"The Moon And Earthshine"

What is Earthshine

This is a smoky glow on the dark portion of the crescent Moon. It is caused by sunlight that reflects off the Earth onto the Moon's night side. Under the earthshine, the Moon's outline and its dark features can be seen, even though only a thin crescent is bright. We see the Moon because of reflected sunlight (the Moon does not generate its own light). At times, however, the dark part of the Moon glows.

Just after the new moon, the first crescent moon produces what we refer to as the "old moon" in the new moon's arms. This occurs a few days after the new moon just after sunset. The same things happens to the old cresent moon in early morning.

Earthshine - A Double Reflection of Sunlight.
Light from the Sun is reflected off of the Earth (See Point A) onto the Moon (See Point B). Some of this light is then reflected off of the Moon back towards the Earth (See Point C). Because of this we see part of the Moon illuminated by the Sun and the rest of the Moon dimly illuminated by this doubly reflected light (which we call earthshine). Each time light reflects off a surface (like a planet), it gets dimmer because some of the light is absorbed by the reflecting surface. This means that earthshine is dimmer than moonlight because earthshine is sunlight that has been reflected twice and moonlight is sunlight that has been reflected only once (off the surface of the moon). In addition, the reflectivity of the moon (its "albedo") is less than that of the Earth, which makes earthshine even dimmer. Earthshine example

Earthshine - An Animated View.
The light of the sun not only reflects off the face of the moon,
it also reflects off the Earth's surface. The reflected sunlight
is referred to as earthshine.

During a crescent moon, only a sliver of sunlight is reflected to Earth
(Part 1 in this animation). But when the Moon is a crescent, the rest of
it is illuminated by sunlight reflecting off the Earth, hitting the night
side of the Moon, and bouncing back to Earth (Part 2). In fact, if you
were on the Moon when it was a crescent, you'd see a full Earth.


The moonlight of a full moon is so bright that even if it were positioned correctly to reflect light back toward Earth, it would completely overpower earthshine. In fact, we only see earthshine during the crescent phases of the Moon because during these phases only a small part of the Moon is lit up, and moonlight doesn't outshine earthshine, and the moon is in the right position to reflect light back to the Earth.

Copyright © 2001 By Keith Cooley     - eMail:  krcool@hiwaay.net  - Return To: Keith's Moon Page