What is Planetshine?

Earthshine blankets the dark side of the moon in a gentle glow.
Earthshine blankets the dark side of the moon in a gentle glow.

Planetshine refers to the lighting up of the dark side of a planet’s moon or moons by reflected light from a planet. This light comes from the reflected light of the sun by the planet and its contents. A well-known example of planetshine is called earthshine. This phenomenon, earthshine, is visible from the surface of the earth provided the moon is new or almost new. When earthshine happens, the dark side of the moon, which is the side facing away from the sun, is blanketed by a gentle glow. The phenomenon has also been observed in other places in the solar system and is used by astronomers to take images of the sides of Saturn’s moons even when they are not facing the sun. An example when planetshine was used to image Saturn’s moons was during the Cassini-Huygens mission led by NASA, Italian Space Agency, and the European Space Agency.

What Is Earthshine?

Earthshine is all the sunlight that is reflected by the surface of the earth and its clouds which in turn bathes the other side not facing the side with a faint glow. The light is not bright. Sometimes, the term earthlight is used in place of earthshine. To observe this beautiful phenomenon, one has to be observant during the periods close to the new moon; before, during, and after. During these times, the moon is shaped like a crescent. During these nights, the dark side of the moon is visible as a faintly glowing part that completes the orb. The dark side of the moon is more dim compared to the crescent moon.

Earthlight is not without its uses. Astronomers use this time to measure the present albedo of the planet earth. In turn, the information is used in the analysis of the climate by determining the cloud cover in the atmosphere and how their quantity varies with time. Water bodies such as oceans reflect the minimum amount of light (10%) when compared to land (10-25%), and clouds (50%).

Earthshine should not be confused with ringshine. When the rings of a planet reflect the light from the sun onto the planet itself or its moons, then that phenomenon is called ringshine.

Effect of Retroflection on Planetshine

Celestial bodies such as the moon, the sun, and others have the above property. When they are hit by light, they will reflect this light back to the source. Just like a mirror which reflects light in a straight line back to the source, so does the moon, sun, and other bodies. The light is not sprayed back in all directions. The moon reflects light from the sun. The nearer to being full the moon is, the higher the amount of reflection. During the full moon, the moon reflects light back to the sun. The sun is roughly in the same direction as the earth. In turn, the moon appears brighter when it is full. The same thing happens during earthshine. While the moon is full, retroflection is also increased. Consequently, earthshine also appears much brighter because of the increased size of the moon.


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