By the Light of the Moon

Mercury, November/December 2004 Table of Contents

Courtesy of NASA

by Arthur Upgren

Moonlight varies over a greater range than the light from any other bright celestial object—planet, satellite, or star—in the entire sky. We all know how bright it seems in our minds, but it is not so commonly known that even at the phase we call full, the variations are extensive.

In our modern world, where gasoline stations raise the light level above their pumps ever higher and street lights blanch the eyes of motorists with direct glare, it is useful to know just how bright is our only luminous natural nocturnal light source, the Moon. Nothing quite like it can be found anywhere in the sky; the Sun, at some 400,000 times its brightness, changes night into day—no basis for comparison there. And no planet or star, not even the dazzling Venus, can light up the whole night sky.

Today the reasons for a full understanding of the brightness of the Moon, particularly the full Moon, have taken on a new level of importance. Recent research on the influence of night brightness upon the human pineal gland near the brain, reveals a marked effect on its secretion of melatonin and its regulation of biorhythms, possibly affecting the incidence of cancer. Like most other creatures, the human being is affected by the diurnal circadian rhythm of light and dark more than was previously recognized, although anyone exposed to jet lag gets the idea that these rhythms are not disrupted without side effects. The level of light that disturbs this gland and its secretions is not yet accurately known, but indications are that this level is not much brighter than the full Moon.

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