Since the night two days ago, the moon here in Israel appears to be orange as it rises, and then gradually becomes white as it continues travelling across the sky.

This phenomenon has a great timing as I am starting to become interested in astronomy (I have no special knowledge in astronomy). So I googled up and came across this explanation.

I'll try to summarize: it says that it happens because of Earth's atmosphere, that tends to scatter short wave-lengths (such as blue and green) and let long wave-lengths (such as red and orange) pass through. When the moon rises, it's in the position where the light coming from it to our eyes goes through the thickest layer of atmosphere (as opposed to when it's high up in the night skies) and that gives the orange effect.

I understand this explanation, but it raises a couple of questions for me.

  1. Why doesn't the orange moon phenomenon occur all the time?
    It does not happen quite often. What's so special about the past three nights? I mean, the rising moon's light always has to go through a thick atmosphere layer, doesn't it?
  2. Maybe that's not entirely astronomic, but, to complete the big picture - why does our atmosphere scatter short wave-lengths?
  3. This explanation suggests that all light coming to us from the sun is (at least) slightly reddish. Is this measurable (if true)? Because to the naked eye, sunlight refracted through a prism shows no tendency towards red.

2 Answers 2

  1. There may be a particular effect. Smoke from wildfires or cities, fine dust in the atmosphere etc. can have the effect of making the moon appear redder. However it is most likely an example of observation bias. You noted that the moon appeared orange, and on subsequent nights you were looking for this effect. In fact it happens each moon rise. You just didn't notice it before (or if you did, it didn't make enough of an impression on you.)

  2. There are two forms of scattering that are applicable. Rayleigh scattering, and Mie scattering. There is also absorption of some colours. Rayleigh scattering occurs as the electromagnetic wave that is light causes the molecules of the air to act like little electic dipoles. The light then bounces of these electrically active molecules. Shorter wavelengths are affected more than longer ones.

  3. The light emitted by the sun is very slightly yellow. It is slightly reddened by scattering through the atmosphere. However it still contains a mixture of all visible colours, and so a prism will still show a complete rainbow. The reddening is not apparent to the eye, but can be measured by instruments.

  • $\begingroup$ Edited, but I don't understand what you mean by "white" as distinct from what we perceive as white. $\endgroup$
    – James K
    Oct 8, 2017 at 6:16
  • $\begingroup$ Sunlight it is whiter then what actually we perceived, actually because of the atmosphere. It is a yellow star anyway, at least we can agree. An important point somehow neglected in question, answer and comments : the same source is perceived differently on different backgrounds. Lighting conditions do affect the appearance of the moon, both in terms of brightness and colour. As for point 3) geometrical considerations are involved too: intensity of scattering is inversely proportional to the fourth power of WL, if the scatter is smaller then WL (order of 1/10). $\endgroup$
    – Alchimista
    Oct 8, 2017 at 9:40
  • 1
    $\begingroup$ "White" can be either a definition based on perception, or a scientific definition based on equal intensity at all frequencies. $\endgroup$
    – ProfRob
    Oct 8, 2017 at 9:46
  • 1
    $\begingroup$ at all frequencies? No black body radiator could ever be white. No white light could be produced from a led, combination of leds or a lamp. In fact I doubt that white light exists. If it does I suspect it wouldn't look "white". $\endgroup$
    – James K
    Oct 8, 2017 at 11:08

Besides the arguments in the previous answer as well as in the comments, consider the following.

1) the rising moon is not always the same orange or red simply because this depends on the height of the sun, as it should be clear if scattering is taken into account (see point 2). This is the major reason / answer to your main question.

2) under approximation, the intensity of scattered depends inversely on the fourth power of the wavelength. This dependency is what does amplify differences between the sun light that reaches our eyes directly (or "straight but reflected" by the moon) and light that has been diffused first. An easy but correct explanation for the ten to - 4 relationship is given by Wikipedia under Rayleigh scattering.

3) Do not neglect the response of our eyes. It is not linear in terms of intensity nor sensitivity is the same at different wavelengths. Whatever you look at, in atmosphere or out of it, will look different to a spectrometer. The maximum number of photons emitted by the Sun is in the region we perceive as green,for instance. This is true both in space or after passing the atmosphere.


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