Shower:
named for the constellation or closest star within a constellation
where the radiant is located at maximum activity.
Activity
Period: the dates when the ZHR (Zenith Hourly Rates) are equal
to or greater than one.
Maximum:
the date on which the maximum activity is expected to occur.
S.L.:
the equivalent solar longitude of the date of maximum
activity. Solar longitude is measured in degrees (0-359) with 0
occurring at the exact moment of the spring equinox, 90 at the
summer solstice, 180 at the autumnal equinox, and 270 at the
winter solstice.
Radiant:
the area in the sky where shower meteors seem to appear from. This
position is given in right ascension (celestial longitude)
and declination (celestial latitude).
Velocity:
the velocity at which shower meteors strike the Earth's
atmosphere. The velocity depends on the angle meteoroids (meteors
in space) intersect the Earth. Meteoroids orbiting in the opposite
direction of the Earth and striking the atmosphere head-on are
much faster than those orbiting in the same direction as the
Earth. This velocity is measured in kilometers per second.
r:
The Population Index, An estimate of the ratio of the
number of meteors in subsequent magnitude classes. Simply stated:
the lower the "r" value, the resulting overall mean
magnitude of each shower will be brighter. "r" usually
ranges from 2.0 (bright) to 3.5 (faint).
ZHR:
Zenith Hourly Rate, the average maximum number of shower
meteors visible per hour if the radiant is located exactly
overhead and the limiting magnitude equals +6.5. Actual counts
rarely reach this figure as the zenith angle of the radiant is
usually less and the limiting magnitude is usually lower. ZHR is a
useful tool when comparing the actual observed rates between
individual observers as it sets observing conditions for all to
the same standards.
Class:
A scale developed by Robert Lunsford to group meteor showers by
their intensity:
Class
I: the strongest annual showers with ZHR's normally ten or
better.
Class
II: reliable minor showers with ZHR's normally three or
better.
Class
III: showers with widely variable rates. They may be strong
one year and totally inactive the next.
Class
IV: weak minor showers with ZHR's rarely exceeding three. The
study of these showers is best left to experienced observers who
use plotting and angular velocity estimates to determine shower
association. Observers with less experience are urged to limit
their shower associations to showers with a rating of I to III.
These showers are also good targets for video and photographic
work.
Moon:
the age of the moon in days where 0 is new, 7 is first quarter, 14
is full, and 21 is last quarter. Meteor activity is best seen in
the absence of moonlight so showers reaching maximum activity when
the moon is less than 10 days old or more than 25 are much more
favorably observed than those situated closer to the full moon.
The
Perseids are Coming
July 31,
2009:
Earth is entering a stream of dusty debris from Comet
Swift-Tuttle, the source of the annual Perseid meteor shower.
Although the shower won't peak until August 11th and 12th, the
show is already getting underway.
Brian Emfinger of
Ozark, Arkansas, photographed this early Perseid just after
midnight on Sunday, July 26th:
"I used an
off-the-shelf digital camera to capture this fireball and its
smoky trail," says Emfinger. "It was a bright one!"
Don't get too
excited, cautions Bill Cooke of NASA's Meteoroid Environment
Office. "We're just in the outskirts of the debris stream
now. If you go out at night and stare at the sky, you'll probably
only see a few Perseids per hour."
This will change,
however, as August unfolds.
"Earth passes
through the densest part of the debris stream sometime on August
12th. Then, you could see dozens of meteors per hour."
For sky watchers in
North America, the watch begins after nightfall on August 11th and
continues until sunrise on the 12th. Veteran observers suggest the
following strategy: Unfold a blanket on a flat patch of ground.
(Note: The middle of your street is not a good choice.) Lie down
and look up. Perseids can appear in any part of the sky, their
tails all pointing back to the shower's radiant in the
constellation Perseus. Get away from city lights if you can.
There is one light
you cannot escape on August 12th. The 55% gibbous Moon will glare
down from the constellation Aries just next door to the shower's
radiant in Perseus. The Moon is beautiful, but don't stare at it.
Bright moonlight ruins night vision and it will wipe out any faint
Perseids in that part of the sky.
Above:
Looking northeast around midnight on August 11th-12th. The red dot
is the Perseid radiant. Although Perseid meteors can appear in any
part of the sky, all of their tails will point back to the
radiant. Image copyright: Spaceweather.com, used with permission.
The Moon is least
troublesome during the early evening hours of August 11th. Around
9 to 11 p.m. local time (your local time), both Perseus
and the Moon will be hanging low in the north. This low profile
reduces lunar glare while positioning the shower's radiant for a
nice display of Earthgrazers.
"Earthgrazers
are meteors that approach from the horizon and skim the atmosphere
overhead like a stone skipping across the surface of a pond,"
explains Cooke. "They are long, slow and colorful—among the
most beautiful of meteors." He notes that an hour of watching
may net only a few of these at most, but seeing even one can make
the whole night worthwhile.
The Perseids are
coming. Enjoy the show.
