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Mercury, January/February 1998 Table of Contents

Michael C. LoPresto
Henry Ford Community College

Planetary alignments let you see for yourself what those textbook diagrams of the solar system are trying to convey.

Every once in a while, the planets line up in the sky in such a way that makes it easy to visualize how Earth fits into the solar system. This happened last July 29, when my good friend Matt Linke, director of the University of Michigan's Exhibit Museum planetarium, and I had the type of observing session we usually only dream about.

What was most exciting was the opportunity to observe all the inner planets (if you include the one we were standing on) at once - and then, later in the evening, the outer planets. The inner planets were near the western horizon shortly after sunset (see Figure 1). What really impressed me was how easy it was to form a mental picture of them in their orbits around the Sun, just like the usual bird-eye's view (see Figure 2).

At about 1 a.m. we saw the outer planets in the eastern sky (see Figure 3). Finding Uranus and Neptune required Linke's 8-inch telescope. Uranus revealed itself easily as a small blue-green disc. We found Neptune easily too, although it took some comparing to a print-out from Carina Sofware's Voyager II to convince us of our success. Jupiter's orange disk with its bands of clouds and moons, and yellow Saturn with its rings, were their usual engaging selves. By looking at Jupiter and Saturn and imagining Uranus and Neptune where we had observed them, we could again picture Earth in orbit around the Sun, which was by then behind us (see Figure 4).

When observing the sky, where Earth fits into the big picture is not always so obvious, so it is important to appreciate such opportunities. Last month, all the planets were lined up in the early evening so they could be seen together - not at two different times, but all at once. On June 5 at sunrise, all five visible planets will be spread out from the eastern horizon. Although Mercury and Mars will be lost in the Sun's glare, Venus will be higher up, with Saturn above and to its right and Jupiter toward the southwest. Uranus and Neptune, although not visible with the naked eye, will be due south and even higher; Pluto will be near the western horizon. Another spectacular event will occur on April 23 before dawn, when the Moon, Venus, and Jupiter will be nearly on top of one another.

That said, you needn't wait for the next conjunction or near-alignment to visualize the solar system. Anytime you observe the Moon or planets, try to relate what you see to a plan view of the solar system; the diagrams in Guy Ottewell's annual Astronomical Calendar are particularly helpful. Such observing will give you a visceral sense that the cosmos is not something "out there," but something we are part of.

MICHAEL C. LOPRESTO teaches physics and astronomy and is the planetarium director at Henry Ford Community College in Dearborn, Mich. He and Linke live in Saline, Mich., with its dark skies. LoPresto's email address is mloprest@mail.henryford.cc.mi.us; his home page is at http://sciweb.henryford.cc.mi.us.