Alfred Lord Tennyson was no keen supporter of equality for women. "Nature made them blinder motions, bounded in a shallower brain," he tells us in his poem Locksley Hall. Mind you, the young protagonist was miffed, having been jilted by his cousin Amy, but he went on to reinforce his point by simile: Woman is the lesser man, and all thy passions matched with mine, Are as moonlight unto sunlight and as water unto wine.
Leaving aside the truth or otherwise of the Poet Laureate's assertions, how meaningful are his celestial comparisons?
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; the illumination falls off rapidly as the angle of reflection becomes more oblique, as in the case of the crescent or the quarter moons.
Moreover, although moonlight may appear very bright indeed on a clear night with a full moon, it has been calculated that even at best its light is only about 1/500,000th of that of the sun on a clear day. Based on this figure, it seems that if the entire sky were packed with full moons, limb to limb, they would give only about one-sixth of the light we normally get from the sun . So there you have a measure of the comparative passions of young Alfred and his donna mobile. Young lovers might find further material for baleful metaphor in earthshine. Soon after the new moon, when it is seen as a thin crescent, the remainder of the moon's surface is very faintly visible on a clear night. This faint illumination is caused by sunlight reflected from the surface of the Earth, and is called "earthshine". It is only noticeable when there is so little of the sunlit portion visible that its light does not overwhelm the much dimmer earthshine.
In recent times scientists have begun to think of earthshine as a tool to study global warming. Since it is a consequence of light reflected from the Earth, its intensity is a measure of the Earth's albedo (its reflectivity) which in turn has a bearing on the Earth's capacity to absorb incoming solar radiation.
Measurements of the intensity of earthshine have been available since about 1925, and by comparing today's earthshine with that of many years ago, scientists hope to be able to quantify any change that may have taken place in the Earth's albedo in the intervening years.