The closest you’re going to get from me on Valentine’s Day is something red, dear readers. As the Moon is now a waxing gibbous and will be for the next week, it will brighten the sky for the majority of the night. But before sunrise, it will dip below the horizon and set. The funny thing is, if you’ve ever watched this (or watched a waning gibbous rise), you’ll notice it changes color! In fact, this composite photo shows you what I’m talking about:
I would like to know why sometimes [the] Moon has [an] almost orange colour.
Well, as you can see, the moon really can have these colors! But how does it happen? It’s all because of our atmosphere. If we lived somewhere without an atmosphere, like say, outer space, and watched the Moon rise then, it would never appear even the slightest hint of orange. In fact, we’ve done this with the space shuttle Discovery, where we took this picture:
You can see right away that the Moon appears much whiter from space than when seen through the Earth’s atmosphere. The Earth’s atmosphere scatters light in all directions. The more of the atmosphere you pass through, the more light gets scattered.
Blue light, since it’s more energetic than red light, has a shorter wavelength and therefore gets scattered more easily. The redder wavelengths travel to your eye without scattering so much, but the bluer wavelengths get scattered in all directions, and only a small percentage reaches your eye. What happens when you take a white light (like Sunlight or Moonlight) and scatter away more of the blues than anything else? It appears redder. See the spectrum at left, where we’ve subtracted a large percentage of the blue light, and compare it with the spectrum on the lower right, which is just a white-light spectrum. The more blue you scatter away (i.e., the more of the atmosphere you pass through, or the lower on the horizon the Moon or the Sun is), the redder it appears. That’s why the Moon and the Sun both look red (or orange) when they’re low in the sky!
And now you know why the Moon appears redder whenever it’s closer to the horizon. Want to know more about the Moon? Check out Keith’s Moon Page, which has a bunch of fun and interesting facts about our neighbor in the Solar System!