Six Planets and the Moon

Week of August 14-20, 2016

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

Stargazers who understand the workings of our cosmos can find it on most nights, but this week even the average sky watcher will spot it after dark, even 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 in their orbits.

Our Solar system includes planets, the sun, moon, comets, asteroids and tons of other stuff like dust and chunks of ice and rock. Because of the way our planetary family 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 a wide 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 is 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 grouping and the celestial bodies that seemed to wander through them.

Normally, the location of the ecliptic isn't obvious to anyone but the astronomically savvy, but this week anyone stepping outdoors at dusk will be able to trace it across the heavens. In fact, even beginning stargazers will be able to see six planets and the moon!

During dusk -- if you have a low horizon to the west -- cast your gaze in that direction. There, you'll find the bright planet Venus, and just above it will appear the fainter worlds Jupiter and Mercury. Follow the approximate arc of these three worlds toward the east, and when you reach the southern sky, you'll encounter the red planet Mars, along with Saturn.

You may notice that the moon also travels close to this path as it swings through our evening sky this week. This should not surprise you, for 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, Aug. 14, you'll notice that the moon lies east of Mars and Saturn, and it will drift increasingly farther to the east as the week goes on. By Wednesday night, Aug. 17, the moon will have reached its full phase and will rise in the east at sunset. The arc of the ecliptic will then stretch completely across the heavens.

So there you have it -- five planets and the moon. But wait. Where's the sixth planet I mentioned earlier?

Think about it!