The Perseid Meteor Shower of 2018

.....Every dark, moonless night not dominated by city lights, we can expect to see a few shooting stars per hour, flashing randomly across the sky.  A meteor shower is an event (more common and often less impressive than one might expect) in which there is an increase in the number of meteors over this random baseline of meteors, and those meteors appear to come from one preferential area of the sky.

.....The most reliable meteor shower of the year is the Perseid meteor shower, usually peaking on the night between August 11th and August 12th, but this year peaking on the evening of August 12th into early on the 13th.  The reason for this unusual variation (read: any variation at all) has to do with the nature of what causes this meteor shower. These typically come from one of three sources: Leftover bits of flotsam and jetsam that have been floating around the solar system for the last five and a half billion years (cool), little bits that have been boiled off of comets as they passed around the Sun (also cool), or nuts/bolts/heat shields/tool boxes that have come off of space craft and are crashing back down to Earth (less cool). For regular meteor showers, we need a regular source,, and that takes us to comets.

Images of Comet Swift-Tuttle taken by the astronomer (born in Tennessee!) E. E. Barnard in 1892

.....These are photographs of the comet (Comet Swift-Tuttle) that gives rise to the Perseid meteor shower.  The reason why comets appear with impressive comas (the area around the head of the comet) and tails is due to ice and particles boiled off the comet as the comet enters the inner solar system.  Sunlight reflects off of the highly reflective (i.e., shiny) ice, and makes the comet far more prominent than it would otherwise be.  The ice is broken down by sunlight, but as the ice boils off, small rocky particles are drawn off with it.  These particles don't just disappear - they can't.   They stay in the same orbit that the particles had when they were part of the comet, spread out by the small relative momentum they got (relative to the comet) as the particles came off of the central cometary body, smeared out a bit more over time as gravitational effects of distant planets.  Over a long enough time frame, this can fill the orbit of the comet with dust and pebbles.

.....If the Earth should pass through this gunk, then when the particles hit the Earth's atmosphere they will light up from the heat of friction generated from going from a temperature of less than three hundred degrees below zero (Fahrenheit) to thousands of degrees. We will get a special effect due to all of these particles following the same path, a particular effect will come from this.  Consider the Earth passing through a cloud of these particles:

.....It might appear from this sketch that since the meteoroids could hit the Earth anywhere on the leading half, that the meteors could come from anywhere in the sky, but what is important to remember is that the "sky", the stars that form the backdrop that we are looking against, are all very, very far away.




.....If one were to run those paths in the picture backwards, all of the paths would track back to a single point, far enough in the distance.  As seen against the stars, that point would be a specific point in the sky.  These meteors will all seem to radiate out from the same point. (Called, reasonably enough, the "radiant".)







.....Each August, the Earth passes through the remnant trail of the comet Swift-Tuttle, generating the Perseid meteor shower because the radiant of the meteors (the dotted circle in the image below) is in the constellation of Perseus.  Meteor showers do not require a telescope or binoculars; just go outside and look (in this case to the northeast, especially after midnight).  In the case of the Perseids, most books state the an observer can see from 60-120 meteors per hour, but that comes with a couple of limitations.

.....That number assumes that you are observing the meteor shower with the radiant directly overhead from a very dark site.  With the radiant low in the northeast, even early in the morning one should expect to see perhaps 40 meteors an hour.  This year, the New Moon is on the 11th, so at least the Moon will have no effect.  If you are watching from an urban or suburban location, the fainter meteors will be lost against background light, so the number could be much less, even if the number of meteors is equal to what is expected.

.....In younger years, this was the one time during the year that I could get the whole family as interested in astronomy as I was, which didn't happen very often.  Those memories are really helpful, of those times at our house and on vacation; of cool August nights in the UP of Michigan with my wife's parents; of nights with our friends at our own house. I have a lot of good memories of this shower.

.....Technically, the peak will be from Sunday afternoon through Sunday evening, but not only are those not good times to see meteors (against a clear blue sky), but I've always found it much easier to get rest on a Sunday than a working Monday, in any event.  Looking for Perseids can still be done in the nights leading up to the peak, although the number drops off pretty quickly after the peak has passed.


.....Do you have any family memories of the Perseid meteor shower?