Meteor Showers
- Comets generate a vast amount of debris in the form of tiny dust grains
as they are evaporated by the Sun.
- Short period comets, in particular, make regular frequent passages by the Sun.
- The resulting debris spreads out along the comet's orbit over time, eventually distributing dust grains all along the orbit.
- The Earth's orbit intersects the orbital path of a few dozen comets
(bad news for the Earth sometime in the distant future).
- For any given intersecting comet orbit, the Earth passes through its debris train once each year.
- Since the intersection point is a fixed point in the Earth's orbit,
the intersection occurs on a particular calendar date.
- For example, every August 11 the Earth passes through the orbital path
of Comet Swift-Tuttle.
- On this night (and a few nights on either side) an observer will see
the typical rate of meteors increase from about 5 per hour to as many as
100 per hour.
- All of the meteors from a given comet will appear to radiate from the same point in the
sky -- the "radiant" of the shower.
-
The radiant is an effect of perspective. All of the meteor trails are
actually parallel to one another, but just like a pair of railroad tracks,
they appear to converge in the distance.
- Meteor showers are often named for the constellation in which
the radiant appears.
- The August meteors radiate from the constellation of
Perseus
and are called the "Perseid" meteors.
- A prominent shower peaking on December 13/14 radiates from the constellation of Gemini -- the "Geminids."
- Immediately after the passage of the parent comet meteor showers can
be greatly enhanced.
- In 1833, 1866, 1933, and 1966 (associated with the 33 year orbital period of comet Temple-Tuttle). the frequency of Leonid meteors at
the shower's peak was reported to be similar to the rate of snowflakes
falling in a snowstorm.
- Despite the huge meteor rate, few if any fragements reach the ground as meteorites -- comets are icy and fragile.
- The 1833 storm sparked a keen interest in understanding
the origin of meteors and meteorites. Prior to 1833 the idea
that rocks fell from the sky was met with great skepticism.
- Leonid activity has been enhanced in the last few years due to the recent passage of the
parent comet.
- How to view meteor showers
- Although a shower's radiant corresponds to a particular location
in the sky, the shower's meteors can appear anywhere in the sky.
- The keys to observing a meteor shower are:
- Find a dark location away from the direct glare of
lights and ideally far from city lights.
- A bright sky will wash out the fainter meteors.
- The best viewing occurs when the Moon is not visible.
- Find a location with unobstructed sky. Ideally you should
be able to seen nothing by sky in your peripheral vision when you look up.
- Get comfortable. Expect to be out for a few hours watching meteors. Bring a lawn chair or blanket. Dress warmly in the winter months.
- Know your shower. How many meteors per hour should you expect. What time does the radiant rise? When is the radiant high in your sky?
- The meteor rate is highest when the radiant is closest to overhead.
- Watch for "earthgrazers" when the radiant is rising.
- Be patient. Most good showers produce only about one meteor per minute (with the exception of a meteor storm, like the Leonids, which can
produce thousands in an hour).
- Locate the radiant and identify "sporadic" meteors which don't
belong to the shower.
- In any hour of viewing you can expect to see about 5 random
meteors not associated with the shower.
Notes
Created Revised November 18, 2002
