When the Moon is a thin crescent, why can see the rest of the Moon dimly lit.

The phenomenon is called Earthshine. This event is present anytime the moon is not a Full Moon, but is most noticeable when the Moon is a thin crescent, or what might be termed a “fingernail moon”.

This event can be seen at First and Third Quarter when the Moon appears to be “half full”. It can also be observed in the Gibbous phases, when the Moon looks like it is about three-fourth full. But again, it is most noticeable at crescent phase.

When you are looking at the thin crescent Moon in the evening, the Sun is located in the general direction you are facing, but below the horizon. Light from the Sun strikes the Earth, and some of the light reflects from the surface of the Earth, strikes the Moon, and reflects back to our eye.

So the side of the Moon that is dimly lit is seen by light from the Sun that has reflected off the Earth. That thin sliver of the Moon is much brighter because it is lit directly by sunlight.  The dimmer portion is lit indirectly by light that has first hit the Earth. The Moon, of course, does not generate any light of its own.

As the Earth travels around the Sun, the tilt of the Earth on its axis points toward the Sun in summer and away from the Sun in the winter. This is what gives us our seasons.

But this tilt also changes how we see the Moon travel across the night sky. In the winter, the Moon sets north of west and moves almost straight down to the horizon. When this happens, the Moon seems to be lit from the bottom and you see a “U” shaped Moon. People have described this as the “horns” of the Moon that point upward.

During the summer months, the Moon sets south of west and follows a more slanted path down to the horizon. The Moon appears to be lit more on the side, and not the bottom.

Albedo is a term used to describe the reflectivity of a surface such as the Moon or planet. A perfect reflection from a white surface is rated as a 1. No reflection from a perfectly black body yields an albedo of  0.

The Moon’s average albedo is .12.  The Earth’s average albedo is .3. The albedo for Venus is .75. Venus has those dense, carbon dioxide clouds made up of sulfuric acid. Those clouds reflect a lot of light.

The greatest  albedo, the highest reflectivity,  in our solar system is a moon of Saturn, called Enceladus. It has an albedo of .99, meaning that it reflects almost all the light that hits it.

Enceladus is one big ice ball and may have liquid water under its surface. This moon of Saturn was discovered by astronomer William Herschel in 1789. This was the same year that America held its first presidential election. George Washington was elected on January 7 and sworn into office on April 30.




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