Sommers-Bausch Observatory - University of Colorado

The Tears of St. Lawrence

Original article by Keith Gleason appeared in The Old Lyons Recorder, Lyons, Colorado, on August 8,1996.

Imagine you're on a spaceship travelling around the Sun at 67,000 miles per hour. As you stare off into space, you are unaware that an object is approaching from the northeast, dropping sunward on a course nearly opposite your own at a speed of 93,000 mph. Unaware, that is, until the two of you attempt to occupy same position in space at the same instant in time. The object suddenly appears "out of the blue" and smashes into your spaceship at an angle 39 degrees off the port bow at a relative speed of 135,000 mph.

Actually, you don't have to imagine it. If the weather is clear late this Sunday night and early Monday morning, August 11th and 12th (NOTE: these days and dates refer to the shower of 1996, not the current year!), you can observe the collision yourself - over and over again. It's the annual Perseid meteor shower.

On that particular night, Earth's orbit places it on a heading towards the Pleiades star cluster in the constellation of Taurus. The other objects are rocky chunks ranging in size from grains of sand to small marbles, which are all moving together as a group towards the southern constellation of Pavo, south of Sagittarius. From our vantage point, the collisions appear to come from Perseus, 39 degrees to the northeast of Taurus. The resulting encounter with our atmosphere vaporizes each rock in a brilliant streak of light, at altitudes ranging from 30 to 80 miles high. The brighter meteors often leave a "train" of glowing gas behind, which persists for several seconds. The glow actually comes from the Earth's own atmospheric gasses, which have been stimulated to radiate light by the intense heat of the meteor's fiery passage.

For over a millenium, English superstition has held that the "burning tears of St. Lawrence" could be seen falling from the heavens every August 10th, the anniversary of the Catholic martyr's death at the hands of the Roman emperor Valerian in 258 AD. Remarkably, this common folklore went unnoticed by astronomers, who stubbornly maintained that meteors were merely random atmospheric phenomena to be ignored by serious observers. Egos and dogma were shattered in 1833, when an intense "storm" lit up the night sky with thousands of "shooting stars" per minute, sparking renewed interest and research into the nature and origins of meteors.

Within a decade, measurements of meteor speeds proved that they must be rocks of extraterrestrial origin. But that still didn't explain why they sometimes appeared in streams, all colliding with the Earth at about the same time, from the same direction, and with the same speed. Then in 1862 a new comet was independently discovered by Lewis Swift and Civil War soldier/embezzler Horace Tuttle. Four years later, the astronomer Giovanni Schiaparelli (who also first sighted the infamous "canals" on Mars) compared the plotted orbit of Comet Swift-Tuttle to that deduced for the particles in the annual August stream - and found them to be identical.

We now know that as a comet approaches the Sun from the depths of space, solar heating vaporizes the ice that makes up the bulk of its nucleus and releases embedded dust grains and gravel in the process. The debris fizzes away into space - giving rise to the tails of gas and dust for which comets are famous. The dust particles continue to orbit the Sun along the same path as the comet itself, and eventually spread out along the entire length of its orbit. There may be hundreds of such dusty loops encircling the Sun - but we only know of the few that we run directly into.

The celestial display is expected to be particularly promising this year for several reasons. First, there will be no moonlight to wash out the fainter events. Second, the Earth should pass through the densest region of rubble about dawn Monday morning - far better than if in broad daylight. Finally, remember Swift-Tuttle? For most of the past 130 years, the parent comet has drifted far beyond the orbit of Saturn in the cold, dark regions of the solar system. But three and a half years ago, the comet returned once again to pass through this patch of space which we now occupy - hopefully, leaving a fresh wake of debris for us to plow into.

The best way to view Nature's fireworks is to simply locate a nice dark site this Sunday night - the darker the better - with a broad panoramic view of the sky. Settle back in an easy chair and keep an eye pointed upwards. It doesn't really matter which direction you look; Perseids can appear anywhere in the sky, although they all seem to radiate away from a point halfway between the wishbone of Perseus and the "W" of Cassiopeia.

Don't expect to see lots of Perseids early in the evening. As is always the case, the evening sky is aimed in the direction where we've been, not where we're headed. Essentially, you're peering out of the rear-view window of spaceship Earth. Most of the meteors you'll see early-on will be "sporadics" - randomly moving rocks that just happen to overtake the Earth and whack us from behind. But by midnight, the rotating Earth will have shifted your view to a side porthole, with our direction of motion just appearing in the eastern sky. After that, you'll have a much better view of the collision angle, well above the horizon in the northeast. The later you stay awake, the more fireworks you'll see, as the Earth turns us to face the radiant nearly head-on. And just before dawn, if the sky remains clear, you should be able to "catch a falling star" at least once every minute.