meteor shower
, temporary rise in the rate of meteor sightings, caused by the entry into Earth’s atmosphere of a number of
meteoroids
(
see
meteor and meteoroid
) at approximately the same place in the sky and the same time of year, traveling in parallel paths and apparently having a common origin. Most meteor showers are known or believed to be associated with active or
defunct
comet
s; they represent Earth’s passage through the orbits of these comets and its collision with the streams of debris (typically of sand-grain to pebble size) that have been left behind. The showers return annually, but, because the densities of meteoroids in the streams (commonly called
meteor streams
) are not uniform, the intensities of the showers can vary considerably from year to year.
A meteor shower’s name is usually derived from that of the
constellation
(or of a star therein) in which the shower’s
radiant
is situated?i.e., the point in the sky from which
perspective
makes the parallel meteor tracks seem to originate. Some showers have been named for an associated comet; e.g., the
Andromedids
were formerly called the Bielids, after
Biela’s Comet
. The
Cyrillid shower of 1913 had no
radiant
(the meteoroids seemed to enter the atmosphere from a circular
orbit
around Earth) and was named for
St. Cyril of Alexandria
, on whose feast day (formerly celebrated on February 9) the shower was observed. The great
Leonid meteor shower
of Nov. 12, 1833, in which hundreds of thousands of meteors were observed in one night, was seen all over
North America
and initiated the first serious study of meteor showers (
see
meteoritics
). It was later established that very strong Leonid showers recur at 33?34-year intervals (the orbital period of its associated comet,
Tempel-Tuttle), and occasional records of its appearances have been traced back to about
ad
902. Since about 1945, radar observations have revealed meteor showers regularly occurring in the daylight sky, where they are invisible to the eye.
Britannica Quiz
Brightest Star in the Solar System
Major meteor showers observable at night and their associated comets are provided in the table.
Principal nighttime meteor showers
shower
|
average date of maximum
|
normal duration (days)
|
visual strength (Northern Hemisphere)
|
entry velocity (km/sec)
|
associated comet
|
*Possible identification.
|
**This body was classified as an asteroid on discovery, but it is now suspected to be a burned-out comet.
|
Source: Data derived primarily from A.F. Cook in NASA SP-319 (1973).
|
Quadrantid
|
January 3
|
1
|
medium
|
41
|
C/1490 Y1
|
Lyrid
|
April 22
|
1
|
irregular
|
48
|
Thatcher
|
Eta Aquarid
|
May 3
|
5
|
weak
|
66
|
Halley
|
Southern Delta Aquarid
|
July 29
|
8
|
medium
|
41
|
Machholz*
|
Capricornid
|
July 30
|
3
|
medium
|
23
|
169P/NEAT
|
Perseid
|
August 12
|
5
|
strong
|
59
|
Swift-Tuttle
|
Andromedid
|
October 3
|
11
|
weak
|
21
|
Biela
|
Draconid
|
October 9
|
1
|
irregular
|
20
|
Giacobini-Zinner
|
Orionid
|
October 21
|
2
|
medium
|
66
|
Halley
|
Taurid
|
November 8
|
30
|
weak
|
28
|
Encke
|
Leonid
|
November 17
|
less than 1
|
irregular
|
71
|
Tempel-Tuttle
|
Geminid
|
December 14
|
4
|
strong
|
34
|
(3200) Phaethon**
|