SkyandTelescope.com August 2014 65Meteors coming from
radiant directionAlan MacRobert
Remember your favorite meteor? Don McCarthy saw
his during the Leonid meteor storm of November 2001.
McCarthy, an infrared astronomer at the University of
Arizona, went out to watch the sky earlier in the night
than all the experts were advising the public. The show-
er’s radiant in Leo was still practically on the eastern
horizon. “I can still see the meteor rising up the eastern
sky like the bright exhaust of a dark-tipped missile,” he
says. “It gained speed and just kept going! We ducked as
it passed overhead and continued westward, seemingly
forever. We saw thousands of meteors later as Leo rose
high, but two of those early ‘earthgrazers’ spoiled me —
until last August’s Perseids.”
The standard advice for skywatchers has always been
to plan your meteor watch for when a shower’s radiant is
high in the sky. This usually means staying up until (or
waking up for) the pre-dawn hours, when your side of
Earth faces forward as Earth fl ies along its orbit. Earth’s
motion, adding to the meteoroid stream’s motion, tends to
push showers’ radiants into the morning sky.
All the particles in a meteor shower travel nearly in
parallel. The radiant is the spot among the stars where
they would all appear to be coming from if we could see
them approaching in the far distance, instead of just in
their last moments as they hit Earth’s upper atmosphere.
When the radiant is high, you see more meteors all over
the sky than you would if the radiant were low.
That’s why we’re always told that the early-morning
hours are “best” for the August Perseids, for instance.
The Perseid radiant, in northern Perseus near Cassiopeia,
doesn’t reach its highest altitude until just before dawn.
But at 10 p.m. last August 12th, on the shore of Lake
Superior in northern Minnesota, McCarthy and his fam-
ily were roasting marshmallows over a campfi re. A hill
blocked the north, and the lights of nearby towns bright-
ened the sky. Nevertheless, out the corner of his eye,
“I saw a long meteor coming from the north and pass-
ing overhead — and it kept going!” He called out to his
family. They turned and saw it. “I knew this was Perseid
night, and it was coming from the right direction, but the
Left: Many meteors change from blue-green to red during their
plunge. This one did so as it skimmed from low in the east
all the way across the zenith and almost down to the western
horizon. Stéphane Vetter was shooting through an all-sky fi sheye
lens from Champ du Feu in Alsace, France, on November 17,
2012, during the annual Leonid shower. But this does not seem
to be a Leonid! The shower’s radiant in the Sickle of Leo was ris-
ing just above the trees off to the left of the meteor’s direction of
origin. The brightest object is Jupiter in Taurus. Below it is Orion.
Above: A shower’s meteors all travel in parallel. Where
they hit Earth’s upper atmosphere straight on, a sky-
watcher sees them numerous but fairly short. In the
parts of the world where they graze in at a low angle —
that is, where the shower’s radiant appears low in the
sky — the meteors are few but long. (Not to scale.)Grazing & Nongrazing Meteors
S&T: LEAH TISCIONEYour Obser vations Wante d!
If you go out watching for earthgrazing meteors, you
can contribute to a project that Don McCarthy is start-
ing. He hopes to measure the frequency of these dra-
matic events and determine several things:
· Do their numbers scale down as geometry would
predict due to their low incoming angle? Or are they
more (or less) visible than expected — perhaps due to
their attention-getting length and duration?
· Are they numerous enough to justify promoting
them to the public? Could they attract new meteor
watchers to the fi eld?
· Can they predict the intensity of a shower’s activity
later in the night?
To collect your observations, McCarthy has set up
an earthgrazer report form at astronomycamp.org, the
website of Arizona’s Astronomy Camp, which he directs.
Click on the “Fireball Meteor Report Form” link on the
home page. Report meteors even if they’re not fi reballs.