Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2021 December 1
$The featured image shows the recent partial eclipse of the Moon with the eclipsed part appearing red and a blue band due to refraction of sunlight through the Earth's atmosphere. Please see the explanation for more detailed information.$

A Blue-Banded Blood Moon
Image Credit: Angel Yu

Explanation: What causes a blue band to cross the Moon during a lunar eclipse? The blue band is real but usually quite hard to see. The featured HDR image of last week's lunar eclipse, however -- taken from Yancheng, China -- has been digitally processed to equalize the Moon's brightness and exaggerate the colors. The gray color of the bottom right is the Moon's natural color, directly illuminated by sunlight. The upper left part of the Moon is not directly lit by the Sun since it is being eclipsed -- it in the Earth's shadow. It is faintly lit, though, by sunlight that has passed deep through Earth's atmosphere. This part of the Moon is red -- and called a blood Moon -- for the same reason that Earth's sunsets are red: because air scatters away more blue light than red. The unusual blue band is different -- its color is created by sunlight that has passed high through Earth's atmosphere, where red light is better absorbed by ozone than blue. A total eclipse of the Sun will occur this weekend but, unfortunately, totality be visible only near the Earth's South Pole.

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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
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