How Does the Moon Shine?
Even though the Moon is the brightest object in the night sky, it does not make its own light. The moon shines by the light it reflects from the sun. The Moon only reflects about seven percent of the sunlight that reaches it. The rest is absorbed by the Moon’s surface. The sharp definition with which, on a clear night, we see the moon is due to the absence of any concealing veil of air or cloud on it.
Since there is no atmosphere on the moon to act as a buffer to the sun’s rays, temperatures there are extreme and sudden. The maximum is 212 °F. (100 °C), the temperature of boiling water, and the minimum is -292 °F. (-180 °C). Understandably, the moon’s hostile conditions cannot support life of any description.
The perceived brightness of the moon from Earth depends on where the moon is in its orbit around the planet. The moon travels once around Earth every 29.5 days, and during its journey, it’s lit from varying angles by the sun. This movement of the moon around the Earth — and the simultaneous orbiting of Earth around the sun — accounts for the moon’s different phases (full moon, quarter moon, etc.). At any given point in the moon’s trajectory around the Earth, only half of its surface is facing the sun, and therefore, only half of the moon is lit up. The other half of the surface faces away from the sun and is in shadow.
The moon is at its brightest when it is 180° away from the sun from our perspective. At this time, the full half of the moon’s surface facing the sun is illuminated and is visible from Earth. This is what’s known as a full moon. At “new moon,” on the other hand, the moon isn’t even visible from our vantage point. This is when the moon is between the sun and the Earth, so that the side of the moon reflecting sunlight is facing away from Earth.
In the days before and after a new moon, we’ll see a sliver of the moon reflecting sunlight. And during those times, the faint brightness of the rest of the moon — the part not brightly lit as a sliver — is a result of what scientists call “earthshine,” in which the moon’s relatively dark disk is slightly illuminated by sunlight that reflects off of Earth, then off the moon, and back to our eyes.