The first days after new moon, the moon is visible as a narrow crescent. Sometimes, however, behind the crescent you see the full moon (in black). Why is that and why is that not the case in the first fifteen minutes?

Answer

Dear Roel,

The explanation for this phenomenon, also known as “the old moon in the arms of the new moon”, is in fact simple: the whole side of the moon that is facing the Earth (that is, the side visible to us) is illuminated by sunlight reflecting off the earth. It is because of this “earthshine” that we can sometimes see even the part of the moon that is not directly illuminated by the sun. In the period when it is new moon from our point of view, it is “full earth” on the moon, and the received amount of reflected light is therefore maximum there. Combine this with the fact that near the new moon you have little interfering light from the crescent moon itself, and you understand why the earthshine is best visible during that period.

The phenomenon, first explained by Leonardo Da Vinci some 500 years ago, depends, among other things, on how strongly the moon-facing side of the Earth reflects sunlight. For example, the earthshine will be stronger when the side facing the moon contains many continents (which reflect better than water) or especially when it is very cloudy. It also occurs in other planet-satellite systems, and is sometimes even used in scientific research: for example, thanks to “planet shine”, the Cassini probe was able to examine Saturn’s moons even when not directly illuminated by the sun. were illuminated.