Will the Leonids roar this year like its namesake constellation (Leo, the Lion), or will we stifle a yawn as we struggle to keep our eyes open looking upward into the sky during the wee hours of November 17 and 18?
The occasionally mind-blowing but never strictly predictable Leonid meteor shower visits Earth every mid-November, delivering anywhere from a peak 15,000 to 100,000 meteors (shooting stars) per hour. The scientific explanation of the Leonid phenomenon attributes the meteors to small particles shed from a particular comet, Comet Tempel-Tuttle, which traces one complete elliptical orbit around the sun every 33 years. Every year, at the same time on the calendar, our planet returns to the site where the Tempel-Tuttle debris streams through. But only at intervals of 33 years, more or less, do we encounter denser strands of this cometary debris.
The Leonid meteor particles travel together, so that if they collide with Earth's atmosphere they do so in a parallel stream. Each particle disintegrates in a blaze of light as it plows through the air some 50 miles up at more than 100,000 miles per hour. Due to perspective, the meteor streaks seem to come from a single vanishing point in the sky -- the shower's "radiant" -- which happens to be in the same direction as the constellation Leo. Leo, and the radiant with it, starts rising at around 11 p.m. Therefore, no Leonids can be seen before 11 p.m.
The checkered history of the Leonid shower includes episodes in 1833 and 1966, during which meteors fell at rates exceeding 20 per second! On the morning of November 17, 1998, meteors flashed over North America at moderate rates of around 100 to 200 per hour. Many of these, however, were brilliant "fireballs" luminous enough to cast visible shadows. This year's Leonids are expected to consist of a larger fraction of fainter meteors caused by the incineration of a greater proportion of smaller particles.
Some meteor specialists, working with fragmentary evidence from past Leonid episodes, think that 1999 should be the peak year of the present 33-year Leonid cycle. Some say the peak of the shower itself, which may last less than an hour, will occur at around 3:00 Greenwich Mean Time on November 18 (7 p.m. Pacific Standard Time on November 17), which favors early-morning observers in Europe and Africa. If this prediction holds up, we in western North America will only be able to catch the "wings" of the shower's most intense activity during the early-morning hours of the 18th and possibly the 17th. If the prediction is incorrect regarding the exact timing of the peak, then our chances of seeing a true meteor storm from the West Coast are improved.
Any meteor watching by San Diegans is enormously aided by traveling east to dark locales such as our local mountains and desert. Assuming clear skies, start your vigil at around 11 p.m. or midnight and continue watching until dawn, approximately 5 a.m. No telescope is required; simply lie back in a warm sleeping bag at any site with an unobstructed view of the whole sky. If there's not much activity going on and you'd rather get some sleep, try setting an alarm every hour or so to monitor the shower's progress.