This week brings the return of the famous Leonid meteor shower, a display that over the past several years has brought great anticipation and excitement to sky watchers around the world.
Solely from the standpoint of viewing circumstances, this is a favorable year to look for these meteors, since the moon is only a thin crescent and sets in the west long before the constellation Leo (from which the meteors get their name) begins to rise. This is in stark contrast to 2002, when a nearly full moon lit up the sky like a spotlight and wiped out all but the brightest meteors.
Last year, conditions were much better; the moon had slimmed to a narrow crescent in the predawn morning sky and offered little hindrance. But this year, the moon will be out of the sky completely, offering perfect viewing conditions.
The Leonid meteors are debris shed into space by Comet Tempel-Tuttle, which swings through the inner solar system at intervals of 33 years. With each visit the comet leaves behind a trail of dust in its wake.
Lots of the comet’s old dusty trails litter the mid-November part of Earth’s orbit, and Earth glides through this debris zone every year. Occasionally we’ll pass directly through an unusually concentrated dust trail, or filament, which can spark a meteor storm resulting in thousands of meteors per hour. That’s what happened in 1999, 2001 and 2002.
Since Comet Tempel-Tuttle passed the sun in 1998, it was in those years immediately following its passage that the Leonids put on their best show.
But now, the comet — and its dense trails of dust — have all receded far beyond Earth’s orbit and back into the outer regions of the solar system. So there is little if any chance of unusual meteor activity.
Peak times
In the 2004 Observer’s Handbook of the Royal Astronomical Society of Canada, meteor experts Margaret Campbell-Brown and Peter Brown, indicated that this year’s peak activity should have come Wednesday morning.
They cited 09:00 GMT, which corresponded to 4 a.m. ET and 1 a.m. PT. This was the moment when Earth passed closest to the orbit of the long-departed comet, and when our planet seemed most likely to encounter some residual comet material. Smatterings of Leonid meteors were indeed sighted at locations ranging from California to Australia to China in the Nov. 16-17 time frame, according to reports posted on the Meteorobs discussion forum.
Other meteor researchers such as Jeremie Vaubaillon of France, David Asher of Ireland and Esko Lyytinen of Finland suggested watching for some meteor activity on Friday as well.
For instance, sometime around 06:40 GMT (1:40 a.m. ET) on Friday, Earth might interact with material that was shed by Comet Tempel-Tuttle back in the year 1333. But at best, only about 10 Leonids per hour may be seen. Lyytinen even suggests that because of gravitational perturbations affecting these particles after making 20 trips around the sun, this sparse activity might actually take place several hours earlier, which would favor observers in Europe.
Later Friday, Earth could sweep through a fresher and somewhat more concentrated trail of material dating back to 1733. Perhaps a brief bevy of 30 to 60 Leonids per hour may be seen, but the time frame when these meteors are predicted to be most numerous (approximately 19:00 to 22:00 GMT) favors observers in Asia and Australia (where it will be early Saturday).
How to watch
The meteors appear to emanate from out of the so-called "Sickle" of Leo, but prospective viewers should not concentrate on that area of the sky around Leo. Instead, they should keep their eyes moving around to different parts of the sky.
Because Leo does not start coming fully into view until the after midnight hours, that would be the best time to concentrate on looking for the Leonid meteors.
The hours after midnight are generally best for watching for "shooting stars" anyway, because before midnight we are riding on the back side of Earth in its orbit around the sun, whereas after midnight we are on the front or advancing side. After midnight, the only meteoroids escaping collision are those ahead of Earth and moving in the same direction with velocities exceeding 18.5 miles per second. All others we will either overtake or meet head-on. But before midnight, when we are on the backside, the only meteoroids we encounter are those with velocities high enough to overtake Earth.
Therefore, on the average, morning meteors appear brighter and faster than those we see in the evening.
And because the Leonids are moving along in their orbit around the sun in a direction opposite to that of Earth, they slam into our atmosphere nearly head-on, resulting in the fastest meteor velocities possible: 45 miles per second (72 kilometers per second). Such speeds tend to produce bright meteors, which leave long-lasting streaks or trains in their wake.
After Friday morning, the Leonids will tail off quickly. A few stragglers may be visible on subsequent mornings.
This report contains updated information from MSNBC.com.