Twilight Zone

Grab this once-a-year chance to picture the disk we spin on

By Bob Berman|Monday, March 01, 1999
New perspectives are rare. How often do we get a chance to see ourselves or our universe in a truly different way? How often do we get to observe something in space that we think can only be sketched or drawn, something like the planets in their orbits around the sun? This month the night sky offers us a chance to see exactly that.

March is the best time to view this because our part of Earth is tilted just right. Also, Earth's position in orbit opens up the whole solar system at an easy time to watch--within an hour after sunset. To understand what you'll see, imagine Earth spinning around the sun along with the other planets on a flat disk like a Frisbee, with the sun at the center. All the planets, except Pluto, orbit on that same flat plane (within 7 degrees). This means that once a year we can look into the sky, find the planets, and easily note that they form an imaginary straight line, the edge of our solar-system disk. Simply face west into the fading evening twilight and look up. The planets, at their highest upward angle from the horizon, are not hard to find.

For example, when sun-hugging Mercury swings to the edge of its orbit, as it does now, it is positioned directly above sunset. The same is true for the other planets--they'll all be above the setting sun. In autumn, by contrast, when Mercury again reaches its maximum separation from the sun, it will be low on the horizon, impossibly buried in our hazy atmosphere.

So during the next 30 days before the spring equinox, any planet visible to us in evening twilight will be higher above the horizon and more easily seen than at any other time of year. But the real fun will be looking for that disk the planets are orbiting on. First, find one or more planets and then use your imagination to line them up with the sun's fading position below the horizon (marked by twilight's glow). That line is the edge of the Frisbee we're all spinning on. You'll immediately be able to visualize the plane of the solar system. Its position will seem weird because it's vertical, at right angles to your normal plane of reference, the horizon. Old textbook drawings of the solar system's structure may rush back to memory, but you'll wonder why the drawings weren't turned on their sides. This edgewise perspective is exhilarating, enlarging our sense of position and place and reminding us once again that there is no up or down in space.

The quickest way to get started is to check the local newspaper for the exact time of sunset, then begin looking for planets 30 to 40 minutes later.

On February 23, Venus and Jupiter, the two brightest objects then in the heavens, almost touch--an awesome conjunction. Bring the kids out to see this one. Halfway between this dazzling duo and the horizon, pick out a solitary small starlike object--that will be Mercury.

From March 6 to 11, Jupiter will be lower and close to Mercury. Brighter Venus will be higher in the evening sky. Facing west, hold a clenched fist at arm's length and sight the horizon below it. Just above the fist, look for Mercury on the right and Jupiter on the left.

On March 18 and 19, Saturn will be the little "star" just to the left of brilliant Venus. By the end of March, Earth will have journeyed farther in its own orbit, leaving behind this once-a-year twilight zone of opportunity.
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