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4 December 1998

Shooting Stars

It was overcast on the first night of the DMS Joshua Tree trip on November 17, but the clouds parted long enough to give us a great view of the promised Leonid Meteor Shower. We saw many shooting stars, including several fireballs that lit up the sky and left glowing trails! Meteors are tiny bits of space debris that usually burn up completely in the Earth's atmosphere. You can see a few on any dark night if you're lucky, but they are unpredictable. So how can a meteor shower like the Leonids predictably return at the same time and place in the sky year after year?

It's estimated that several hundred tons of meteoroids fall onto the Earth every day, but we only notice the big ones. Meteor showers like we saw in Joshua Tree are the remnants of comets that partially vaporize as they approach the sun like melting dirty snowballs. This leaves a long thin trail of rocky debris that follows the comet in its orbit. The material is there in space, just waiting for the Earth to cross its path. The Geminid shower is expected to peak on Sunday night! (13 Dec 98) Look south towards Orion any time after 10 p.m.

Here's a Leonid 2001 update:

We had a perfect view from our porch for the 2001 Leonid Meteor shower on November 18! The action peaked about 2:00 A.M. PST as predicted. We saw hundreds of meteors, sometimes 4 or 5 within seconds. The tails were not as lingering as I recall from Joshua Tree in 1998. I set up my Olympus 2040 digital camera on my tripod pointed at Orion, with manual exposure set to f/1.8 @ 16 seconds, hoping to catch something. That must have been too much exposure since the dark sky came out speckled. It's my first try so I'm not complaining.

There are two parallel meteor streaks in this photo. It's easier to see in this enhanced negative image. There are better Leonid pictures here.
Lenoid Meteors, November 18, 2001


Of course I have a Leonid meteor stumper (4 Dec 98)

Note: The Greek philosopher Aristotle thought that the unpredictable motions of things on the Earth arise from interactions of the four elements earth, air, fire, and water which have different up/down natural motions. He argued in De Caelo that the heavenly bodies move only in simple circular paths, so they must be made of a different substance altogether, which he called "aither." Meteors are unpredictable, so they must be earthly, atmospheric phenomena, like the weather. Hence the connection between meteors and meteorology, the study of weather. Aristotle was at least part right!

Graybear sent in this clear explanation of meteor showers:

Meteoroids are pieces of space debris. This debris is in orbit around the sun, and so is Earth. When the two orbits intersect and the timing is right, they become meteors. If they are big enough not to burn up completely in the atmosphere, they become meteorites. The occasional meteor was a random meteoroid, but the grouping of meteors, such as the Leonids, the Perseids, the Orionids, the Geminids, etc., obviously must come from concentrations of meteoroids. Scientists in the 1800's realized this and determined that these concentrations of debris were associated with the orbits of comets, and thus theorized that the debris was left by the 'tail' of the comet. The Leonids are associated with the Tempel-Tuttle comet, which has an orbit of 33 years. Therefore the meteoroids which become Leonids are replenished every 33 years. That is how we predicted that this year's shower should be a good one.

During a meteor shower, shooting stars can be seen anywhere in the sky, but if you follow their paths back, they all seem to originate from one place in the sky called the radiant. This shows where in space the meteors are coming from.

This stunning image was captured in the Slovak Republic (where the Leonids peaked) at the Astronomical Observatory Modra (Astronomical Institute of the Faculty of Mathematics and Physics, Comenius University Bratislava) using a Zeiss fisheye lens with a 180 degree view, on the night of November 16th, 1998. It shows over 150 meteors!

I got this image from http://www.uniba.sk/~ago_modra/english.htm. It clearly shows the radiant in the middle-left, like a duck's view of a shotgun blast!

The real stumper about meteor showers for me is why so many meteor showers come at us from the ecliptic, in the plane of the solar system. The solar system is almost flat, like a pizza with planets instead of pepperoni. Looking out, we only find the planets (and the sun and the moon) along a particular narrow band of the sky against the background of the zodiac constellations. (See my stumper Pretty Planets all in a Row.) But comets fall into the solar system from the outer reaches of the spherical Oort cloud beyond the planets, so they can be found anywhere in the sky.

There are meteor showers like the midsummer Perseids that do not originate on the ecliptic. But most do come from the zodiac, including the Aquarids, the Taurids, the Capricornids, the Leonids, and the Geminids.

This is certainly not coincidence. Astronomers distinguish between long-period comets, with long orbits that take hundreds or thousands of years, and short-period comets like Halley's Comet that return every few decades. Long-period comets that pass by Jupiter are snagged by gravitational forces into orbits that remain in the inner solar system along the plane of the planets. Because these comets pass the sun relatively frequently, they develop significant streams of meteoroids in their wake. We're more likely to encounter these streams as meteor showers because the Earth orbits the sun in roughly the same plane, so we're more likely to cross their paths. The solar system would be a different place without massive Jupiter redirecting the traffic. (Now there's a real stumper!)

There's lots of good info on the Web about meteors and meteor showers:

There are many more sites that you can easily find with a Web search and some surfing.

You can play with celestial orbits on your computer using my DOS/BASIC program ORBIT, available for download from Treebeard's Basic Vault.

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Copyright © 1998 by Marc Kummel / mkummel@rain.org