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Above: Meteoritic particles belonging to the Perseids travel in nearly parallel paths when they encounter the Earth. The radiant, like the vanishing point of a perspective drawing, is the direction from which they seem to come. This plot tells how the Perseid radiant migrates eastward as the shower progresses, owing to the Earth's motion around the Sun. Credit: Sky & Telescope diagram. The Perseids are the Old Faithful of meteor showers. That's worth remembering, even if the last-quarter Moon will interfere somewhat on August 12th, when the height of this year's display is due. The shower actually lasts two weeks or so, but in the days leading up to the peak the shower will be largely wiped out by an even brighter Moon. Our best bet in 2001 is to catch the Perseids on the morning of the 12th, as the peak of the shower is expected to arrive between 8h and 10h Universal Time. Good views may also be had on several mornings thereafter, when the waning crescent Moon will produce less glare. Activity increases sharply in the hours after midnight, so plan your observing sessions accordingly. We are then looking more nearly face-on into the direction of the Earth's motion as it orbits the Sun. The radiant, or point from which the Perseids appear to come, is also rising higher in the sky as dawn approaches for observers in midnorthern latitudes. In a dark, moonless sky, when the radiant is 30 degrees or more above the horizon, observers often see 60 or more Perseids per hour at the shower's peak. Careful counts, even when made under less-than-ideal conditions, can be corrected by standard methods to yield a zenithal hourly rate (ZHR) telling how many meteors would have been seen if the radiant were directly overhead. The ZHR is the bread and butter of meteor astronomy, a crucial parameter for studying long-term changes in the shower's strength and for ranking it among other showers. The Comet Connection The name "Perseids" was coined
in 1866 by the Italian astronomer Giovanni V. Schiaparelli, who urged
that all meteor showers be identified by the constellation containing
their radiant. But Schiaparelli also noticed a surprising fact: the particles
responsible for the Perseids, before entering our atmosphere, had to be
traveling in nearly the same orbit as Comet Swift-Tuttle of 1862. It was
this insight that led to our modern understanding of meteoritic particles
as the debris shed by comets, and it also explains why a given shower
occurs on nearly the same calendar date, year after year. Over many centuries
the particles wander throughout a parent comet's orbit (or are perturbed
into slightly different orbits of their own). We see a meteor shower whenever
the Earth reaches the point in its orbit that intersects that of the meteor
stream - in other words, once each year. So when exactly will the Perseids
occur in 2001? For a first stab at answering that question, a computer
program by Southwest Texas State University astronomer Donald W. Olson
predicts the annual peaks of the Perseids and eight other well-known meteor
showers. Olson's program predicts the Perseids will peak in 2001 on August
12.5, meaning about 12h UT on the 12th, coinciding with the predawn hours
of Sunday morning for western North America. But because of the shower's
cometary origins, that may not be the best (or only) time to look, nor
the most favorable geographical location. Perseids in 2001 When Comet Swift-Tuttle returned
in 1992 after a lapse of 130 years, meteor enthusiasts wondered if the
"traditional" Perseid maximum (the one predicted by Olson's program and
similar techniques) would remain the shower's only feature. Might the
comet be accompanied by a beehive of newly released particles, either
heralding its arrival or tagging along closely in its wake? In fact, observations
collected by the International Meteor Organization (www.imo.net)
show that the comet has brought with it a "new" Perseid maximum in all
years since 1988. The new peak has been sharper
and more intense than the traditional one, preceding it by several hours.
However the new peak seems to have shifted closer to the traditional one
in the last few years and may arrive between 8h and 10h UT on August 12th
this year. If so, the eastern half of North America is best placed for
a view during the favorable predawn period. Meteor watching is one of the simplest observing projects. Here are some tips on how to observe and record meteors.
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