I'm unable to locate physical display observatories for the 19 year Metonic cycle. How do I display this information & calendar detail on a concrete floor near the tropic of cancer?

I seek a floor display so the 19-year cycle can be marked by the full moon at 19 year, and other influx, peak, or minimum points. These points would be displayed at local lunar midnight during full moon. Perhaps calendar events like solstice & lunar months are available information as well.

I hope to display several inflection points in a measurements similar to the link posted by PM 2ring below. The camera obscura approach may not work, as I don't intend a cathedral height ceiling over the concrete.

The optical component and a floor component are both part of my question. I have not seen an example of what I'm trying to do, nor find examples of metonic display to copy.

The optical component may be a focusing device or lens to allow for non cathedral height (from floor to hole in roof). Do lensing schemes allow for movement multiplication? A normal height ceiling may not allow this approach.

The floor display that this would be projected onto would hold points where the lunar light would annotate its place in the cycle. The cycle map or maridan line itself would be a component of the floor, perhaps artwork of some sort.

As to the use of the term physical display; I hope to use moonlight and possibly lenses to register the movement on the floor artwork. I have no display to copy, and don't understand what exact information the moon position provides during it's cycle. I understand the cycle, that data is available by phone. I want to see the display & lunar light physically interact.

The site is about 30 mi north (correction) of the tropic of Cancer. In Baja California.

  • $\begingroup$ It is very unclear what you are asking. The metonic cycle is a period of 19 years or 235 lunations (it is the smallest number of days that is a whole number of years and a whole n umber of months.... I have no idea what you are doing on the "concrete floor", or what you mean by a "physical display observatory". Can you clarify? $\endgroup$
    – James K
    Dec 22, 2022 at 21:17
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    $\begingroup$ I suspect you're talking about something like this: philipsteadman.com/blog/noon-on-the-church-floor but for the Moon. $\endgroup$
    – PM 2Ring
    Dec 22, 2022 at 22:44
  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Dec 23, 2022 at 9:29
  • $\begingroup$ Can you please clarify it? i.e what do you mean by concrete floor near the tropic of cancer. Thanks! $\endgroup$
    – user47732
    Dec 23, 2022 at 10:04
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    $\begingroup$ Just want to let you know that by removing the link from your question, if and when the comments get deleted, we won't know what you are referring to. $\endgroup$ Dec 23, 2022 at 14:50

1 Answer 1


This is an interesting idea, but I fear that there may be problems with the implementation. In particular the metonic cycle is not synchronized with the draconic cycle. This means that although there wad be a full moon on Christmas day 2015 and there will be again in 19 years time, and again 19 years after that, the location of the moon in the sky won't be the same.

Here is a simulation

enter image description here

The moon's position is marked at 19-(julian)-year intervals. Note that the ecliptic longitude is fairly constant, but there is a greater variation in latitude: up to about five degrees from the ecliptic.

Now, my understanding of your plan is that it is based on the moon being in exactly the same position every 19 years, but due to the draconic variation, this isn't the case.

  • 1
    $\begingroup$ Wow, thank you for the words! My trouble is each time I get close to an answer it's because I don't understand the the correct question. $\endgroup$
    – Eric Snow
    Dec 23, 2022 at 20:48
  • $\begingroup$ How does your simulation work? I can only see 1 image of the Moon, which is surrounded by 4 short red lines. $\endgroup$
    – PM 2Ring
    Dec 23, 2022 at 23:20
  • $\begingroup$ Eqch of the yellow dots is then location of the moon at 365.25 day intervals. (the red lines are just cursors in the soft ware to highlight the selected object. ignore them) The small white dots are stars. $\endgroup$
    – James K
    Dec 24, 2022 at 8:16

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