Bob Moler's Ephemeris Blog

This post from the Grand Traverse Astronomical Newsletter “Stellar Sentinel” was written for August of 2018. The dates and times of the peak may change a bit from year to year.

The Perseid meteor shower is the second most active annual meteor shower. The most active is the Geminids of December during a period that’s cold and generally very cloudy here in Northern Michigan. Consequently, I’ve never seen a Geminid meteor.

The Perseid meteor shower is the most famous as the August meteor shower, coming on the warm summer month. In Northern Michigan the radiant of the shower, the point in the sky from which they appear to come, is circumpolar, which means they are visible anytime in dark skies from dusk to dawn.

The Perseids are so named because they appear to come from near the constellation of Perseus the hero, an autumn constellation that starts the evening low in the northeast and rises and moves to high in the east near dawn. In earlier times these meteors were called the Tears of St. Lawrence, who was martyred in AD 255. His Feast day is August 10th, the day he died, which falls very close to the peak activity of the shower.

The Perseid meteors are visible for over a month from about July 17th to August 24th, with peak activity between August 12th at 4 p.m. to August 13th at 4 a.m. EDT. So the peak activity will partially be during our night hours, and the one day old Moon will not interfere at all. The peak hourly rate may reach 100 per hour at times. All things being equal, the higher the radiant is in the sky the greater the numbers of meteors seen. The Perseid radiant will be rising all night, being highest as the first light of dawn appears. Even though the numbers of meteors are fewer I like to start looking by 10:30 p.m. With the radiant low in the sky, the meteoroid particles we see are almost skimming the atmosphere, lasting longer. There’s is nothing so cool as to see a bright Perseid meteor seeming to fly along the Milky Way. The radiant point is in the Milky Way between Perseus below, and Cassiopeia above.

The Perseid radiant is located off the highest star is Perseus as it rises about 11 p.m. August 12, 2018. Click on the image to enlarge. Created using Stellarium.

Observing this meteor shower is very easy and one needs no special equipment. A blanket to lie on, mosquito repellent, warm clothes, some water and snacks, if staying the night, and a dark location. My preferred location is the Dune Climb at the Sleeping Bear Dunes National Lakeshore. It has no light, except the occasional car lights and has modern restroom facilities and a low horizon except in the west. I supposed one could climb up the dune to get rid of even the car lights. Even though the radiant is in the northeast, the meteors will appear all over the sky.

Binoculars are nice to take a break to explore the Milky Way and to observe the smoky train left by a particularly bright meteor. These can be viewed for a minute of more and deform and twist due to the different wind directions and speeds at different altitudes.

What causes the Perseid meteor shower and why does it occur at the same time every year?

The Perseid meteor shower, like all meteor showers are caused by the debris left along the orbits of comets. If the comet’s orbit crosses close to the Earth’s orbit we can get a meteor shower. Comets spend the majority of their time far from the Sun, where it’s very cold, and are in very elongated orbits.

Comets are made from rocky bits, dust and frozen gasses. As the comet comes into the inner solar system the Sun heats it up and the frozen gasses sublimate, are ionized by the Sun’s radiation and are caught into the thin ion tail. This liberates the comet’s fine dust which is blown away from the Sun by the pressure of sunlight into a broad dust tail. Larger particles end up traveling in the comet’s path, and are affected mainly by the Sun and the various gravitational tugs of the planets.

The comet responsible for the Perseids is 109P/Swift-Tuttle. It was independently discovered by L. Swift and P. Tuttle in 1862. It was recorded as being seen in 69 BC by, you guessed it, the Chinese. It’s a big comet, with a nucleus some 16 miles in diameter, and it crosses the Earth’s orbit, so it is a potentially hazardous object, and if it hit the Earth, would wreak more damage than the asteroid that killed the dinosaurs. From the 1862 appearance the comet was given a period of 120 years. It didn’t show in 1982. An observation of the previous appearance of the comet in 1737 allowed a recalculation of the orbit and a new return year of 1992. That was correct. The comet was recovered that year.

The comet will return in 2126. The calculations used to predict the 1992 return suggested that the comet could possible collide with the Earth. However observations of the 1992 appearance of the comet determined that the comet, though it would pass close to the Earth, is not a hazard. But it should be really bright. I can’t wait!