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This morning I was on the bus and as I looked out of the window I saw that the moon was larger than usual and red. I live in eastern Norway and it was about -20ºF / 30ºC outside. There's no forest fires, and I live in a small village. There's no eclipse scheduled either. The moon went behind the mountain line before I got to school ( bus ride happened 7:03 - 7:20 ), and nobody there had seen it. Can you explain this?

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    $\begingroup$ Was it close to the Horizon? (I'm guessing it was, cause you said it went behind a mountain). $\endgroup$ – userLTK Mar 1 '18 at 21:56
  • $\begingroup$ Every time we spot a rising or setting moon, than it is redder as for Rayleigh scattering, the same that makes red sunrises and sunsets. The answer is yes. Look here in SE or Google atmospheric scattering for various regimes and phenomena $\endgroup$ – Alchimista Mar 2 '18 at 10:45
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    $\begingroup$ To see it really red the sun must be low or down, as contrast plays a role. $\endgroup$ – Alchimista Mar 2 '18 at 10:57
  • $\begingroup$ Pine pollen will do it, but wrong time of year? $\endgroup$ – Wayfaring Stranger Mar 2 '18 at 14:51
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What causes the moon to be red is our own atmosphere. It's the same effect that causes a sunset to be red. As the sun nears the horizon, the amount of air that we're looking through increases - and with it everything in the air such as dust, pollution, humidity, etc.... If you look at the moon near the horizon, you'll notice it is significantly yellower/oranger than when it's high in the sky, where it is normally whiter and bluer. Again, this is all caused by the air and the stuff in it.

When the moon turns read during an eclipse, it is because it is passing through the shadow of the atmosphere, which filters the light passing through and on to the moon, leaving more red light to make its way to the moon and, then, to reflect off of it.

A similar action happens when smoke from fires gets into the air - it filters out the shorter wavelength, bluer light and lets the red pass. Other pollutants besides smoke particles can have the same effect.

So, the only way for this to happen is for something to pass between the sun and the moon, or the moon and the earth - between the source of light and where it is received.

As for the moon appearing larger: this is an optical illusion. No one is entirely sure why it happens, but the moon does APPEAR larger on the horizon than when it is high in the sky, but this is merely an illusion. To prove it, next full moon, go outside with a ruler and hold it up at arm's length and try to measure the apparent size of the moon (this will vary from person to person due to the length of your arm). Then, wait until it's high in the sky and do the same thing. It will measure the same size (give or take a small amount due to the change in position affecting the distance from your eye to the ruler).

If you have the right kind of telescope and mount, you could connect a camera, take a picture on the horizon, wait for it to pass close to its highest point and take another picture, then compare. Assuming no changes were made to the camera or telescope, the images should show the same apparent size.

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I assume that the Moon was near the horizon. This is just my assumption, since there kind of needs to be more detail, but the light scattering at the horizon probably was the cause. It's the same light scattering that makes sunset that pinkish color. Hope that helps!

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