Week of May 11-17, 2014

One of the coolest sights of the nighttime sky is one that's completely invisible ... unless, of course, you know just where to look.

Stargazers who know the workings of our cosmos can find it on most nights. But this week even the average sky watcher may notice it after dark, though most will not realize what they're seeing.

I'm referring, of course, to the geometric plane of our solar system — the arc along which all of our celestial neighbors travel around the sun.

Our solar system includes the planets, the sun, moon, comets, asteroids and tons of other stuff like dust and chunks of ice and rock. Because of the way it collapsed and flattened as a result of the sun's birth some five billion years ago, nearly everything orbits our parent star along this geometric plane.

From within, we see this plane as an arc extending across our sky; it represents the path along which the planets journey in front of the more distant stars. Astronomers call it the "ecliptic" because it's along this path that the sun and moon also appear to travel and, therefore, the only locations in the sky where eclipses can occur.

The ancients recognized this arc as well but, of course, didn't understand its physical significance. They instead devised twelve stellar groupings (the zodiac) to mark its location, and assigned mystical properties to each and to the planets that seemed to wander through them.

Normally, the location of the ecliptic isn't obvious to anyone but the astronomically savvy. However, this week anyone stepping outdoors at dusk will be able to trace it across the heavens.

During dusk, cast your gaze low toward the west-northwestern sky. There you'll find the elusive planet Mercury, and just above it brilliant Jupiter. Connect these two with an imaginary line, and you've got the beginnings of the ecliptic. Now continue that arc eastward toward reddish-orange Mars and, low in the southeast, the ringed world Saturn.

You may notice that the moon lies close to this path as it swings through our evening sky this week. This should not surprise you; it, too, is part of our solar system and travels along roughly the same path. I say "roughly," because the moon's orbit is actually tipped by about five degrees to the plane of the ecliptic, so it does appear to stray slightly above or below it. And that's why we don't experience eclipses every time there is a new or full moon.

If you're out on Sunday night, May 11, you'll notice that the moon lies just south of Mars. By Tuesday, May 13, the moon will appear just above and to the right of Saturn and, on the following night will lie just below and to its left. And by the end of May, the entire cycle will begin anew, as the waxing crescent moon will appear to pass Jupiter low in the west.

Folks who glance only occasionally at the night sky will often interpret such a planetary "alignment" as something of supernatural significance. I'm not too worried though; if ever these worlds do not align, that's when I'll become concerned!

Visit Dennis Mammana at www.dennismammana.com. To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.