Morning Of The Blue Moon

What - a blue moon? Thanks to scattering, you can see a colored moon during the morning hours this week. Photo: Bob King

A whirlwind of work pins me to the wall today, so I’ll have to postpone writing about my favorite molten rocks – the tektites – until tomorrow. Did you see the moon this morning? The last quarter hung high in the southwestern sky around 8 a.m. Very easy to see even with the sun well up in the east. Looking more closely, I couldn’t help but notice that the lunar seas, those big gray spots that are actually vast volcanic plains, were tinted blue like the waters of nearby Lake Superior.

Shorter wavelength blue light is scattered efficiently by air molecules, while the other colors can better penetrate the air and travel directly to your eye. Credit: Pingry School at pingry.org

We all know that white light is composed of all the colors of the rainbow from violet to red. When sunlight strikes the atmosphere, tiny molecules of nitrogen and oxygen scatter the blue part of sunlight much more than the other colors, painting the sky its familiar blue.

The moon, which is not bright enough during the day to overwhelm the scattering effect, is likewise tinted as if viewed through a blue filter. Take a look for yourself at the moon’s appropriately sky-blue waters the next clear morning.

The moon has returned to easy visibility in the daylight hours because it’s headed in the sun’s direction as it thins from last quarter to morning crescent on its way to the next new moon. And since the sun is approaching its highest point in the sky (summer solstice), it follows that the moon also gains altitude as it tracks sunward.

Visitors to Mars will someday see with their own eyes its butterscotch skies caused by suspended dust kicked up by Martian winds. Credit: NASA

Mars’ sky is very different from Earth’s, colored instead by iron oxide dust kicked up by the planet’s incessant winds. The dust absorbs the blue end of the rainbow and reflects back the red and orange light, giving the sky a pink or butterscotch hue. Mars atmosphere is much thinner than Earth’s. If you could somehow vacuum up all the dust, it would appear very dark blue, similar to what you see out the window of a high-altitude aircraft.

3 Responses

  1. caralex

    Bob, I read somewhere that the Martian sky only looks red when looking horizontally through the layers of air. If you looked directly upward, you’d see it as deep, dark blue.

    How true do you think that is?

    1. astrobob

      Hi Carol,
      I had not heard that but it rings true because you’d be looking through much less dust in the zenith direction. With the Martian air so thin, especially zenithward, I think you’re probably right.

      1. caralex

        Well, that’s good to hear! It satisfies both those who say the sky is red, and those who say it’s blue, and shout ‘hoax’! (that the evil NASA is lying and deliberately making the sky red in spite of the evidence to the contrary!)

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