Originally posted on Dark Sky Diary by Steve Owens @Darkskyman

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).

* 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.