# Annual moon photo question **URL:** \ **Category:** Factual Questions\ **Created:** [May 29, 2021, 3:20am UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152 "2021-05-29T03:20:59Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![pjd](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pjd/32/3009_2.png) [@pjd](https://boards.straightdope.com/u/pjd)\ **Post date:** [May 30, 2021, 6:24pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/21 "2021-05-30T18:24:27Z")
I don’t know nuffink about subtended angles, but i like the picture. Why not sell the NTF to the house’s owner. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 30, 2021, 6:33pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/22 "2021-05-30T18:33:22Z")
> [@DPRK](#): > > Did you compute these “large fluctuations”? Taking for example, L.A., the moon rises on 31 May at 7:37 UTC (00:37 local time) at an azimuth of 115 degrees, 30 minutes. On May 31, 2040 it rises at 7:41 at 115 degrees, 19 minutes. So a moonrise photo would be off by only 11 minutes of arc. Certainly there’s a repeating pattern to the beat frequency between the two inclinations. Did you check the azimuth on all the other 240-ish moonrise dates within that 20-ish year span? ---
**Author:** ![pjd](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pjd/32/3009_2.png) [@pjd](https://boards.straightdope.com/u/pjd)\ **Post date:** [May 30, 2021, 7:07pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/23 "2021-05-30T19:07:56Z")
I meant NFT, of course. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 30, 2021, 9:09pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/24 "2021-05-30T21:09:00Z")
> [@LSLGuy](#): > > Certainly there’s a repeating pattern to the beat frequency between the two inclinations. Did you check the azimuth on all the other 240-ish moonrise dates within that 20-ish year span? There is, of course, the issue with the moon not rising on certain calendar days, and 2040 being a leap year. A typical sort of “large” fluctuation would be e.g. the moon rising in Los Angeles at 10:36 in the morning on Jan. 18, 2021 at 92.9 degrees, while on Jan 18, 2040 it will rise at 10:24 at 95.4 degrees (nor does it help to try to match it up with Jan 19, 2040 instead since the corresponding values are 10:54 and 88.5 degrees). 2097 is a little better (94.6 degrees). Or something like March 11, when in 2021 the moon rises at 109.9 degrees (versus 109.1 degrees in 2040), but the next day only at 104.2 degrees (vs. 102.3 degrees in 2040). ---
**Author:** ![lobotomyboy63](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lobotomyboy63/32/2900_2.png) [@lobotomyboy63](https://boards.straightdope.com/u/lobotomyboy63)\ **Post date:** [May 30, 2021, 9:27pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/25 "2021-05-30T21:27:20Z")
> [@squeegee](#): > > So does that mean I can take the exact same photo of the moon at the same place and same day+time every 19 years? Easter dates repeat on a 19 year cycle. I wonder if you could apply the gap between the dates to predict. This chart probably won’t reproduce right but… > The epacts for the current Metonic cycle, which began in 2014, are: > > | Year | 2014 | 2015 | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 | 2026 | 2027 | 2028 | 2029 | 2030 | 2031 | 2032 | > | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | > | Golden | | | | | | | | | | | | | | | | | | | | > | number | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | > | Epact[[h]](https://en.wikipedia.org/wiki/Date_of_Easter#cite_note-43) | 29 | 10 | 21 | 2 | 13 | 24 | 5 | 16 | 27 | 8 | 19 | \* | 11 | 22 | 3 | 14 | _ **25** _ | 6 | 17 | > | Paschal | | | | | | | | | | | | | | | | | | | | > | full moon | | | | | | | | | | | | | | | | | | | | > | date[[36]](https://en.wikipedia.org/wiki/Date_of_Easter#cite_note-44) | 14 | | | | | | | | | | | | | | | | | | | > | April | 3 | | | | | | | | | | | | | | | | | | | > | April | 23 | | | | | | | | | | | | | | | | | | | > | March | 11 | | | | | | | | | | | | | | | | | | | > | April | 31 | | | | | | | | | | | | | | | | | | | > | March | 18 | | | | | | | | | | | | | | | | | | | > | April | 8 | | | | | | | | | | | | | | | | | | | > | April | 28 | | | | | | | | | | | | | | | | | | | > | March | 16 | | | | | | | | | | | | | | | | | | | > | April | 5 | | | | | | | | | | | | | | | | | | | > | April | 25 | | | | | | | | | | | | | | | | | | | > | March | 13 | | | | | | | | | | | | | | | | | | | > | April | 2 | | | | | | | | | | | | | | | | | | | > | April | 22 | | | | | | | | | | | | | | | | | | | > | March | 10 | | | | | | | | | | | | | | | | | | | > | April | 30 | | | | | | | | | | | | | | | | | | | > | March | 17 | | | | | | | | | | | | | | | | | | | > | April | 7 | | | | | | | | | | | | | | | | | | | > | April | 27 | | | | | | | | | | | | | | | | | | | > | March | | | | | | | | | | | | | | | | | | | | > **[Date of Easter](https://en.wikipedia.org/wiki/Date_of_Easter)** > > As a moveable feast, the date of Easter is determined in each year through a calculation known as computus (Latin for 'computation'). Easter is celebrated on the first Sunday after the Paschal full moon, which is the first full moon on or after 21 March (a fixed approximation of the March equinox). Determining this date in advance requires a correlation between the lunar months and the solar year, while also accounting for the month, date, and weekday of the Julian or Gregorian calendar. It was... This year was March 28…next year is April 16, eighteen days later. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 30, 2021, 9:28pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/26 "2021-05-30T21:28:43Z")
