The Leonids are back—and they're not a family circus group from Russia! This is the annual meteor shower of November that offers us the chance to see a bit of very ancient history disintegrate in a fiery second. What an opportunity! And, this meteor shower has a personal connection for me, touching on my childhood as well as the young adulthood of my grandfather.
First, the facts for those of you who just want to know where to go and when to do it.
The peak of the annual Leonid meteor shower occurs in the early morning hours this Sunday, November 18. The shower has actually been going on for about a week and will continue for a few more days, but November 18 is when you can expect to see the most meteors per hour. The shower will become visible post-midnight and into the early morning hours of Sunday as its radiant point (the patch of sky they appear to fly out of, in this case the constellation Leo) rises in the eastern sky.
This year, the anticipated meteor rate—the "ZHR," or "Zenith Hourly Rate"—is somewhere in the 10 to 15 range. That is, under good viewing conditions (clear sky, low light pollution levels and minimal interference from moonlight) and if the shower's radiant point were positioned directly overhead at the zenith, you could expect to see 10 to 15 meteors each hour. That may not sound like a lot, but really that's a meteor every 4-6 minutes!
The moon is not an issue this year; it will be setting before midnight, leaving moonless skies for the rest of the night. The trick is to have clear skies and to get away from city lights as much as possible. If you live in the city, but want to see Leonids, plan a trip to a spot away from major congestion. In the San Francisco Bay Area, there are a few good places to try: Henry Coe State Park near San Jose, Skyline Blvd. on the peninsula south of San Francisco, the Santa Cruz Mountains, Mount Diablo and quite a few points north in Marin and Sonoma counties. In the East Bay, the hills from Berkeley down through Hayward have a lot of roadside light-sheltered areas to find.
The Leonid shower occurs each year when the Earth passes through the trail of dust left behind by comet Tempel-Tuttle, which orbits the Sun every 33 years. When the Earth plunges through the dust trail at its orbital speed of 18 miles per second, tiny bits of rock and metal -- typically anywhere from pebble-sized to dust grains -- burn up in our upper atmosphere from friction. In a mere second or so a bit of rock or metal that has been around for billions of years -- either as a free bit of material or locked up in the ices of the parent comet -- burns up in a flash.
The reason that meteor shower aren't visible until after midnight is that you need to be on the side of the Earth that is moving forward in space, into the dust—similar to how in a car on the highway if you drive through a cloud of flying insects, you need to look at the windshield to see the bug streaks on the glass. The morning skies are Earth's "windshield" in this case.
Now for the family reminisce. Comet Tempel-Tuttle passes by our part of the Solar System every 33 years, laying down a fresh trail of dust for us to pass through and causing an increase in the ZHR of the Leonid shower. The comet passed through a little over a decade ago, in 1999, so in the early 2000's there was an upswing in the Leonid rate that made for some spectacular shower viewing.
In 1966, after the next previous passage of Tempel-Tuttle, when I was 4 years old, we were told to expect a spectacular meteor show—unfortunately, it was overcast in Oakland that evening and my parents only took me outside in the evening hours when you can't expect to see a Leonid anyway. I remember seeing the glow of city lights reflected off the overcast and thinking that was the light of the shower shining through! Ah, four-year-olds.
In 1933, after the next previous passage by the comet, my grandfather witnessed a superb Leonid meteor storm. He described it as seeing meteor after meteor radiating from a point in the sky. That shower was estimated to have produced around 200 meteors per hour! Lucky you, grandpa -- I have yet to see anything like that!
So, happy primordial dust speck incineration watching, everyone!