So, what causes the moon to look red occasionally when it first rises? I understand that it looks larger when it first rises because one then has a frame of reference (the skyline), but that doesn’t seem to explain the color.
Does the fact that the light has to travel through more of the atmosphere at moonrise make a difference? If not, what does? Thanks.
As far as I know, dust and other things susended in our atmosphere cause the color.
Yup. In particular, ash from volcanic eruptions does a lovely job of it, but garden-variety dust or pollution will work, too. As noted in the OP, a nice red moon is most often seen close to the horizon, when the moon is seen through more atmosphere (and dust). Short-wavelength (blue) light gets scattered out of our line of sight, leaving behind the long-wavelength (red) light.
(The fact that it looks larger near the horizon actually has to do with a complicated optical illusion. Search for “moon illusion” on the web for more info.)
Same reason the sun looks reddish at sunrise and sunset.
If I remember correctly, the main contribuion comes from molecular oxygen, not dust particles. It’s the same reason the sky looks blue - while the light travels through the air, blue light is scattered in all directions (including towards the ground), and red light continues in a straight line.
It takes a bit more than just air molecules to turn the Moon red, though. Otherwise the Moon would look red every moonrise/moonset, which is not the case.
What follows is just an educated guess. The Moon is very gray, whereas the Sun peaks in the green, and in its spectrum, longer wavelengths are brighter than than shorter wavelengths. Rayleigh scattering removes enough of the blue light to bring out the Sun’s redder color. The Moon, though, doesn’t have such a deficiency of blue light, so it’ll still look pretty white unless the atmosphere is particularly grungy that day-- conditions which will, incidentally, lead to a more spectacular sunrise/sunset as well.
also a lunar eclipse will cause the moon to turn reddish
www.xlcus.co.uk/photos/2001-01-09-totallunar/anim.html
