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The tropics is region of the Earth in which the Sun could be directly overhead. What is the corresponding region for the Moon? Is it the same or slightly larger?

I live in the UK and hence have to travel quite far to see the Sun directly overhead. If I am content to see the Moon directly overhead, would my trip be much shorter?

My experience suggests so. Once long ago, I travelled to India without flying. I "saw" the Moon directly overhead while in Iran and hence not yet in the Tropics. However, I had no way to accurately measure this and I may have mistaken the usually high (for me) Moon for being exactly overhead.

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  • $\begingroup$ I originally read this question title to mean "where on the moon can the Sun be directly overhead". $\endgroup$
    – user21
    Aug 5, 2018 at 16:39
  • $\begingroup$ Not my question but it would also be interesting. A rather long trip to go to see it though. I was not sure of the correct term for what I wanted hence the quotation marks which were intended to indicate that it was only my guess of the appropriate term. $\endgroup$
    – badjohn
    Aug 5, 2018 at 16:49
  • $\begingroup$ Verne put the giant cannon of From the Earth to the Moon south of the 28th parallel so it could be vertical. $\endgroup$
    – Mike G
    Aug 5, 2018 at 18:34

1 Answer 1

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The orbital inclination of the Moon is 5.1°, which means it can be that amount above or below the ecliptic. That means it can theoretically reach a declination of 23.4° + 5.1° = 28.5° (North and South), so Chabahar, in the south of Iran, lying 25° North of the equator, is within the "lunar tropics".

This will not happen every month or even year; sometimes, the orbital inclination of the Moon (around the Earth) and the Earth (around the Sun) cancel out and the maximum declination will be 23.4° - 5.1° = 18.3°. This is a minor lunar standstill. One could say that at that moment, the "lunar tropics" are smaller than the "solar tropics". The last minor lunar standstill happened in October 2015; we're slowly moving towards a major standstill which will occur in April 2025.

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  • $\begingroup$ I did not go to the very south of Iran. As well as Tehran, I went to Isfahan (32°38′N 51°39′E) and Zahedan (29°29′N 60°51′E). So, it seems that I cannot have seen the Moon directly overhead but I might have been lucky and seen it close. The time that I saw this "high moon" was during a rest stop on the bus trip between those cities, I guess that the road may have dipped skightly south but probably not. Certainly, it was much higher than I ever see it here in the UK. $\endgroup$
    – badjohn
    Aug 5, 2018 at 10:54
  • $\begingroup$ I saw the term "lunar standstill" first only just now while researching this question. I'll read up on it. $\endgroup$
    – badjohn
    Aug 5, 2018 at 10:55
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    $\begingroup$ After some reading, it seems that I was mistaken. My trip was not near a major standstill, it 5.5 years before one. So, the Moon was maybe much higher than I was used to but not directly overhead. $\endgroup$
    – badjohn
    Aug 5, 2018 at 19:57

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