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Updated February 14, 2004
Perseids: July 23 - August 22;
Peak August 12
The Perseids are the best-known of any meteor shower. Occurring as they do in mid-August, they can be observed under the warm and comfortable yet darkening skies of late summer. Because of the consistent strength of the shower, and its convenient time of year, the Perseids are observed by more astronomy enthusiasts than any other shower, and are by far the most likely display to be observed by the general public.
The Perseid meteoroid stream was the first to be specifically associated with the orbit of a "comet", namely Comet Swift-Tuttle which was discovered in 1862. The largest comet which passes this close to Earth, Swift-Tuttle has a period of 130 years, and made its first return appearance in 1992. While the counts of Perseids were indeed higher in 1861-63 and 1992-94 when their parent comet was in Earth's vicinity, there was no evidence of the sort of clumping that would result in a meteor storm as can happen with the Leonids. Instead, the meteoroid stream appears to be fully mature and is the source of a pleasing display of meteors every summer. At the peak, a Zenith Hourly Rate approaching 100 per hour can be expected.
Like its parent comet, the orbit of the meteoroid stream is retrograde - the opposite direction to that of Earth and the other planets - and as a result the meteors hit Earth's atmosphere at the relatively high speed of 60 kph. Furthermore, at the point where it crosses Earth's orbit the stream is significantly tilted from north to south, meaning that northern hemisphere observers are favoured.
The radiant is the most northerly of all major showers, in the northern arm of Perseus near the famous Double Cluster. (If you're unfamiliar with the constellations, look just to the lower left of the easily recognizable W of Cassiopeia.) Only 32° from the north celestial pole, the radiant is therefore "circumpolar" - permanently above the horizon - for all of Canada. It rises in the northeast throughout August nights, and crests near the zenith just as morning twilight puts an end to visual observing of the shower.
In 2002 the primary peak of the Perseids will occur around 22h UT (4:00 p.m. MDT) on Monday August 12, when western North America will be on the trailing hemisphere of the rotating Earth in its orbit. There will be two fairly equal opportunities to observe from the 'shoulders' of the peak, both Monday morning August 12 and Tuesday morning August 13. As with most showers, the highest rates are seen in the predawn hours. This year the waxing crescent Moon will cause minimal interference, and will set before the prime viewing hours between midnight and 5 a.m. An observer at a dark rural location can expect to see about five times as many meteors as one situated in a moderately light-polluted urban environment.
Looking ahead to 2003, the Moon will be full right at the Perseids maximum, which will reduce observed rates by a factor of three or so. While the Moon will cause less interference in subsequent years, not until 2007 will its phase be more favourable than that of 2002.
Due to the circumpolar position of the radiant, in theory it will be possible to make radio observations of Perseids around the clock. While such monitoring can be done independent of daylight, moonlight, cloud cover or other mitigating factors, like all points on the sky the radiant undergoes diurnal motion, which will result in a maximum of meteor activity around 6 a.m. and a minimum at 6 p.m. (This is analogous to the front windshield of a moving car collecting many more raindrops than the rear.) However, due to the northerly declination of the radiant it should be possible to detect a reduced number of meteors even at the most unfavourable time of day.
For further information
Much more on the Perseids is available at the following links: