimated to be around 100,000 per hour across North America.
To view the Leonids, find a dark spot, away from light pollution, sit on a reclining deck chair facing as large an area of the sky as you can manage, wrap yourself in a blanket, and enjoy the view. For observers in the UK the meteor shower radiant will rise around 2200 GMT on 17 November and will be high in the SE by 0400 on 18 November.
If you want to make observations of the Leonids that might help scientists better understand the shower, you can do so via the Society of Popular Astronomy, or the British Astronomical Association. Lots more info can be found at the Meteorwatch website.