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So the question is clear from the title itself. Only sometimes does the moon when it just rises above the horizon, looks reddish in colour. This mostly as I have seen occurs when the moon rises when it has gotten already a bit dark. When the moon rises in the evening, when there is still a bit of sunlight, the moon looks whitish. So there are 2 parts to the question.

  1. Why does the moon look reddish sometimes when it rises above the horizon and keeps on looking the same till about an hour or so but then becomes whitish.

  2. Why does this occur only sometimes, particularly when the moon rises when it has already got dark.

Please answer both the sub questions and if there is a problem with any of them please let me know.

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I believe this is due to Rayleigh scattering. The light from the moon passes through a thicker layer of atmosphere when the moon is low in the sky than when it is high. The light gets scattered along the way, with shorter wavelengths being scattered the most. That means bluish colors are scattered away more than red, so you see the moon as appearing reddish.

Rayleigh scattering is also the reason why the sun looks red just before sunset and why the sky looks blue during the day.

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  • $\begingroup$ Yeah I think what you say is the reason. But any suggestions you would like to give as to why it happens when the moon rises in the night and not when it rises in the evening $\endgroup$
    – Shashaank
    Jun 11, 2020 at 18:21
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    $\begingroup$ I don't know the answer to that, and I have not noticed that myself. I can only guess that perhaps the color of the moon that you perceive is influenced by scattered sunlight which is bluish; or the moonlight is still reddish, but it is difficult to tell that when the moon is close to the setting sun because the sunlight is reddish as well. $\endgroup$
    – Puk
    Jun 11, 2020 at 20:22
  • $\begingroup$ yeah I liked your argument $\endgroup$
    – Shashaank
    Jun 12, 2020 at 15:12

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