Anyone gazing at the summer
night sky for even a short length of time is likely to spot a few
"shooting stars" darting across the sky.
Meteors are typically bits
of material left behind by comets. They're often no larger than sand grains,
and they vaporize as they enter our atmosphere. In general, the Earth
encounters richer meteoric activity during the second half of the year. Between
August 3 and 15, there are a half-dozen different minor displays that are
active.
The best display of the
summer comes during the second week of August: the annual Perseid meteor shower. At its peak around the nights of Aug. 11 and 12,
this display can produce 50 to 100 fast, bright meteors per hour.
This will be a fair-to-good
year to watch for the Perseids. A bright gibbous moon, which initially will
interfere with observations, will set at around 1:30 a.m., leaving the rest of
the night dark for prospective meteor watchers. The only equipment you'll need
is your eyes and a modest amount of patience.
Early morning is best
The main trick is to plan
your meteor-watching for the pre-dawn hours. Not only will the moon have set,
leaving skies darker, but there are simply more meteors then. This is due to the fact that during the
pre-midnight hours we are on the "trailing" side of the Earth, due to
our orbital motion through space. So any meteoric particle generally must have
an orbital velocity greater than that of the Earth to "catch" us.
However, after midnight when
we are turned onto the Earth's "leading" side, any particle that lies
along the Earth's orbital path will enter our atmosphere as a meteor.
These objects collide with
our atmosphere at speeds of 7 to 45 miles (11 to 72 km) per second, their
energy of motion rapidly dissipates in the form of heat, light, and ionization,
creating short-lived streaks of light popularly referred to as "shooting
stars."
Already begun
The very first forerunners
of the Perseid shower began to appear around July 17. Unfortunately, that
virtually coincides with a full moon, but even without any interfering
moonlight you would only see a few per hour at best.
The numbers will begin to
noticeably ramp up during the second week of August. The last Perseid
stragglers may still be noted as late as Aug. 24.
To go along with the
Perseids, however, there are at least ten other minor meteor displays that are active at various times during July and
August. While the hourly rates from these other meteor streams are but a
fraction of the numbers produced by the Perseids, combined, overall they
provide a wide variety of meteors of differing colors, speeds and trajectories.
Among these are the Southern
Delta Aquarids, which reach their peak around July 28 and can produce faint,
medium speed meteors; the Alpha Capricornids, which arrive at their maximum a
couple of nights later on July 30 and are described as slow, bright, long
trailed meteors and the Kappa Cygnids, peaking near Aug. 17 and have been
classified as "slow moving and sometimes brilliant."
Earlier this year, Peter Jenniskens of the SETI Institute announced he had
identified the probable breakup of a comet from several thousand years ago that
may be responsible for the Kappa Cygnids; the asteroid 2008 ED69 may be a
fragment from that breakup.
As meager as the individual
hourly rates are with the minor displays running from mid-July through the
third week of August, collectively they become strikingly augmented with the
annual August Perseids. British observational meteor astronomer, Alastair
McBeath comments that August is Perseid month, with " ... rising sporadic
meteor rates, mild weather overnight, several other minor showers on show and
it's vacation time. With the Perseids partly moon-free, all we need are clear
skies!"
SPACE.com will provide a
complete Perseids viewing guide on Aug. 8.