Sky Lights

The orbit of Earth's enigmatic satellite tilts, wobbles, and rolls like an egg

By Bob Berman|Sunday, June 05, 2005

The moon is the most recognizable object in the night sky, yet for most people its movements remain a mystery. Take the full moon that will occur on June 21, for example. It will ride strangely low in the sky—so low that it will be lost among the background of buildings and treetops for viewers in the northern United States. In Fairbanks, Alaska, the moon will not even clear the horizon. For everyone there, this will be a June without a full moon.

There is a method to this lunacy, of course. The moon’s place in the sky is determined by its orbit, which in turn is rigorously controlled by gravity acting primarily among the moon, Earth, and the sun. These seemingly simple elements combine to produce an oddly elaborate piece of celestial choreography.

The first surprise is that the moon does not orbit above Earth’s equator. Unlike every other major satellite of every other planet in our solar system, our moon ignores the axis of its parent planet and instead circles in nearly the same plane that Earth and the other planets orbit around the sun, offset by slightly over five degrees. This explains why the sun and moon take such similar paths across the sky.

The moon’s orbit also changes over time because of the endless interplay of solar, lunar, and terrestrial gravity. The egg-shaped lunar orbit keeps shifting counterclockwise as seen above the North Pole; an imaginary line drawn through the length of its orbit would swing completely around every 8.9 years. Meanwhile, the direction of the tilt of the orbit wobbles clockwise on an 18.6-year cycle. (If you find it hard to picture how a tilt can wobble, drop a pizza pan on the floor and watch how it moves. If it lands slightly on its side, it will clatter around for a while. This is an example of a flat plane changing orientation, just as the moon’s orbit does.)

Fortunately, the visible consequences of all these motions are relatively straightforward. During certain parts of the tilt cycle, the moon’s nighttime path exactly repeats the path the sun follows during the day. At other times, the moon is more moderate, never rising as high in the sky as the summer sun but never sinking as low as the winter sun. The 1990s were one such temperate lunar period.

From now through 2007 the moon is at the opposite end of the cycle: Its orbit tilts in the same direction as Earth’s axis but even more so, sending the moon both unusually high and low in the sky. Consequently, it will stray outside the zodiac—the zone where the sun and the major planets are always found—to spend time in such off-the-beaten-path constellations as Auriga the Charioteer, where it will be on June 8.

In the Northern Hemisphere, June’s full moons are always the year’s lowest because that is when they lie in the most southerly part of the moon’s course. This month the full moon lands exactly on the solstice, meaning it will be as far south as it can go. Toss in the present direction of the tilt of the lunar orbit and the result is the lowest full moon since 1987.

See for yourself early in the morning of June 22, around 1 a.m. This is when the moon will reach its highest point of the night. Yet in Boston it will climb only 19 degrees above the horizon, about one-fifth of the way to the zenith. In Juneau it will be up just 3 degrees. From Fairbanks the full moon will not rise at all. Because it will float so low, the full moon will shine through an abnormally thick expanse of atmosphere. The resulting absorption will tint it yellow or reddish, especially if the weather is humid.

With the moon at its southernmost point and the sun at its northernmost point, ocean tides at midlatitudes—where most Americans live—will be unusually strong. Amplifying the effect, the moon reaches perigee (the point at which it is closest to us) just 32 hours after it is full. People living by the seashore should prepare for unusually high water.

At least we have plenty of advance warning. The moon may seem mysterious, but it is reassuringly predictable, once you decode its tricks.

What’s up in the June sky
This is a good month for planet watching, as Venus and Mars return to prominence. Even Pluto makes its best appearance of the year, but you’ll need at least a six-inch telescope and a detailed star chart to locate this distant, miniature world.

June 7: Venus moves back into the evening sky; it shines next to the low, thin crescent moon.

June 14: Pluto sits at opposition, its closest approach of 2005, in the constellation Serpens.

June 21: Summer begins at 2:46 a.m. EDT. Earth’s North Pole is tipped maximally toward the sun.

June 25: Venus, Mercury, and Saturn gather together low in the west 40 minutes after sunset.

June 26: Venus and Mercury move so close together tonight that they may appear as a single star.

June 30: Mars, drawing nearer, now rivals the brightest stars in the sky. See it in the east after 2 a.m.

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