why is the moon yellow and big
(Source, Wikipedia Commons)
The moon is generally called a " " when it appears that way (i. e. large and red) in autumn, amongst a few other names. There are other names that are associated with specific timeframes as well. The colour is due to (Also known as may have noticed that they always occur when the Sun or Moon is close to the horizon. If you think about it, sunlight or moonlight must travel through the maximum amount of atmosphere to get to your eyes when the Sun or Moon is on the horizon (remember that that atmosphere is a sphere around the Earth). So, you expect more blue light to be scattered from Sunlight or Moonlight when the Sun or Moon is on the horizon than when it is, say, overhead; this makes the object look redder. As to the size, that is commonly referred to as the " ", which may be a combination of many factors. The most common explanation is that the frame of reference just tricks our brains. Also, if you look straight up, the perceived distance is much smaller to our brains than the distance to the horizon. We don't perceive the sky to be a hemispherical bowl over us, but rather a much more shallow bowl.
Just ask anyone to point to the halfway point between the horizon and zenith, and you will see that the angle tends to be closer to 30 degrees as opposed to the 45 it should be. Gray is not a color - it is a shade. A shade of white. We perceive something as gray when it has no obvious "color" (that is, the amount of red, green, blue stimulus that the cones in our retina receive are roughly the same), and Our eyes adjust our perception of gray or white relative to "something else". You eyes receive exactly the same information from a dimly illuminated object with a reflectivity of 1. 0 as from a more brightly illuminated object with reflectivity of 0. 1 (gray). When you see the moon "by itself", brighter than anything else in the night sky (or brighter than the blue sky in the day) our brain says "that's white". But if you put a big white sheet on the moon it would be a lot whiter - as you see in the photo of the astronaut whose white suit makes the moon look gray. It is reasonable to say that the little rectangle on the left looks white, while it looks gray on the right; and whether you consider it gray or white in the middle depends on your screen brightness, probably.
But they are in fact all the same shade of gray. There is a good description of the composition of the moon and its color But there is one other very cool thing. When you look at the intensity of the moon as it goes through its phases, you expect there to be more moonlight when the moon is full - obviously, since the "disk" of light is bigger. BUT! The intensity of moonlight does not scale with the phase (apparent area) of the moon. If it did, you would expect the intensity of moonlight to follow some smooth curve - but in particular you would not expect the intensity to change much right around the full moon (because the illuminated area doesn't change a lot). But with information from. Borrowing a curve from this link: The moonlight gets considerably brighter (the moon "whiter") right around the full moon. There are two effects in play: Since the moon surface is rough, when the sunlight is shining across the surface at an angle, you will see some illuminated surfaces and some areas that are in the shade.
This means that the average intensity will seem lower - an effect that is stronger as the light is more glancing: you see this very clearly in this image The surface of the moon (moon dust) has a lot of tiny quartz spheres on it: these are the result of meteor impacts that heated the soil to melting point, after which the molten droplets fall back to the surface as little "glass spheres". And these have the interesting property of retroreflectivity: when light come from directly behind you when you look at a sphere, you will see a very bright reflection - because the sunlight is focused onto a point right behind the sphere, and you are looking at this bright spot "through a lens". This second point explains the really sharp peak right around full moon - and this is why the moon looks particularly bright and white at that time (barring atmospheric effects). This is described in more depth (and I just discovered it has a name: Heiligenschein which is German for "saintly glow") at
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