Sky Watch: Orbit shape spins moon closer

When the full moon rises around sunset on Saturday, May 5, it will appear much larger than usual.

Experienced sky watchers and regular readers of this column might know that a large-appearing moon is caused by an optical trick known as the "moon illusion." It's a startling effect, and most people swear that the rising moon must be immense.

Wait a minute, how can this be? The moon is a solid body; it can't change its actual size. True, but it can change its apparent size.

This effect occurs because the moon doesn't orbit Earth along a circular path, but moves instead along an ellipse that at times carries it closer to us and at times farther from us during its monthly cycle.

Fortunately, this change isn't much, because, without the moon's relatively constant gravitational pull, the tides might have never made it possible for life to emerge from the seas eons ago.

Astronomers who measure the moon's distance tell us that this is constantly changing. On the night of May 5/6, the full moon will lie at its nearest for the year -- only 221,800 miles from Earth. That's 30,870 miles -- or about 12 percent -- closer than on the night of November 27/28, its farthest approach to Earth this year.

detect this difference with the unaided eye. However, by comparing two full moon images -- one taken when the full moon is at its nearest (perigee) and another at its farthest (apogee) -- we can easily see a dramatic difference in its size.