Copyright ©2003 by Paul Niquette. All
...but it's farther away.
-- Paul Niquette, "Measuring the Moon"
do we begin our analysis of the
an idle non sequitur. As you know, the earth is
about 25,000 miles
in circumference, 8,000 miles in diameter. Taking
the word "size"
to mean surface area, we estimate that the moon must be
about 2,000 miles
in diameter. Space-age measurements, in fact, have
the moon is actually 2,162 miles in diameter and it is
indeed 238,900 miles
A circle with a radius of 238,900 miles would have a diameter of 477,800 miles and a circumference of 1,501,053 miles ( times the diameter). That's the distance the moon travels in one 29-day orbit around the earth -- 694 times its own diameter for 360o. The diameter of the moon itself, therefore, corresponds to 360/694 degrees. That's 0.52o as viewed from the surface of the earth.
Meanwhile, a penny measures 3/4th of an inch in diameter. A circle made of $6.94 worth of pennies would measure 520 inches in circumference. Viewed from the center, each penny subtends an angle of 0.52 degress, just like the moon as viewed from the surface of the earth. A circle of 520 inches has a radius of 6 feet 11 inches. So the solution for the puzzle is...
Sophisticated solvers know that this is a mere illusion -- that, if anything, the moon ought to appear slightly larger overhead, since it is nearly 4,000 miles closer. In fact, an objective measurement of the angle subtended by the moon at midnight will confirm that it is nearly 3% larger than it is at moon-rise (0.5273o degrees vs 0.5185o).
The pictures above illustrate what is known as the The Moon Illusion.