Sep 27, 2000

Bright Planets and Random Meteors

Bright Planets & Random Meteors

This week's new Moon sets a dark stage for a sporadic meteor show featuring a cast of eye-catching stars and planets.

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September 28, 2000 -- It can be hard to wake up on dark autumn mornings, especially if what lies ahead is a long commute to work or school. But a slow-rising Sun isn't all bad. This week it offers drowsy commuters something to look forward to -- a breakfast buffet of morning stars, planets, and meteors.

At 5:30 a.m. local time the planets Jupiter and Saturn, the red star Aldebaran, and the delicate Pleiades form an eye-catching rectangle nearly overhead at mid-northern latitudes. It's enough to make hurried travelers forget their cell phones! Southbound motorists are likely to see the constellation Orion astride the southeastern horizon not far from low-hanging Sirius, the brightest star in the sky. Sirius is so bright that it's often mistaken for an airplane, but watch carefully. If it doesn't move it is probably the Dog Star.

Right: This fanciful graphic by Duane Hilton shows the pre-dawn sky as it might appear this week to a southbound freeway commuter.

on the image to view a larger version with objects in the sky labeled.

The bright planets and stars make it a good time for freeway stargazing -- at least for passengers. Swerving drivers are better off with their eyes on the road.


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It's also a rewarding week for early-rising meteor watchers. Wednesday's New Moon sets the stage for the annual maximum of sporadic meteors in the northern hemisphere. The absence of moonlight won't make much difference to drivers on city-lit freeways, but rural observers can expect to see 5 to 15 meteors per hour. A few bright shooting stars, like the spectacular fireball many people saw in Southern California on Wednesday, will likely dazzle urbanites as well.

Sporadic meteors are caused by random bits of comet debris spread throughout the inner solar system -- the same cloud of debris that gives rise to the elusive Zodiacal Lights. As our planet plows through this dusty background, meteoroids hit the atmosphere and burn up, resulting in a low-level meteor shower that lasts all year long. Usually sporadics play second fiddle to short-lived, intense meteor showers like the Leonids, but this month they take center stage with a modest peak of their own.


Above: The faint trangular glow in the center of this 1997 picture is sunlight scattered from interplanetary dust particles, the same particles that cause sporadic meteors. The diffuse "Zodiacal Lights" are best seen from dark-sky observing sites before dawn in autumn or just after sunset in the spring.

To understand why sporadic activity is greatest near the beginning of Fall, simply recall the last time you drove through a swarm of insects. (Splat! There goes another bug on the windshield...) Bugs rarely "splat" on the rear window because it's hard for insects to overtake a fast-moving car from behind. They accumulate instead on the front glass, in the direction that the car is moving.

The same holds true for sporadic meteoroids. They usually meet the Earth in a head-on collision from the direction of our planet's orbital motion around the Sun, a direction that astronomers call "the Earth's apex." The region of sky around the apex is our planet's "front windshield." In late September the apex, as seen from northern latitudes, lies 70 degrees above the horizon at dawn -- that's its highest altitude of the year. With the Earth's "front windshield" so favorably placed, sporadic meteors are easy to see. Northern observers usually count twice as many sporadics in September as they do in March, when the apex has a lower declination.


Above: This sky map shows the direction of Earth's apex on Sept 22, 2000, the date of the northern autumnal equinox. The apex is always 90 degrees westward from the Sun along the ecliptic plane.

The situation is reversed in the southern hemisphere where the apex is highest in March and lowest in September. This month happens to be a poor one for sporadic meteor watching south of the equator.

No matter where you live, north or south, the best time to spot sporadic meteors is during the hours just before dawn around early autumn in your hemisphere. Although sporadics tend to come from the direction of the apex, they can appear in any part of the sky, headed in any direction. Unlike organized cometary debris streams that give rise to meteor showers such as the Perseids or Leonids, sporadic meteoroids are truly random. You never know what you will get from a sporadic meteor -- you might spy a glorious fireball, or a flurry of shooting stars, or nothing at all. The only way to find out is by looking.

If, however, you happen to be looking from inside your car, we suggest pulling to the side of the freeway. Watching meteors and chatting on the cell phone at the same time is simply too dangerous!

Web Links

More information about sporadic meteors -- from

Moon and Planet Games for the Last Week of September -- Jack Stargazer episode # 00-38

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