One of the most frequent questions and comments surround the Moon. And, every 2 1/2 years we are treated to what is referred to as a “Blue Moon”. We describe this on our website but here is an interesting question/ comment about the “true” blue moon and a response form my capable astronomer, Joe Rao.

Dear Farmers Almanac:

I found your article “What Is a Blue Moon?” to be fascinating. For some time I have nursed my own definition of a blue moon. Calling a full moon “blue”, whether it is the thirteenth in the year, the second in the month or the fourth in a season, has always seemed a little off to me. Like any full moon it will be white, or perhaps yellow or red when near the horizon, but hardly ever blue.
However, there is a real blue moon. If you observe a crescent moon while the sky is still blue, you can often see the outline of the rest of the ball, blue on blue in the late afternoon or early morning sky. In between the old moon and the new moon, there is a day when no crescent is revealed. The moon is very close to the sun, from our point of view. However, it can never be behind the sun because the sun is too far away. Occasionally it can be right in front of the sun, which is an eclipse of the sun.
Most months, however, the new moon is up there in full view. We can’t pick it out, because it is blue. If we knew just where to look, some late afternoon when the sun was just past the horizon, we would see a true blue moon.

Thank you,

J.D.

You are correct. And that blue color is actually Earth’s reflection. If you were on that part of the Moon where the blue color showed up, you would be looking up at a sky with a nearly full-Earth. Indeed . . . as seen from the Moon, our Earth goes through phases, but they are opposite to the lunar phases. So, if we are looking at a thin crescent Moon on Earth, from the Moon, the Earth would appear nearly full (a phenomenon called “complementary phases”).

So . . . when a nearly full Earth is in the lunar sky, there is no other illumination . . . no Sun, just the background stars. Our Earth appears nearly four times larger than the Moon looks for us. And because of the clouds and oceans, it appears about 45 to 90 times brighter than our Moon. So, a bluish-gray illumination (reflected from the clouds and oceans) illuminates the lunar landscape. From here on the Earth, we see this as a faint bluish color, lighting up the “dark side” of the Moon. We call it Earthshine, although it is also popularly referred to as “The Old Moon in the New Moon’s Arms.”

See: http://tinyurl.com/3dlcos

By the way . . . during total eclipses of the Sun, the features on the entire lunar disk can be brought out on a photograph. We don’t see them with our eyes — the Moon appears jet black against the Sun — but New Moon is when Earthshine is brightest. And if the camera is opened long enough, it can actually capture the lunar features on the dark disk of the Moon.

See: http://tinyurl.com/229×7h

Thanks for writing!

– Joe Rao

Astronomer

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blue moon, full moon, crescent moon, lunar phases, Farmers’ Almanac