This is the most famous
of all meteor showers. It never fails to provide an impressive display
and, due to its summertime appearance, it tends to provide the majority
of meteors seen by non-astronomy enthusiasts.
This meteor shower gets
the name "Perseids" because it appears to radiate from the
constellation Perseus. An observer in the Northern Hemisphere can start
seeing Perseid meteors as early as July 23, when one meteor every hour
or so could be visible. During the next three weeks, there is a slow
build-up. It is possible to spot five Perseids per hour at the
beginning of August and perhaps 15 per hour by August 10. The Perseids
rapidly increase to a peak of 50-80 meteors per hour by the night of
August 12/13 and then rapidly decline to about 10 per hour by August
15. The last night meteors are likely to be seen from this meteor
shower is August 22, when an observer might see a Perseid every hour or
so.
For observers in the
Southern Hemisphere, the Perseid radiant never climbs above the
horizon, which will considerably reduce the number of Perseid meteors
you are likely to see. Nevertheless, on the night of maximum, it is
possible to see 10-15 meteors per hour coming up from the northern
horizon.
There are other, weaker
meteor showers going on around the same time as the Perseids, but the
Perseids will generally appear to move much faster across the sky than
meteors from the other showers. In fact, the Perseids are among the
fastest moving meteors we see every year. Another way to know if the
meteor you saw was a Perseid is to mentally trace the meteor backwards.
If you end up at Perseus then you have probably seen a Perseid meteor!
If you are not sure where Perseus is in the sky, the following charts
will help you find it from both the Northern Hemisphere and Southern
Hemisphere:
Location of the Perseids
For Northern Hemisphere Observers
Location of the Perseids
For Southern Hemisphere Observers
History
The earliest record of Perseid
activity comes from the Chinese annals, where it is said that in 36 AD
"more than 100 meteors flew thither in the morning." Numerous
references appear in Chinese, Japanese and Korean records throughout
the 8th, 9th, 10th and 11th centuries, but only sporadic references are
found between the 12th and 19th centuries, inclusive. Nevertheless,
August has long had a reputation for an abundance of meteors. The
Perseids have been referred to as the "tears of St. Lawrence", since
meteors seemed to be in abundance during the festival of that saint in
Italy on August 10th; however, credit for the discovery of the shower's
annual appearance is given to Adolphe Quételet (Brussels,
Belgium), who, in 1835, reported that there was a shower occurring in
August that emanated from the constellation Perseus.
The first observer to provide
an hourly count for this shower was E. Heis (Münster), who
found a maximum rate of 160 meteors per hour in 1839. Observations by
Heis and other observers around the world continued almost annually
thereafter, with maximum rates typically falling between 37 and 88 per
hour through 1858. Interestingly, the rates jumped to between 78 and
102 in 1861, according to estimates by four different observers, and,
in 1863, three observers reported rates of 109 to 215 per hour.
Although rates were still somewhat high in 1864, generally "normal"
rates persisted throughout the remainder of the 19th-century.
Computations of the orbit of
the Perseids between 1864 and 1866 by G. V. Schiaparelli (Italy)
revealed a very strong resemblance to periodic comet 109P/Swift-Tuttle,
which had been discovered in 1862. This was the first time a meteor
shower had been positively identified with a comet and it seems safe to
speculate that the high Perseid rates of 1861-1863 were directly due to
the appearance of 109P/Swift-Tuttle,
which has a period of about 135 years. Multiple returns of the comet
would be responsible for the distribution of the meteors throughout the
orbit, but meteors should be denser in the region closest to the comet,
so that meteor activity should increase when the comet is near
perihelion.
During 1973, the astronomer
Brian G. Marsden examined the orbit of periodic comet 109P/Swift-Tuttle
to determine when it was likely to return. The observations from the
1862 return were not the best and the uncertainty in the orbital period
amounted to several years. His best bet was to try and identify a
previous return. He found two good options: a comet in 1737 and one in
1750. Marsden chose the 1750 comet as the best candidate for a previous
appearance of comet 109P/Swift-Tuttle
and predicted the comet would return in 1981. This immediately
generated excitement among meteor observers as the potential for
enhanced activity unfolded. This excitement seems to have been fully
justified, as the average rate of 65 per hour during 1966-1975 suddenly
jumped to over 90 per hour during 1976-1983---with the high being 187
in the latter year. Although meteor observers seemed content with their
observations of the enhanced activity from 109P/Swift-Tuttle,
comet observers were less enthusiastic as the comet was not recovered.
Following the 1983 peak, hourly rates for the Perseids declined. With a
full moon occurring just a day before maximum in 1984, the Dutch Meteor
Society still reported unexpectedly high rates of 60 meteors per hour.
In 1985, reported rates generally fell between 40 and 60 meteors per
hour in dark skies, and results were generally the same in 1986.
As the 1990s dawned, Marsden
published a new prediction. If comet 109P/Swift-Tuttle
was actually seen in 1737, then the comet might pass perihelion during
December 1992. The comet was recovered late in the summer of 1992.
Although not one of the most spectacular apparitions, the comet was
well observed. But meteor observers were more interested in the Perseid
display of 1993. Predictions indicated Europe was the place to be
during August of 1993. Observers from around the world flocked into
central Europe and were met with hourly rates of 200 to 500! High rates
were still present during 1994, this time with the peak occurring over
the United States.
The Perseid radiant turns out
to be complex. The main radiant is situated near the star Eta Persei,
but other radiants appear to be active at the same time. As long ago as
1879, W. F. Denning (England) pointed out that he had "detected the
existence of two other simultaneous showers from Chi and Gamma Persei."
This latter shower is one of the most active of the secondary radiants
and seems to have been frequently observed during the twentieth
century---especially with telescopic aid. One of the most recent
examples of the complexity of the Perseid meteor shower was revealed in
three studies of the radiant conducted during 1969 to 1971, by
observers in the Crimea. In addition to the main radiant near Eta
Persei, they confirmed the existence of the major radiants near Chi and
Gamma Persei, as well as minor radiants near Alpha and Beta Persei.
These meteor showers are generally short-lived and exhibit radiants
that move nearly parallel to the main radiant.
There is an uneven size
distribution within the stream. One very interesting characteristic of
the Perseids is that there are times when larger, brighter meteors are
much more plentiful than smaller, fainter meteors. In 1953, A. Hruska
(Czechoslovakia) found that Perseids were brighter during August 8-12,
slightly fainter on August 12/13, and notably fainter by August 14/15.
In 1956, Z. Cephecha (Czechoslovakia) found the meteors were brightest
on the night of Augsut 6/7 and faintest on the night of August 13/14. A
similar pattern has been noted by more recent studies during the 1980s
and 1990s. All of the magnitude studies have one thing in common---they
point to an irregular mass distribution within the Perseid stream. Some
of this is most likely due to the Earth encountering filaments of
material representing different that comet Swift-Tuttle
has moved in during the last 2000 years.
There is an odd variation from
year to year in the number of Perseids exhibiting persistent trains.
One of the first astronomers to study this was M. Plavec
(Czechoslovakia), who examined 8028 Perseids seen during the period
spanning 1933 to 1947. He noted the 45% of Perseids exhibited
persistent trains in 1933, while this was value changed to 60% in 1936,
35% in 1945, and 53.3% in 1947. Plavec noted that he could not
correlate the variations to sunspot numbers. It could be that this is
also tied in to Earth encountering different orbital filaments
perviously shed by comet Swift-Tuttle.
