A lunar eclipse happens at least twice a year, whenever Earth's shadow falls on the moon. Interestingly, the moon is always full during eclipses. When the moon passes through the Earth's shadow (the umbra is the darkest part and the lightest part of the shadow is the prenumbra) the moon will begin to glow an eerie red color, and at full eclipse, the moon will appear to be surrounded by a glowing ring, called an annulus. The color of the shadow and the ring around the moon is caused by the amount of dust in the atmosphere. Bet you didn't know that! Now you do.
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