During each lunar cycle, the full moon is technically full for one instant, but to the human observer, the moon appears full for 2-3 days and nights in a row, making it hard to tell on which date and time the full moon is, just by looking at how round the illuminated area of the moon appears.

For example, in the image below, the moon appears to be at its fullest on the 23rd or 24th, but it's hard to say exactly when.

Moon phases

Image from https://upload.wikimedia.org/wikipedia/commons/9/9d/Moon_phase_calendar_May2005.jpg

But if you were in Miami, Florida, USA at the right hour on the dates below, with a clear view of the entire sky, you could tell with only simple, natural sky observations during that hour that the moon was full “around now, plus or minus a couple of hours”. What type of observation could you make on each date? It is different on each date. No instruments, calculations, or reference data are required during the time of the observations (you'll probably need to look up some info to solve this puzzle though).

March 1, 2018
January 21, 2019
May 7, 2020


1 Answer 1


On January 21, 2019,

there was a lunar eclipse; the totality phase lasted one hour and the 'full moon moment' happens during that hour.

On May 7, 2020,

the sun rose at 6:40am local time, five minutes before the 'full moon moment'. At the same moment, the moon set. If you had an unobstructed view, you could see both happening at the same time, proving that it was full moon at that moment or very close. (How close would still need some calculations.)

On March 1, 2018,

the reverse situation happened (compared to May 7, 2020): the moonrise happened about the same moment (6:14pm local time) as the sunset.

  • $\begingroup$ Your are correct for January 21, 2019 and May 7, 2020. What is the answer for March 1, 2018? $\endgroup$
    – FlanMan
    Mar 3, 2021 at 18:05
  • $\begingroup$ I don't know, otherwise I would've posted it :) I thought of rot13(yhane bpphygngvba bs n fgne) but couldn't find it. $\endgroup$
    – Glorfindel
    Mar 3, 2021 at 18:18
  • $\begingroup$ The answer for March 1, 2018 uses a method similar to one of your other two answers, but at the opposite time of day. $\endgroup$
    – FlanMan
    Mar 3, 2021 at 20:00
  • $\begingroup$ Really? I ruled that out because the data here doesn't align with the data here. $\endgroup$
    – Glorfindel
    Mar 3, 2021 at 20:10
  • $\begingroup$ It aligns closely enough. Local times on March 1, 2018 are: Moonrise 6:14pm, Sunset 6:22pm, Full moon 7:52pm (0:52 UTC next day). Full moon is 1.5 hours after sunset, which is "around now, plus or minus a couple of hours". $\endgroup$
    – FlanMan
    Mar 4, 2021 at 13:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.