> [@DPRK](#): > > There is, of course, the issue with the moon not rising on certain calendar days, and 2040 being a leap year. I believe we are talking past one another. The Moonrise azimuth as seen from any given spot on the Earth varies in a complex non-sinusoidal way across a range of 56-some degrees over a period of years. That range constitutes a large fluctuation as measured from extreme to extreme. 56 degrees subtends a substantial space on your local horizon, ~15% of the 360 degree total. I was saying exactly zero more than that. I was saying nothing about how that may connect with specific dates. As applied to the OP, the point I’m making now here in this post is that to recreate the pic he needs to get the azimuth right AND also get the rise to be a full moon AND to have this happen on the magic day of year he arbitrarily chose. Those three things are each necessary conditions that are each unrelated to the other two. You are certainly correct that since all three are periodic phenomena, AND since they occurred simultaneously once (we have the picture to prove it) we can safely conclude that they will coincide again at some future date. With room of course for anyone to quibble about exactly how precisely is precisely enough to constitute a “recurrence”. I _beleive_ you are pointing out the quibbling-level almost-recurrence values and mistakenly thinking that was the large fluctuation I was talking about. Not so. I make no claim as to how far in the future that reasonably close recurrence date will be. If I’ve mis-stated your case please correct me. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 30, 2021, 10:05pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/27 "2021-05-30T22:05:53Z")
I was suggesting (in support of something @Chronos said) that if the OP wants the same phase of the moon to recur on the same calendar day, the answer is the 19-year Metonic cycle. I did, in fact, check that his conditions (full moon rising in the evening on April 26, USA time) do recur in 2040 and not sooner, but I encourage everyone to double-check. The “large fluctuations” I was talking about concern whether the rising full moon on his birthday in 2040 will again rise directly over that house, or be a degree or two off. I apologize if I misunderstood your comments. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 31, 2021, 1:09am UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/28 "2021-05-31T01:09:44Z")
All good. The Metonic cycle explicitly is about phases and calendar dates only and doesn’t address azimuth variations. So you and I have been talking about two different things. I’d goofed up some of my part too, without any help from anyone else. 🤦‍♂️😉 Upon further reading … There is an 18.6 year cycle of whether the Moon’s orbital inclination and Earth’s obliquity aggregate or offset from each other. So on that not-quite 19 year cycle the spread of azimuths goes between the max of roughly 23 + 5 = +/- 28 degrees versus the min of roughly 23 - 5 = +/-18 degrees. But whatever the azimuth range is this month, the Moon will cycle through that complete range of azimuths in just ~27 days ~= 1 lunar orbit ~= 1 lunar month. So for very _very_ round numbers from one date to the next, moonrise (of whatever phase and time of day) will slide about 1-1/2 degrees = 3 moon diameters north or south of where it rose last time. This is an approximately sinusoidal change over the complete cycle, so as the Moon approaches the northern or southern extent of its range this month the rate of shift N or S will smoothly decline from the average to zero before reversing direction. And conversely the rate of change will be maximized as the Moon passes the midpoint of the cycle. Although the duration of this cycle is approximately the same as the lunar month - phase cycle, the two do not have a consistent relationship. i.e. you can’t say the full moon always happens in a consistent place in the north / south azimuth cycle. Here’s more reading for orbit nerds: > **[Lunar standstill](https://en.wikipedia.org/wiki/Lunar_standstill)** > > Pages for logged out editors learn more > > > A lunar standstill or lunistice is when the moon reaches its furthest north or furthest south point during the course of a month (specifically a draconic month of about 27.2 days). The declination (a celestial coordinate measured as the angle from the celestial equator, analogous to latitude) at lunar standstill varies in a cycle 18.6 years long between 18.134° (north or south) and 28.725° (north or south), due to lunar precession. These extreme... ---
