http://www.msnbc.msn.com/id/20187093?GT1=10252

Your guide to the summer's best meteor show
Viewing conditions could make for perfect Perseids
this weekend
 
By Joe Rao
Skywatching columnist

Updated: 7:10 a.m. MT Aug 10, 2007
Every August, just when many people go vacationing in
the country where skies are dark, the best-known
meteor shower makes its appearance.

The annual Perseid meteor shower is expected to be at
its best this year, producing one or two meteors per
minute during peak hours.

"It's going to be a great show," said Bill Cooke of
NASA's Meteoroid Environment Office at the Marshall
Space Flight Center in Alabama.

August is also known as the month of "The Tears of St.
Lawrence." Laurentius, a Christian deacon, is said to
have been martyred by the Romans in 258 AD on an iron
outdoor stove. It was in the midst of this torture
that Laurentius cried out: "I am already roasted on
one side and, if thou wouldst have me well cooked, it
is time to turn me on the other."

The saint's death was commemorated on his feast day,
Aug. 10. King Phillip II of Spain built his monastery
place, the "Escorial," on the plan of the holy
gridiron. And the abundant shooting stars seen
annually between approximately Aug. 8 and 14 have come
to be known as St. Lawrence's "fiery tears."

The reality
We know today that these meteors are actually the
dusty remains left behind by Comet Swift-Tuttle.

Discovered back in 1862, and most recently observed in
1992, this comet takes approximately 130 years to
circle the sun. And in much the same way that Comet
Tempel-Tuttle leaves a trail of debris along its orbit
to produce the Leonid meteors of November, Comet
Swift-Tuttle produces a similar debris trail along its
orbit to cause the Perseids.

Indeed, every year during mid-August, when Earth
passes close to the orbit of Swift-Tuttle, the
material left behind by the comet from its previous
visits rams into our atmosphere at approximately 37
miles per second (60 kilometers per second) and
creates bright streaks of light in our midsummer night
skies.

Perfect prospects
According to the best estimates, this year Earth is
predicted to cut through the densest part of the
Perseid stream sometime around 2 a.m. ET on Monday.
That corresponds to 11 p.m. PT Sunday for those living
in the western United States or Canada.

The interval when the meteors will be falling at their
highest rates will likely last several hours or more
on either side of these times.

As a result, it is the late-night hours Sunday, on
through the first light of dawn Monday, that holds the
greatest promise of seeing a very fine Perseid
display.

The moon, whose bright light almost totally wrecked
last year's shower, will have zero impact this year.
The moon will be new on Sunday, meaning that there
will be no interference from it at all.

Perfect!

What to expect
A very good shower will produce about one meteor per
minute for a given observer under a dark country sky.
Any light pollution or moonlight considerably reduces
the count.
 
The August Perseids are among the strongest of the
readily observed annual meteor showers, and at maximum
activity the display nominally yields 90 or 100
meteors per hour. Observers with exceptional skies
often record even larger numbers. Typically during an
overnight watch, the Perseids are capable of producing
a number of bright, flaring and fragmenting meteors,
which leave fine trains in their wake.

On the night of shower maximum, the Perseid radiant is
not far from the famous "Double Star Cluster" of
Perseus (hence the name, "Perseid"). Low in the
northeast during the early evening, it rises higher in
the sky until morning twilight ends observing. Shower
members appearing close to the radiant have
foreshortened tracks; those appearing farther away are
often brighter, have longer tracks, and move faster
across the sky.

About five to 10 of the meteors seen in any given hour
will not fit this geometric pattern, and may be
classified as sporadic or as members of some other
(minor) shower.

Plan your time
Perseid activity increases sharply in the hours after
midnight, so plan your observing times accordingly. We
are then looking more nearly face-on into the
direction of the Earth's motion as it orbits the sun,
and the radiant is also higher up.