![]() ![]() Full moon – The Sun, the Earth and the Moon are all in a line, with the Earth between the sun and the moon.Phase angle – The angle between the Sun-moon line and the Earth-moon line.What’s wrong with this picture? Is the Moon not a diffuse reflector? Apparently not! The moon would look brightest at the center, gradually becoming dimmer at the edge of the disc. radiance) should likewise fall off as a cosine from the center of the “disc” towards the edge. In fact, it falls off as a cosine function. However, we know that it is spherical, and the irradiance (power per unit area) from the sun falls off as we go from the surface directly facing towards the sun around to the terminator. That would be OK if indeed it were a flat disc. The full moon looks sorta like a flat, uniformly illuminated disc. Here is some of the stuff I have come up with. So I have been making measurements for the past several years to attempt to determine the amount of this bizarre behavior. ![]() I have not yet found a good model to determine how much brighter. We can’t see full opposition, because we get in the way, casting our big ol’ shadow on the Moon (a lunar eclipse).Īnother important component is shadow-hiding, where shadows cast by finite objects are not seen at opposition. The closer we get to the line of sight between the Sun and the Moon, the brighter the Moon appears. If the material has a relatively high index of refraction and the particles are sufficiently small that they are partially transparent, then they act like cats eyes, and reflect a disproportionate amount of radiant power back to the original source, our Sun. When a meteorite strikes the lunar surface with sufficient energy, the ejecta will melt, and then harden into quasi-spherical glassy particles before landing on the surface. We also are pretty sure of the causes a fraction of the lunar dust covering the surface came from ejecta from meteorite collisions. We know the reason, it’s the so-called opposition effect, also known as retroreflection. If the moon were a perfectly diffuse (i.e., lambertian) reflector, the ratio of apparent intensity of the full moon to the quarter moon would be pi, but in reality the ratio is more like 10. In addition, some full moons are quite a bit brighter than others. Specifically, why is it so much brighter at full moon than on the one or two adjacent evenings waxing or waning? The Moon still appears full, but is nowhere near as bright. I have a long-term interest in our nearest neighbor, the Moon. ![]()
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