**Author:** ![pjd](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pjd/32/3009_2.png) [@pjd](https://boards.straightdope.com/u/pjd)\ **Post date:** [May 31, 2021, 6:43pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/29 "2021-05-31T18:43:28Z")
> [@DPRK](#): > > I don’t know where you are, but you can download Stellarium > [https://stellarium.org/](https://stellarium.org/) That’s brilliant, thanks ! ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [June 1, 2021, 5:05am UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/30 "2021-06-01T05:05:57Z")
I feel like this numerology is getting somewhere, but we should keep in mind that the Metonic cycle is not precisely an integral number of days. The number (on Wikipedia) is 235 synodic months = 6939.688 days. And the OP is observing the moment of moonrise, not the precise moment of maximum phase. Furthermore, if we look at the periodicity of the moon around his azimuth, it’s not so far off in tropical and draconic months, but the anomalistic month, during which the moon speeds up and slows down, does not go into 6939.688 particularly evenly, inducing “fluctuations” on the order of half a day. But we still do not know the circumstances at the OP’s location; maybe he is lucky and it will still hit the building. I still suspect the difference between 2021 and 2040, at his location, will not be off by more than a degree. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [June 1, 2021, 11:14am UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/31 "2021-06-01T11:14:44Z")
> [@DPRK](#): > > I still suspect the difference between 2021 and 2040, at his location, will not be off by more than a degree. Agree completely. Whether by coincidence or by cosmic design the 2040 alignment will be as you say. ---
**Author:** ![Spiderman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/spiderman/32/230_2.png) [@Spiderman](https://boards.straightdope.com/u/Spiderman)\ **Post date:** [June 3, 2021, 9:35pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/32 "2021-06-03T21:35:50Z")
As others have already stated, a lunar month is not the same length as a calendar month; therefore, if there was a full moon on your b-day this year, there won’t be one on your b-day next year. Further, full moon rise is fairly close to sunset time but moonrise time is _approximately_ 50 mins later each night until it lines up close to sunset at the next full moon. Notice I stated ‘close to’ & not ‘exactly the same minute as’; usually ±15-20 mins. Finally, the (sun &) moonrise & sets change location a little bit each day. In a couple of months the full moonrise won’t line up the same way. Actually the full moon closest to mid-Aug will line up the same as your shot in late April since that is an equal number of days away from summer solstice. Think of it like a pendulum; it slowly moves one direction until summer solstice & then moves the other direction until winter solstice when it reverses back toward summer solstice again. The full moon closest to your b-day (& it’s offsetting counterpart in Aug) should line up fairly close to what you shot but if it’s a clear night & you remember to go out there in Oct, Nov, &/or Dec, you’ll see that it won’t line up over that house again; it’ll be further to the north/left. There are a number of websites/apps that will calculate sun/moon rise/set time & placement for you so you can plan out shooting sun/moon rise/set exactly where you want it, whether that’s behind (or between - [Manhattanhenge](https://en.wikipedia.org/wiki/Manhattanhenge)) a building or statue or wherever else you may want it. ## Spidey - sky photographer ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [June 3, 2021, 10:12pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/33 "2021-06-03T22:12:01Z")
Just a nitpick: you surely mean to say that the _Sun_ is like a pendulum that swings north until the winter solstice and then swings south until the summer solstice. The declination of the Moon will, _very_ roughly speaking, have two big components, one big swing with period 27.32158 days (tropical month), and a smaller one with period 27.21222 days (draconic month) resulting from the tilt of the orbit of the moon with respect to the ecliptic. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 3, 2021, 10:22pm UTC](https://boards.straightdope.com/t/annual-moon-photo-question/943152/34 "2021-06-03T22:22:06Z")
As an aside, I’m somewhat of a nerd about astronomical cycles and periods, in particular the ones that almost but not quite exactly match (resulting in longer cycles as they beat against each other)… but I somehow never before encountered the term “draconic month”. So thank you for that. [Previous page](https://boards.straightdope.com/t/annual-moon-photo-question/943152.md?page=1)