Friday, October 26, 2012
The Leonids are coming.
During the late-night hours of Nov. 17, the Leonid meteor shower should peak with a rate of 15-20 meteors per hour at a dark site.
As I explained in the August column, meteor showers occur when the Earth, in its orbit around the Sun, passes through the path of a comet. When the comet is close to the Sun (perihelion), it heats up, which causes the outer ice to melt. The ice that melts contains small particles of rock and metal, which become meteors when they enter the Earth’s atmosphere.
In the case of the Leonids, the comet is 55p/Tempel-Tuttle, discovered by Ernst Tempel and Horace Tuttle about 1865. This comet orbits the Sun every 33 years.
Predicting the meteor rate for a shower is like predicting the weather — you only know for sure after it’s over. The Leonids are a highly variable shower, with rates ranging from 5-10 per hour to over 1,000. The rates go up when the comet passes the Sun and the stream is resupplied with new material. Comet 55p/Tempel-Tuttle was last at perihelion in 1998, so the rates have been falling over the past few years. But the debris in the comet’s orbit is not evenly distributed, so you never know how many there will be.
This year we will encounter the debris stream on Nov. 6 and leave the stream near the end of the month. The prediction is that the peak rate will be at 1:30 a.m. Pacific Standard Time on Nov. 17. This timing is ideal. Morning hours are best for viewing because the Earth is facing its direction of motion of the orbit, this meaning that the sky above us is where the meteors are entering the atmosphere. Another reason this time is good for us is that on that date, the moon is a three-day-old crescent that will set a few hours after sunset, giving us dark skies for most of the night.
The Leonids are the fastest moving meteors of all the major showers. They enter the atmosphere at about 44 miles per second. This means they produce a large percentage of fireballs, meteors as bright as planets that can even cast a shadow at a dark site. Most are bluish-white, but some yellow-pink have been reported in the last few years. The Leonid debris is very small, always burns up in the atmosphere, and never reaches the ground, so no meteorites result from this shower.
Here are some suggestions on how to observe the meteor shower. First, you need as dark a site as possible. Because many of the meteors will be dim, the darker the site, the more you will see. The number you see will really increase even a few miles out of town. The site should also have a clear view of the sky away from buildings, trees and other obstructions that would block the view.
For comfort, dress warmly and bring a lawn chair and some snacks; some cookies, fruit, and a hot drink will make the event more enjoyable. Nonalcoholic beverages are best because alcohol interferes with the eye’s dark adaptation and visual perception.
Where in the sky should you look for the meteors? If you view before midnight, look to the east about halfway up. After midnight, look straight overhead; and as time passes, look more to the west. As dawn approaches, you should be looking about halfway up to the west.
It is fun to count the number of meteors you see. Write the starting time on a sheet of paper and make a tick mark each time you see a meteor. When you stop, note the ending time on the same sheet.
An even easier way to count is with a tape recorder. Turn the recorder on and state the time, then just repeat “there’s one” each time you see a meteor. Say the ending time just before you turn the recorder off. When you play the recording back you can count the number of meteors during any time interval.
So watch for the Leonids, and remember — if you miss this one or we get weathered out, the Geminids are coming on Dec. 14 and the Quadrantids on Jan. 4.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.