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I have recently learnt to determine moonrise and moonset times using the Nautical Almanac, as part of my maritime navigation course. I have a query regarding the determination of moonset times. I will use an arbitrary date and an arbitrary latitude to illustrate my confusion. On 3rd February 2020, at the equator, the Nautical Almanac tells me that the moon will set at 0041 hrs. Does this mean that the moon that rises on 3rd February 2020 sets at 0041 hrs on 4th February 2020 or does this mean that the moon that rose on 2nd February sets at 0041 hrs on 3rd February 2020?

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It means that there is a moonset at that time on the third of February.

At the equator, it is certain that the moon that set at 0041, rose the previous day (Feb 2nd), at about midday. (the moonset is about 50 minutes later each day, which means that there can be days with no moonset, but you can't have a day with two moonsets)

On days with no moonset the time of moonset in your almanac might be indicated with a time that is more than 23:59

For example:

Date of month
1    2    3    4
h m  h m  h m  h m 
2052|2250|2500|0100

Here you see on the first day of the month the moon sets at 20:52, on the second it sets at 22:50, on the third day there is no moonset. The moon sets one hour after midnight. The moonset 25:00 and 01:00 refer to the same event.

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  • $\begingroup$ Strangely, the Nautical Almanac I use also shows times in the form 24XX and also 25XX. Why don't they report these times as 00XX and 01XX? $\endgroup$ – Tan Yong Boon Apr 30 at 10:39
  • $\begingroup$ no idea, can you link to that? $\endgroup$ – James K Apr 30 at 10:42
  • $\begingroup$ Our instructor only gave us copies of a few pages of the book. The publisher is British Crown. Not all the times were reported as 24XX and 25XX. There is a combination of both 24XX/25XX and 00XX/01XX, which leads me to believe that this was intentionally done. $\endgroup$ – Tan Yong Boon Apr 30 at 11:27
  • $\begingroup$ Refer to page 11 of books.google.com.sg/… $\endgroup$ – Tan Yong Boon Apr 30 at 11:36
  • $\begingroup$ Thanks for your answer anyway! I believe that when they write "moonset on 3rd February", they are referring to the moonset on that day. It only makes sense that way. $\endgroup$ – Tan Yong Boon Apr 30 at 12:08

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