Surely you have view the phases of the moon. You know, growing, waning, full moon, new moon. In all of them what we see is the light reflected from our Sun on the Moon.

If what we see is the reflection of the light of the Sun on the Moon, thinking a little, we will realize that the Full Moon (we go, when it is whole) is when it receives “all light”. That is, the incident angle of the Sun’s rays and its surface is 90 degrees (average pi). That is to say, properly speaking, it is a 3-point system where one of them is the omni light emitter (that is, everywhere equally, a source as it is called physically), a point that receives light (point A) And another point where the observer (point B).

If we call the angle between the ray of light to the focus (F) on point A and the ray bounced from point A and point B, we call it “phase angle” (to call it somehow, so by chance) We have the so-called Seeliger effect.

The Seeliger effect is the one we have on an object when the focus (F) is behind the observer and therefore the phase angle is zero. This effect tells us that object A (which has to be a surface of a certain size, although for simplicity has said that it is a point) will shine reflecting the maximum even reaching to hide the shadow of point B. The amount of reflected light When approaching the point where the phase angle is zero is exponential (not linear, as expected) due to the sine of the angle, but, as we know from the sine graph, the point of the maximum or the point of approximation to the maximum at To be a continuous function, the approximation to the maxima has to be smooth, which, with the Seeliger effect, proves the opposite since the rapidity with which it grows or decays, the variation of the derivative of the function, does not fully coincide With the derivative of the sine function (which is the least cosine) being, the exponential variation.

And why is this? It mainly passes through the surface of the object/point A. If the object has a porous surface, the light that reaches the pores is not reflected as if it were a polished surface until the time at which the phase angle is zero is reached. For this reason the variation of the derivative is not of the sine function itself.

This effect can be observed in our Moon, on Mars or in the rings of Saturn composed by porous rocks (or powder).

The case is that explains why, when there is a full moon, it shines so brightly, and also, by the brightness, we do not see the shadow of the Earth in it. How nice!.