Al Stahler: The sky show |

Al Stahler: The sky show

With hopes that the River Fire will be well on its way to full containment, and that skies are clear, here’s something to look for, next Wednesday night.

Toss a ball into the air … watch it loop around … catch it on its way down. Playing with a ball, we’re playing with gravity.

A juggler also plays with gravity, but with three … four … five balls at a time. Each of the three, four, or five balls loops around, and falls back down.

Our sun, at the center of the solar system, also plays with gravity, juggling planets, and asteroids, and comets. But whereas the balls tossed by the human juggler all share the same loop – up and around – each planet, asteroid and comet has its own, individual loop. With just a few exceptions, every object in the solar system has its own orbit.

Every now and then, though, a planet’s orbit will cross the orbit of another object. We cross orbits at one point in space, and share – just for a moment – the same “address.”

Space is big … really big. The odds of being in the same place at the same time … the odds of sharing one address at the same time … are really small. Space is big enough for all of us …

… most of the time. The odds may be small, but they are not zero. Every now and then, we do share an address.

Earth crossed orbits with one particular asteroid – a rock maybe six miles across – for millions, maybe billions of years, yet we never showed up at the crossing point – at that one address – at the same time … until one day, sixty-six million years ago.

Until that day, for hundreds of millions of years, dinosaurs had roamed the world. And then … sixty-six million years ago … Earth and asteroid showed up at the same address … same place, same time.

The smash-up drilled a humongous crater into what is now Mexico … melted and vaporized gazillions of tons of rock … ignited forests everywhere. The energy of bazillions of nuclear bombs did major damage to the environment, and thus erased dinosaurs from the surface of the Earth.

Asteroids are made of rock and metal, but comets are made mostly of ice. Ice is not as hefty as rock … but a ball of ice twenty-something miles across – comet Swift-Tuttle – would carry a considerable punch … especially, coming at us at thirty-seven miles per … not thirty-seven miles per hour, but thirty-seven miles per … not thirty-seven miles per minute, but … relative to each other, Earth and comet are moving thirty-seven miles per second … which translates to over one-hundred thirty thousand miles an hour.

We’ll cross orbits with Swift-Tuttle next Wednesday night (as we do, mid-August, every year). Fortunately, the comet is elsewhere in its orbit, nowhere in sight. But that doesn’t mean it’s left no trace.

Frozen into their ice, comets carry tons of dust … dust that they shed, whenever they get close to the sun. This dust, pushed outward by sunlight, forms the comet’s “tail.” The dust wafts off the comet, and continues to circle the sun, following the comet’s orbit. Having circled many, many times, the comet has turned its orbit into a donut of dust.

Every time Earth crosses the comet’s orbit, we encounter that donut of dust. Hitting our atmosphere at a hundred-thirty-plus thousand miles an hour, the dust does some damage to the air … it makes the air glow, and we see a meteor shower.

The Perseid meteor shower (named for the constellation Perseus, from whose direction the meteors will fall) will be visible, Wednesday night, as soon as the sky grows dark … though the best to watch any meteor shower is after midnight.

Don’t expect fireworks – a good shower (the Perseid shower is usually the best of the year) might provide a few meteors, one after another … and then nothing, for some minutes. Not unlike fishing, the major point of watching a meteor shower is to enjoy the surroundings – the night sky. If you happen to catch some nice meteors, that’s great. And, if you’re really lucky, you might catch a fireball … an especially large bit of dust, streaking thru the atmosphere, making the air glow bright, maybe in color.

The Perseid meteor shower peaks – as we pass through the center of the dusty donut – this Wednesday night, as soon as the sky grows dark … and should be best, after midnight.

One more thing: Tonight, Saturday, Aug. 7, local astronomers will set up scopes and invite the public to view the stars, where the Old Downieville Hwy enters SR49, beginning 9 p.m.. It’s free, and fun for all ages. Bring a sweater. We’ll likely see a few early Perseids, as Earth passes through the outer edge of the dusty donut.

Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM.  He teaches classes for both kids and grown-ups, and can be reached at

Several years ago, the European Rosetta spacecraft visited a comet — a large lump of dusty ice. Notice the hazy dust, leaving the ice.
Photo credit ESA, Rosetta, NAVCAM

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