Tears of Saint Lawrence

By Kit Hayden | Jul 24, 2012
Photo by: scienceguy288.wordpress.com

Newcastle — The meteor shower of the Perseids will occur on August 11 this year.  The moon will be a sliver rising three hours before the sun, so the viewing should be good, provided it’s a clear night.  These “shooting stars” all move from north to south, and if you were to follow their trajectories back to origin you would find that they converge at a single point, the radiant, in the constellation Perseus; hence the name.

The Perseids are the jetsam of the comet Swift-Tuttle, discovered in 1862.  All comets are believed to originate in the Oort cloud where aggregates of ices and cosmic dust are loosely held by the gravitational pull of our sun, nearly a light-year distant.  Because of the great distance, it is believed that the Oort cloud can be affected by passing stars whose gravitational attraction may kick loose a “comet” which then is pulled into an orbit around the sun.  As the comet approaches the sun its nucleus is heated and volatile gases are sublimated to form the comma and the tail, the visible parts of the comet that are pointed towards the sun which attracts them.  Included in the sublimated gases are the dust particles to be scattered along the orbit as the nucleus plunges forward at speeds of as much as 100 km/sec (when close to the sun).

Because comets are in elliptical orbits about the sun should they not “return” with a predictable period?  Not so.  Swift-Tuttle was expected to return in 1982.  It didn’t keep the appointment.  Comets are inherently unpredictable, because they shed mass causing perturbations in their orbits.  Swift-Tuttle was rediscovered in 1992, ten years tardy.  The discrepancy led astronomers to determine that Swift-Tuttle was indeed not “discovered” in 1862. That event was simply the return of Comet Kegler, observed in 1737, and Swift-Tuttle is now identified with observations going back 2000 years; lots of orbits and lots of debris.

In August, the earth, thundering along its own orbit at about 68,000 mph, intersects the orbit of Swift-Tuttle, and the plenteous dust particles left by the comet interact with our atmosphere to produce the misnomered “shooting stars” of variable brightness according to their mass.  Should one glow more brightly that the planet Venus it is called a “fireball.”  Should one survive to make contact with the earth’s surface it is a meteorite.  In no case is it a star.

Just how much mass has this Swift-Tuttle?  One doesn’t really know, because we don’t know the density of the nucleus and the mass changes as the nucleus sublimates.  It is postulated, however, that it is much bigger than the comet/meteor that eliminated the dinosaurs when colliding with our planet.  It is known, from the Perseids, that Swift-Tuttle is in a near-earth orbit, so there is the chance that it might run us down and eliminate us when it returns somewhere around 2026.  We won’t know until we see it, but chances are it will spare us, if we have not already eliminated ourselves with the advances of science and their mismanagement.

On the subject of mismanagement, a persistent peeve of mine is the loss of the dark occasioned by our excessive use of electric lights.  I join the eccentrics who call for the return of night, at least on occasion, by extinguishing those lights.  How about August 11 this year?  We could lie back in an open place and gaze up at the million points of light that stunningly reinforce our realization of all that we do not know.  Perhaps we can take some satisfaction in understanding just a little bit about why some of those points are moving brightly across the sky, north to south, and seemingly plunging into the earth.

Not to worry, just enjoy.  And if the science is unimportant to you, think of the spectacular shower, as many do, as the Tears of Saint Lawrence.  Compassion beats science every time.

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