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Perseids: Last Chance to See?

Tonight (9/10 August 2011), between Moonset and the start of twilight, might present the best opportunity to view the Perseids meteor shower, at around 0200, although the peak of the shower doesn’t occur until the night of 12/13 August 2011.

The International Meteor Organisation has been recording the increasing ZHR (activity level) of this year’s Perseids meteor shower, and it currently stands at ZHR=20. It is expected to increase over the next four nights to ZHR=50-200 at the peak on 12/13 August.

IMO's ZHR for Perseids 2011

However, tonight is the last night of the shower where the Moon will be absent from the sky for part of the night, setting before 0200. After the Moon sets there will be a period of time before twilight begins where the sky will be free of any natural light pollution, so assuming you can get away from man-made light pollution you’ll maximise your chances of seeing meteors.

How long you will have under dark skies will depend on where in the UK you are, with observers in the far south of England having until 0330, while those in southern Scotland having until 0230. Observers north of central Scotland will not experience truly dark skies for several days yet. (To find your local Moonset and twilight times visit timeanddate.com)

If you can get somewhere very far from light pollution (like Galloway Forest Dark Sky Park, or Exmoor National Park) then you’ll have the best chance of seeing lots of meteors tonight. Let’s take these locations as demonstrations of how the actual hourly rate will vary between tonight and the peak at the weekend.

Observing Night Hours of true darkness
(Glentrool)
Hours of true darkness
(Exmoor)
Limiting Mag
(due to Moon)
ZHR (estimate) Actual Hourly Rate
 09/10 August  0148-0225  0207-0322  6.5  25  22
 10/11 August  none (due to Moon)  none (due to Moon)  4.5  30  7
 11/12 August  none (due to Moon)  none (due to Moon)  4.0  35  5
 12/13 August  none (due to Moon)  none (due to Moon)  3.5 (Full Moon)  100 (peak)  11* (peak)

*The peak ZHR has been taken here to be 100, although it may be as much as twice this, meaning that even under a Full Moon you’ll see the same at the peak as you will under dark skies tonight.

So if you can get somewhere truly dark tonight between Moonset and the start of astronomical twilight (and you might get less than an hour of these perfect conditions) then your Actual Hourly Rate tonight may be better than that during the peak, due to the Full Moon then.

For those observing in towns and cities this won’t be such a big issue, as man-made light pollution will impose a limiting mag on the sky tonight, which will reduce tonight’s Actual Hourly Rate rate to ~3, compared to ~11 at the peak.

So if you get the chance head out tonight, and indeed every night this week, to look for Perseids in the wee small hours. This is the most reliable shower of the year, and one that even the Full Moon cannot ruin completely!

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