Sky watch: Gauging moon's distance

How far is the moon?

It's a simple question, and probably one we all learned to answer in elementary school. I remember reciting it in fifth grade: "The moon is 240,000 miles from Earth."

On the night of Oct. 11 and 12, the full moon will lie near "apogee," its farthest point from Earth. On that night, it will be the smallest-appearing full moon of 2011. The exact point of apogee occurs around 7:44 a.m. on the morning of Oct. 12, when the moon will lie 252,546 miles away.

The next time a full moon will experience "perigee" -- its closest point to Earth -- will be at 11:34 p.m. on May 5, 2012, when it will lie about 221,800 miles from us. This will appear as the largest full moon of that year.

So will you notice that the Oct. 11 full moon appears smaller than "normal"? Perhaps, but only if you're used to viewing the full moon regularly. It won't be obvious to average stargazers since it will appear less than 6 percent smaller than average. But we can see the difference by comparing two full moon photos -- one shot at apogee and another at perigee.

You can create such photos for yourself. Use a camera with a long telephoto lens (300-400 mm will do nicely), and set your camera to manual mode (M) and ISO 200; then, try 1/250 second at f/8 to start, but try some other exposures to make sure that something turns out well. Capture one image on Oct. 11 and another on May 5, 2012. By comparing them, you'll discover that this month's full moon appears about 12 percent larger than that of next May.Maybe not enough for beginning moon gazers to notice with just the eye, but more than enough for your photos to show quite a dramatic effect.