What is Earthshine?
This is a dim glow which lights up the part of the Moon without light because of the reflection of the Sun's light off the surface of the Earth and back onto the Moon.
Why does it happen?
When there's a reflection of sunlight off the Earth's surface and the dark portion of the Moon's surface lights up, Earthshine occurs.
Given that the light that produces earthshine reflects twice - first, off the surface of the Earth and secondly off the surface of the Moon, this light is not as bright as the part of the Moon that is lit.
When this phenomenon happens on other planets' moons, it is called planetshine.
Ability to Reflect Sunlight
The brightness of the Earthshine is also affected by the albedo of the Moon. Albedo is a measure of the amount of sunlight an extraterrestrial object is capable of reflecting. It is measured on a scale ranging from zero to one. An object that has zero albedo is completely dark and does not reflect sunlight. An object that is from out of space having an albedo of one reproduces every Sun rays that reaches it.
On the average, the Earth has an albedo of 0.3, while the Moon's albedo is 0.12. This simply means that only 12% of the sunlight that gets to the Moon is reflected. On the other hand, the Earth mirrors up to 30% of the total sunlight that reaches it. For this reason, when you view the Earth from the Moon, it would seem about a 100 times brighter than a full Moon that is viewed from the Earth.
Best Times to See Earthshine
Earthshine is most apparent 1 - 5 days before and after a new Moon. The best time of the day to see it is after sunset or before sunrise.
Sometimes, it is described as the old Moon in the arms of the new Moon, 'ashen glow', or the glow of Da Vinci, after Leonardo da Vinci, who was the first person in recorded history to explain the phenomenon.
According to scientists studying global warming, the Earthshine is more intense in the months of April and May.