What is earthshine?
If you look closely at a crescent moon just after dark (for a waxing crescent) or just before dawn (for a waning crescent), you’ll notice the unlit part of the moon. While the brightly lit portion receives direct sunlight, the unlit part is illuminated by earthshine—light reflected off Earth.
Earthshine is the dim glow visible on the unlit part of a crescent moon, caused by sunlight reflected from Earth.
Why is earthshine visible when the moon is a crescent?
Just as a full moon can illuminate the night side of Earth, a full Earth can illuminate the night side (the unlit part) of the moon.
When we look from Earth, we see the phases of the moon. Similarly, if we were on the moon, we would see the phases of Earth. However, the phases of the moon and Earth are opposite to each other.
So, when the moon appears as a slim crescent in our sky, a full Earth appears in the moon’s night sky. This full Earth is bright enough to faintly illuminate the night side (the unlit part) of the moon.
When to see the next earthshine in 2026?
As the new moon occurs on October 10, 2026, the next opportunity to see earthshine will be from October 6 to 8, 2026, on the unlit part of the waning crescent moon. Look for it in the eastern sky before dawn.
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