What Is Earthshine?
Earthshine is a dull glow which lights up the unlit part of the Moon because the Sun’s light reflects off the Earth's surface and back onto the Moon.
It is also sometimes called ashen glow, the old Moon in the new Moon's arms, or the Da Vinci glow, after Leonardo da Vinci, who explained the phenomenon for the first time in recorded history.
Best Times to See Earthshine
In 2021, the best time to see earthshine is a few days before and after the New Moons on April 11/12 and May 11/12.
Why Does it Happen?
Earthshine occurs when sunlight reflects off the Earth's surface and illuminates the unlit portion of the Moon’s surface.
Since the light that generates earthshine is reflected twice – once off the Earth’s surface and then off the Moon’s surface, this light is much dimmer than the lit portion of the Moon.
This phenomenon is called planetshine when it occurs on other planets' moons.
Ability to Reflect Sunlight
Earthshine's brightness is also affected by the Moon's albedo. Albedo is a measurement of how much sunlight a celestial object can reflect. It is measured on a scale, which ranges from 0 to 1. An object that has albedo of 0 does not reflect sunlight and is perfectly dark. A celestial object with an albedo of 1 reflects all of the Sun's rays that reach it.
The Moon has an average albedo of 0.12, while the Earth's average albedo is 0.3. This means that the Moon reflects about 12% of the sunlight that reaches it. The Earth on the other hand, reflects about 30% of all the sunlight that hit its surface. Because of this, the Earth, when seen from the Moon would look about a 100 times brighter than a full Moon that is seen from the Earth.