Every
year in late April Earth passes through the dusty
tail of Comet Thatcher (C/1861 G1), and the encounter
causes a meteor shower--the Lyrids. This year the
shower peaks on Thursday morning, April 22nd. The
best time to look, no matter where you live, is during
the dark hours before dawn. Forecasters expect 10
to 20 meteors per hour visible from dark-sky sites.
Lyrid
meteors appear to stream from the bright star Vega
in the constellation Lyra:
In
fact, Lyrids have nothing to do with Vega. The true
source of the shower is Comet Thatcher. Every year
in April, Earth plows through Thatcher's drawn-out
dusty tail. Flakes of comet dust, most no bigger than
grains of sand, strike Earth's atmosphere traveling
49 km/s (110,000 mph) and disintegrate as streaks
of light--meteors!
Lyrid
meteors are typically as bright as the stars in the
Big Dipper, which is to say of middling brightness.
But some are more intense, even brighter than Venus.
These "Lyrid fireballs" cast shadows for
a split second and leave behind smokey debris trails
that linger for minutes.
Occasionally,
the shower intensifies. Most years in April there
are no more than 5 to 20 meteors per hour during the
shower's peak. But sometimes, when
Earth glides through an unusually dense clump of comet
debris, the rate increases. Sky watchers in
1982, for instance, counted 90 Lyrids per hour. An
even more impressive outburst was documented in 1803
by a journalist in Richmond, Virginia, who wrote:
"Shooting
stars.
This electrical [sic] phenomenon was observed on Wednesday
morning last at Richmond and its vicinity, in a manner that
alarmed many, and astonished every person that beheld
it. From one until three in the morning, those starry
meteors seemed to fall from every point in the heavens,
in such numbers as to resemble a shower of sky rockets..."
[ref]
What
will the Lyrids do this year? The only way to know
for sure is to go outside and look.
Experienced
meteor watchers suggest the following viewing strategy:
Dress warmly. Bring a reclining chair, or spread a
thick blanket over a flat spot of ground. Lie down
and look up somewhat toward the east. Meteors can
appear in any part of the sky, although their trails
will tend to point back toward the radiant--i.e.,
toward Vega.
Vega
is a brilliant blue-white star about three times wider
than our Sun and 25 light years away. About 14,000
years ago Vega was the North Star. Earth's spin axis
wanders: Now it points at Polaris, then it pointed
at Vega. You might have seen Vega in Carl Sagan's
movie Contact.
It was the source of alien radio transmissions to
Earth.