What size is the moon, really?

Sunday, Oct. 4, at 2:10 a.m. MDT is when the moon will be officially a full moon. Since this is the full moon closest to the Autumnal Equinox, it will be a Harvest Moon.

A Harvest Moon is unique in that the angle of the moon's path is almost parallel to the horizon. Normally the moon will rise about 50 minutes later each evening. However, because of the smaller angle in the moon's path, a Harvest Moon will rise only 20 to 25 minutes later each evening for three nights.

Before the days of mechanized farming and artificial lighting, this extra light from the full moon meant farmers could be in their fields longer bringing in the fall crops, hence the name "Harvest Moon."

As with most full moons, the Harvest Moon will appear much larger when rising than when overhead. This is the result of an optical illusion for which, so far, no one has really been able to provide a satisfactory explanation.

Some say it is because the moon is so low to the horizon that we are comparing it to foreground objects like houses and trees. Others propose that since it is so low we are looking through so much more of Earth's atmosphere and that air acts like a lens and magnifies the moon. Neither of those is a really satisfactory answer.

In reality the moon is the same size when it rises as when it is overhead even though it won't look like it. That is where the optical illusion comes in. It only looks like it is bigger.

There are a several ways this sameness in size can be demonstrated. One is to hold up your little finger at arms length. Since your pinky finger is about one-half degree wide, which is the same as the width of the moon, the tip of your little finger will cover the moon.

If you do this when the moon rises and again when it is high overhead, you will see they are the same size. Another way is to hold a dime up at arms length and compare it to the moon. Do it again when the moon is overhead and the sameness in size will be demonstrated.

A third way is one I only recommend for the physically fit among us. Bend over at the waist and look at the rising moon from between your legs, it will suddenly appear smaller and more regular in size. I know if I tried that, I would probably hurt myself.

If you do want to try this last method, find something to recline on and hang your head over the end. That sounds safer to me.

SKY WATCH: Full moon Sunday, Oct. 4. That full moon will wipe out most early evening observing. Oct. 6-7 the moon will be playing tag with the Pleiades star cluster in Taurus, the Bull. Look at about 10 p.m. MDT. In the early morning hours of Oct. 8, Mercury and Saturn will be in an extremely close conjunction an hour before sunrise. Saturn is currently on the far side of the sun from Earth. The pair will be about five degrees, about the width of a binocular field of view, below bright Venus. Mercury will fall toward the horizon over the next few mornings but Venus and Saturn will move closer together toward an early morning meeting on Oct. 12-13. On the 11th and 12th a slender crescent moon will bracket reddish Mars and wander into the Beehive star cluster.

NEXT TIME: More astronomical blathering.