Tuesday, May 11, 2004

Don't Take the Moon for Granted

Don't Take the Moon for Granted
by Dave Adalian

The Moon may be the most taken for granted object in the night sky.

It just sits there. Always has, always will.

Or will it?

Actually, the Moon is slowly moving away from Earth, but no need to panic. The rate of creep is just over one and a half inches a year. Since the Moon has an average distance from Earth of about 239,000 miles, it takes lots of years before those inch-and-a-halves make any difference.

Nice to know Luna’s not just sitting there.

It’s really pretty lively. The Moon moves closer and further from the Earth each month because its orbit is not perfectly round. These extremes--called apogee at the furthest and perigee at the closest--are noticeable in the Moon’s size and brightness, especially in photographs.

It’s also nice to know the Moon will never break free.

The complicated ballet of physical forces caused by the Moon’s orbit around the Earth is also making Earth turn more slowly as the Moon moves away. A similar effect between the Earth and Sun will eventually force the Moon back toward Earth.

If left alone long enough, the Moon would eventually move too close and be destroyed, however, the Sun will expand into a red giant star, swallowing Earth and Moon long before that can happen.

Earth hasn’t always had a moon. When Earth was very young, some four billion or more years ago, an object about half its current size struck it. According to this account, the Moon formed from the debris.

The Moon, at 2,000 miles wide, is by far the largest satellite in the Solar System compared to its parent body. Because of this similarity in size, Earth and the Moon are considered a two-planet system.

Research suggests the Moon was once much closer to the Earth and appeared much larger in the sky. Since then, it has drifted to its current spot and its spin has slowed so much the Moon turns only once every time it moves around the Earth and we see only a single side. The other remains forever hidden to the earthbound.

The most obvious change in the Moon are its phases, which are listed each day in this paper. The phases are a result of the changing relative positions of Earth, Moon and Sun. Try looking in on Luna every night through May and into June, witnessing her transformation as it happens.


This column originally appeared in the Visalia (Calif.) Times-Delta on May 11, 2004.