Blue-looking Moons
 Q:
What causes a moon that looks blue? A couple of years ago, we saw a crescent
that appeared to be attached to or just behind the moon for a distance of from
about "4:00 to 6:00". The night was very clear and we saw it in upper New York
state and in North Carolina?
(Dennis Kortyka, © 1998, used with permission) Earthshine on the Moon
A: "There are rare occasions when the disk of the Moon appears to take on a bluish tint," says the Royal
Observatory Greenwich (ROG). The observatory explains how tiny particles in the air can scatter the
moonlight and make the moon appear blue much in the way our sky appears blue. A violent volcanic
eruption or a big forest fire can fill the air with tiny particles that scatter moonlight.
There's a couple of problems with this explanation, though: you say the night was very clear and you saw
the crescent attached to or just behind the moon.
Another thought from ROG: Maybe your eyes were "fooled" by a bright red light, like the setting Sun or a
campfire. By contrast, the Moon would appear green. After the Sun's reddish light was gone or when the
campfire went out, the Moon should look white again. Of course, that's green, not blue.
"The description of the moon sounds very unusual," says Dr. Robert Massey of ROG. "Blue moons literally
change color rather than shape so it seems unlikely that you saw one." Perhaps you observed a penumbral
eclipse of the Moon. Such an ecllipse took place, he says, on 13 March 1998, visible from North and South America. The Moon enters the lighter area of
Earth's shadow, the penumbra, and appears to dim and to turn a gray or yellow color.
A final thought: maybe you saw Earthshine lighting the Moon. See figure. The Sun, hanging low above the western horizon, illuminated the crescent part so it
was bright white. You saw the rest of the Moon like a ball dimly lit in shadowy gray. Reflected light from Earth (Earthlight) illuminated the ball and, perhaps,
made the crescent appear as you say "attached to or just behind the moon".
Further Surfing:
When the Moon appears blue, Once in a blue moon, Royal Observatory Greenwich
A collection of Dennis Kortyka's astro-photographs sponsored by the Kingville Public Library, Ohio
USATODAY.com: More to blue moons than not being blue
Introducing the night sky, Part 4 by John Harper, The Observatory, England
The Kingville Public Library, Ohio
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