Ups and Downs

By Bob Berman|Wednesday, May 01, 1996
Our solar system is almost flat, so the Earth, moon, and other planets orbit the sun in pretty much the same plane. But the orbits of the moon and planets aren’t perfectly aligned, and this month the variations caused by that lack of alignment brew up some noteworthy sky antics you won’t soon see again.

The plane of Earth’s orbit around the sun is called the ecliptic. If the orbits of the moon and the other planets weren’t tilted to varying degrees, they too would glide on the ecliptic as they move through our sky. But instead our solar neighbors ride on, above, or below our plane at different points in their paths.

The easiest way to note the effect is by watching the moon rise and fall like a carousel horse each month as it follows its own tilted orbit. The moon starts this month high above the Earth-sun plane, crosses it on the 13th, and then dips down below. By the 19th the moon is almost at its maximum distance south of the ecliptic, lying so low in the sky--5 degrees below the ecliptic--that it’s beneath the zodiacal constellations. And that’s when it meets Venus, the evening star.

The two light up the western sky and share the same constellation (Taurus), but they don’t brush side by side. Because they are at opposing points in their tilts--the moon far beneath the ecliptic just as Venus reaches its highest perch of the century, some 4 degrees above--the two miss each other by a whopping 18 moon diameters (about the width of a clenched fist held at arm’s length). It’s a conjunction worthy of a refund.

But keep your eye on the moon. By month’s end it will again have soared above the ecliptic, and on June 3 it floats above Jupiter in the southeast. The giant planet serves as a superb marker since it now sits precisely on the ecliptic.

We’ve watched the moon’s tilted orbit send it bobbing above and below the ecliptic. But the moon’s orbit itself changes orientation, like a dropped plate wobbling around on the floor. That wobble (due mostly to the gravitational pull of Earth’s equatorial bulge) completes one revolution in 18.6 years and also plays tricks on our view of the moon.

Currently the tilt of the moon’s orbit is opposite the tilt of Earth’s axis--which means that the Northern Hemisphere tilts away from the moon when the moon is highest above the ecliptic, and toward the moon when the moon is farthest below it. This moderating effect boosts the normally low full moons of late spring and early summer (look at the full moon on May 2), and drops the usually high full moons of winter. After next year, though, the orbit begins to roll around toward the opposite orientation, until in 2005 it will align itself with Earth’s tilt, with dramatic consequences.

At that point the Northern Hemisphere will lean in toward the moon when the moon is highest above the ecliptic, making winter moons appear much higher in the sky. Full moons in December, always the year’s highest, will rise 22 moon diameters above this past winter’s. The moon will even hover straight overhead from southern states. And those always low full moons of May and June will never climb more than 20 degrees above the horizon as seen from most of the country and will simply not rise from northern Europe and Canada. Imagine: A moon that doesn’t rise at all!

But if talking about orbital tilts makes your brain tilt, just step outside and observe the moon, Venus, and Jupiter as they demonstrate that the heavens, too, have their ups and downs.
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