It's been 24 years since
Halley's Comet last passed through the inner solar system, but remnants from
the icy wanderer will light up the dawn sky this week in the Eta Aquarid meteor
shower.
The meteor
shower is predicted to peak early on Thursday morning, May 6. Under
ideal conditions (a dark, moonless sky) about 40 of these very swift meteors
can be seen per hour. The shower appears at about one-quarter peak strength for
about three or four days before and after May 6.
The famous Halley's Comet
takes roughly 76 years to circle the sun and last passed through our cosmic
neighborhood in 1986. Halley's orbit closely approaches the Earth's orbit in
two spots, offering two chances each year to see meteor showers left over from
the comet's cosmic "litter."
One point is in the middle to
latter part of October, producing a meteor display known as the
Orionids. The other point comes in the early part of May, producing the
Eta Aquarids.
When and Where to Watch
There are, however, two drawbacks
if you plan to watch for the Eta Aquarids meteors this year. [Meteor shower map.]
First, there is the moon, which
will be at Last Quarter on the peak morning and will likely "muscle
in" on the fainter meteor streaks by brightening the early morning
sky with its bright light.
The other obstacle – at least for
those watching from north of the equator – is that the radiant (the
emanation point of these meteors) is at the "Water Jar" of the
constellation Aquarius, which comes above the southeast horizon around 3 a.m.
local daylight time, never gets very high as seen from north temperate
latitudes, so the actual observed rates are usually much lower than
the oft-quoted 40 per hour.
In North America, typical rates
are 10 meteors per hour at 26-degrees north latitude, half this at 35-degrees
latitude and practically zero north of 40-degrees.
Conversely, those who live in the
Southern Hemisphere, where Aquarius rises much higher into the sky, consider
this to be one of the best meteor showers of the year.
Catch an Earthgrazer
For most, perhaps the best
hope is catching a glimpse
of a meteor emerging from the radiant that will skim the atmosphere
horizontally – much like a bug skimming the side window of an automobile.
Meteor watchers call such shooting stars "Earthgrazers." They leave
colorful, long-lasting trails.
"These meteors are extremely
long," said Robert Lunsford, of the International Meteor Organization.
"They tend to hug the horizon rather than shooting overhead where most
cameras are aimed."
"Earthgrazers are rarely
numerous," cautions Bill Cooke, a member of the Space Environments team at
the Marshall Space Flight Center. "But even if you only see a few, you're
likely to remember them."
Comet Crumbs
If you do catch sight of one early
these next few mornings, keep in mind that you'll likely be seeing the
incandescent streak produced by material which originated from the nucleus
of Halley's Comet.
When these tiny bits of the
comet collide with Earth, friction with our atmosphere raises them to white
heat and produces the effect popularly referred to as "shooting
stars."
So it is that the shooting stars
that we have come to call the Eta Aquarids are really an encounter
with the traces of a famous visitor from the depths of space and from the dawn
of creation.
Joe Rao serves as an
instructor and guest lecturer at New York's Hayden Planetarium. He writes about
astronomy for The New York Times and other publications, and he is also an
on-camera meteorologist for News 12 Westchester, New York.
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