The moon shines only by reflected sunlight. It is at its brightest at the time of the full moon, not only because the whole hemisphere is illuminated, but also because the proportion of light reflected is greatest when the sunlight falls upon it perpendicularly.
Even then, however, it has been calculated that at best its light is only about 1/500,000th of that of the sun on a clear day. And the illumination falls off rapidly as the angle of reflection becomes more oblique, as in the case of the crescent or the quarter moons.
At this end of the lunar cycle, when the moon at first glance is only visible as a thin crescent, the outline of the remainder of the moon's surface can be seen dimly on a clear night. It was to this, for example, that the sailor in The Ballad of Sir Patrick Spens was referring when he told Sir Patrick of his premonitions:
Late, late yestreen I saw the new moone,
Wi' the auld moone in hir armes; And I feir, I feir, my master deir,
That we will come to harm. This faint illumination, called "earthshine", is caused by sunlight reflected from the surface of the Earth. Traditionally, as in Sir Patrick's case, it has been of mainly superstitious interest, but in recent times scientists have begun to think of it as a tool to study global warming.
One of the factors that control the average temperature of the Earth is its albedo, the amount of incoming solar radiation that is reflected back to space: obviously the more energy the Earth reflects, the less is absorbed, and the less its temperature will be inclined to rise.
Nowadays the Earth's albedo can be measured using satellites, but it would be useful to have information about how it may have changed in the last, say, 60 years, since a time long before the era of the satellite.
As it happens, for no particular reason that we know of, a French astronomer called Andre Danjon, using a specially adapted telescope, took monthly readings of the intensity of earthshine for nearly 30 years in the first half of this century.
But his readings may yet serve a purpose. By duplicating Danjon's experiment, and comparing the intensity of today's earthshine with that of many years ago, scientists hope to be able to quantify any systematic change that may have taken place in the intervening period.
And since earthshine is a consequence of light reflected from the Earth, any change discovered in its intensity will be a measure of changes in the Earth's albedo.