Imagine you’re on a spaceship traveling around the Sun at 67,000 miles per hour. As you stare off into the void, you’re unaware that there is an object approaching from the northeast, dropping sunward on a course nearly opposite your own and moving at a very respectable clip of 89,000 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 a relative velocity of 130,000 mph.
Actually, you don’t have to imagine it. If the weather is clear on the night of August 12th/13th, you can watch the collision yourself, over and over again. It’s the annual Perseid meteor shower.
On that particular night, Earth is zipping through space towards the Pleiades star cluster in the constellation of Taurus the Bull. The objects are particles ranging in size from grains of sand to small peas, moving together as a group on a path that crosses ours from north to south. Just as snowflakes appear to radiate from a certain point in the sky when driving in a snowstorm (the location of which depends on the speeds and directions of both wind and car), from our viewpoint on the moving Earth the objects appear to originate from the constellation of Perseus, 39 degrees off the port side of Earth’s imaginary bow.
The resulting encounter with our atmosphere vaporizes each speck of dust in a brilliant streak of light. 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.
The Geometry of Collisions
Although meteor showers have long been observed by humanity, early astronomers dismissed them as mere “exhalations of the Earth’s atmosphere” and thus unworthy of serious astronomical study. The halitosis hypothesis had to be abandoned, however, when in the early morning hours of November 13th, 1833, the skies over North America lit up with literally tens of thousands of “shooting stars” per hour, all appearing to radiate from the constellation of Leo the Lion. Subsequent studies showed that the incandescent objects must have come from deep space, were all moving with identical direction and speed, and burned up at altitudes between 30 and 80 miles. Still, why such a phenomenon would occur was not understood until 33 years later, when Comet Tempel-Tuttle was discovered, and it was soon realized that the orbits of the comet and the particle showers of 1833 and 1866 were one and the same. Meteors were simply the dust particles cast off in a comet’s tail that burned up in the Earth’s atmosphere as we plowed through the debris. Because the intersection of two orbits occurs only at a single point in space, and because the Earth arrives at that point only once a year, so that the geometry of the collision is always the same, showers become quasi-predictable and are named for the constellation (the Leonids, in this case) from which they appear to radiate.
Visualizing what is happening in the sky with meteor showers is useful in understanding celestial motions in our solar system in general. Because Earth orbits the Sun in an almost circular fashion, we move at a fairly constant speed around it but in an ever-changing direction with respect to the stars: if you stand and look at the Sun (or where you know the Sun must be, even if it is below the horizon), Earth’s movement through space will always be 90° to the right of where you are looking, measured along the line of the ecliptic (the Sun’s apparent path through the sky, equivalent to our real path around it). Thus at noon our direction of motion through space is towards the western horizon, but that changes as Earth’s rotation constantly alters our viewing angle of the heavens.
Around sunset, then, we are moving through space roughly in a direction that is “downward”, towards our feet. When we look up at the early evening sky, we’re watching from the aft deck of Spaceship Earth and are watching its wake. You won’t see many meteors at that time of night, simply because they would have to be moving in the same direction as we, albeit faster, so as to overtake the Earth and whack us from behind. But by midnight, rotation brings us a view out of the port side windows of our spacecraft, while our motion shifts towards the eastern horizon. And by dawn, the Earth is rushing pretty much straight up, where things just above our heads – such as dust particles from comets – can be readily smashed into.
The Perseid Shower
In 1862, four years before he discovered the aforementioned comet that bears his name, astronomer Horace Tuttle (along with Lewis Swift) discovered a new comet falling in from deep space, nearly intersecting our orbit but missing us by about a third of the Earth-Sun distance. Just as Comet Tempel-Tuttle was deduced to be the source of the November Leonids, the new comet was eventually realized to be the parent of the annual August Perseid meteor shower.
But instead of a 33-year orbit, Comet Swift-Tuttle had a 133-year orbit, and wasn’t seen again until 1992 … the same year we built our new home under the dark skies of the foothills above Lyons. Over the next few years the Perseids consistently put on a remarkable show for us that included numerous bright fireballs. While I initially attributed the spectacle to the darker skies of the foothills, it sadly became apparent that, although the dust from a comet spreads itself out into a loop along its entire orbit, the receding nucleus was taking some of its best dusty fireworks with it.
Nevertheless, the August Perseids remains one of the three best and most reliable showers of the year (along with the Leonids in November and the Geminids in December) and the only one which can be comfortably observed for any length of time without getting chilled. Best of all (for someone who desperately needs his beauty sleep) the shower’s radiant is so far north that it is circumpolar from our latitude; it is never below our horizon. That means the radiant is already spewing out an occasional visible meteor even at the onset of darkness.
Lyons, Colorado Sighting
Although it’s always a bit uncertain when – or even if – Earth will plow into a particularly dense swirl of debris left behind by the comet, the best estimate for Coloradoans this year is sometime during the night of August 12th/13th. By 10 p.m. Thursday evening the 12th, the crescent Moon will have set just as the last hint of evening twilight fades, and the radiant of the Perseids will already be 16° above our northeastern horizon. While the geometry at the time dictates that the atmosphere overhead presents a small cross-section of area for the incoming to hit – and hence we’re likely to see only a few streaks – those that do encounter us will be earthgrazers, ones that skim across a large swath of the sky, and are my favorites to observe. As the Earth’s rotation brings the radiant higher in the sky, our visible sky presents a larger target and hence the number of observable meteors tends to increase.
By 4:30 the morning of the 13th, just at the crack of dawn, the radiant will be a mere 28° from directly overhead. But no matter when you look, you need not keep your eyes fixed on Perseus and the radiant. While the meteors seem to come from that direction, they don’t necessarily appear in that direction: they can and will enter our atmosphere and materialize anywhere at random in the sky overhead. So … grab a lawn chair and some mosquito repellant, pick out a nice, dark direction in the sky to watch, let your eyes become dark-adapted, and enjoy the spectacle as Comet Swift-Tuttle chucks a bunch of little rocks at us.