Long time exposure from one of NASA's All Sky Cameras displaying Perseid fireballs on the morning of August 13, 2017.

The bright fat arc is the Moon that morning.

The Tears of St. Lawrence

By Bob Moler

Ever before astronomers recognized the meteor shower that occurs in August each year, faithful Christians saw the bright streaks in the sky as the Tears of St. Lawrence. This saint was martyred on August 10th of AD 258 by being roasted to death. The faithful saw the bright streaks in the sky as a sign from heaven: St. Lawrence’s fiery tears.

We call these streaks Perseid meteors, and the event that stretches from July 17th to August 24th the Perseid meteor shower. They are so named because they appear to come from the constellation of Perseus the hero, near the more recognizable constellation of Cassiopeia the queen, which looks like the letter W. The radiant is marked on this month’s star charts.

For 2021: The actual peak of the shower is expected to be August 12th, between 3 and 6 pm EDT (19h to 22h UT) according to the International Meteor Organization. So the nights of August 11-12th or 12-13th seem about equally promising. The Moon won’t interfere much either night. The 13% illuminated, 3 day old Moon will set at 10:50 pm on the 11th; and a 22%, 4 day old moon will set at 11:13 pm on the 12th.

At peak a single observer of the Perseids could see as many as 110 meteors in an hours time when the radiant, where the meteors seem to come from, is near the zenith. That’s called the zenithal hourly rate (ZHR). The lower the radiant is in the sky, the less the rate of meteors is. By 5 am the radiant is the highest before twilight really interferes. The radiant at that point is at 64° altitude high in the northeast.

For the Grand Traverse region the radiant is circumpolar, so it’s up all night, though it’s quite low in the northeast in the evening. The cool thing though, when the radiant is low, the meteor streaks are longest, as the meteoroids hit the atmosphere above us at a very low angle. In the early morning they auger in at a steeper angle. It’s really neat to see one glide down the length of the Milky Way in the evening.

I’d better stop and explain the three terms meteor, meteoroid and meteorite. I used two of these already. Meteor is the flash of light we see in the sky when a small meteoroid the size of a grain of sand to larger hits the Earth’s atmosphere at tens of miles per second. Trading kinetic energy of the meteoroid’s velocity in the upper atmosphere into heat quickly vaporizes it. The Perseid meteoroids hit the atmosphere at 37 miles per second (59 km/s). As far is known no Perseid has ever survived this entry to reach the ground to become a meteorite. A really bright Perseid meteor could be caused by a tiny body the size of a pea.

The reason that meteor showers have a radiant is that the meteoroids are traveling in parallel paths in pretty much the same orbit, so they seem to come from the same spot in the sky. Each shower is caused by a comet. These bodies originate from the far regions of the solar system called the Oort Cloud named after Dutch astronomer Jan Oort who hypothesized its existence.

Comets are notorious litter bugs, being made of frozen gasses and bits of rocky and metallic material. When falling into the inner solar system they heat up, sublimating the frozen gasses and liberating the embedded dust and bits of material. The dust and gas ends up in the comet tails. The larger bits end up orbiting the Sun near the comet’s path. When one of these comets comes close to Jupiter its orbit can be altered to stay within the orbit of Neptune or closer. The repeated passes close to the Sun will build up debris streams that approximate the comet’s orbit. If the streams intersect the Earth’s orbit we will see a meteor shower at the same dates each year.

The comet the Perseids are caused by is 109P/Swift-Tuttle discovered by Lewis Swift on July 16th, 1862 who thought he was viewing another comet. Horace Tuttle spotted the comet and reported the discovery three days later. Early estimates of the comet’s orbital period of 120 to 125 years proved incorrect when the comet didn’t show in the early 1980s. A modern recalculation of the comet’s orbit put the orbital period closer to 130 years.

Image from the interactive and animated chart on the International Meteor Organization website https://www.imo.net/ showing where the

Perseid meteoroid stream intersects with the earth's orbit.

The comet was finally recovered on September 26, 1992 by Tsuruhiko Kiuchi. With the comet in the neighborhood of the inner solar system in the 1990s Perseid numbers had increased. Comet Swift-Tuttle has been traced back to prior observed returns: 1737, and by the Chinese in AD 188 and 69 BC. It’s current orbital period is 133 years.

Reprinted from the August 2021 issue of the Stellar Sentinel, the newsletter of the Grand Traverse Astronomical Society. Educators may request a free electronic subscription by requesting it from the society: info@gtastro.org.

Questions? Send Email to me at brelom@gmail.com

Updated: 10/15/21