Hi! I live in a house with a narrow valley behind me, with a clear view of a prominent house a couple miles away on the next ridge:
On the evening of my birthday in late April 2021, I shot the full moon rising behind that house:
Question: can I expect to be able to shoot that picture every year on my birthday, weather permitting? Do I have my own personal Stonehenge calendar for my birthday?
Extra credit: how much would the moonrise move on the day before or after my birthday to the left/right of that house?
Of course not. The moon will not be full every year on your birthday. The moon will not rise at the same time every year on your birthday, and it will not rise in the same place every year on your birthday. The second might not matter to you. The first might not matter to you if you don’t care if the moon is full. But the third definitely does matter.
The first column on this page Moonrise, Moonset, and Moon Phase in Compass, May 2016
shows the time and location (as an angle) of the moon rise for each date. The last column shows the percent illuminated with 100% being a full moon.
According to the table that OldGuy linked to, if I set the month to April 2021, we can see that (for the specific location in PA that OldGuy selected), the moon is basically full (99.9%) on April 27th.
If I set the month to April 2022 and look at April 27th, the moon is only 10.8% illuminated.
Therefore, the rising moon will not be full on the same date every year.
(QED is usually used when you’ve proven something, not just stated your belief.)
As for the full moon part (ignoring of location), the synodic period of the Moon is 29.5305 days. The question then is how closely does that fit into a calendar year (spoiler: it doesn’t because the lack of synchronization between lunar and solar calendars have been known for quite a while).
365 ÷ 29.5305 = 12.36 (rounded). Meaning on your next birthday after a full moon, the Moon will be about 10 days 16 hours past the full moon (subject to leap years).
The first statement is true. However, the moon will not rise at sunset on your birthday every year.
The location and month and year can be changed on that table. If you set it to May 2021, you’ll see that on the 26th, the moon rose 99.9% full at 8:59 pm at an angle of 121 degrees; that is 31 degrees south of due east. (Your numbers will vary if you change the location.)
If you change to 2022, you’ll see that on May 26th, the moon rises 14% full at 3:45 am at 84 degrees; that is 6 degrees north or due east. It will rise at about the same spot as this year (120 degrees) on May 21, 2022 at 1:27 am and be 65.2% full.
To get your same picture, you want to look through the tables for a rise at 121 degrees and 100% full, which will need to be at about the same time of day that you took the picture, but there would be seasonal effects. Actually the picture would be a bit better if the moon were a little to the right, which would be south. So you cold look for risings at angles a bit above 121. I can’t tell scale from the picture so I can’t tell you how much more than 121 would be best.
The answer to your bonus question is also in the table, sort of. The moon rose 6 degrees further north the day before and 3 degrees further south the next day. Again I can’t convert that to distance without knowing exactly how far away the house is and how wide it is.
OK, thanks @OldGuy for the detailed reply. I can see this is far more complex than I’d anticipated and I really should have anticipated that, and thanks for your detailed reply and your patience.
I do wonder if there’s some standard interval for replicating a rising/setting moon picture, same day, same place, same time. But that’s a bit beyond the scope of the OP parameters.
@oldguy I used Google Earth to measure and that house is .77 miles from me.
GE also tells me the house is .2 miles wide, which seems like absolute bullshit, it’s not 1000 feet wide. I’d guess 100-180 maaaybe feet wide from my angle.
IIRC, the Moon’s various cycles all line up after about 19 years. But I might not be remembering the right cycles-- The Moon actually has several different cycles.
No, you are right— the Moon will next be full on April 27 in 2040 (Metonic cycle).
In 2029 it will be full on April 28. In 2051 there is a full moon on April 26th. (According to the almanac. Universal time, so keep in mind your time zone)
If it’s 200 feet then at .77 miles it subtends an angle of 2.9 degrees. Probably 1 degree to the right would put the moon just about over the center of the house. That would be 1 degree further south which if I recall the OP would me you want the moonrise at 122 degrees true.
The complete disc of the Moon (whether fully lit or not) subtends an arc very close to 1/2 a degree. So from the OP’s pic we note that the part of the house we see unobscured by trees, projected perpendicular to the line of sight, subtends a bit over 2 Moon diameters = a bit over 1 degree. As a very rough rule of thumb, 1 degree at 1 mile subtends a chord of 100 feet. So we’re looking at 100+ feet of house frontage as projected into our eyes/camera’s plane of sight.
As folks have said, the time interval from one full Moon to the next doesn’t fit neatly into months or years. This explains the fact that the dates of the full Moons over months and years is a moving target, albeit one that repeats over a span of many years.
Not mentioned explicitly that I noted, is a separate factor - the plane of the Moon’s orbit is not aligned with the Earth’s equator.
Compared to the solar system’s “ecliptic”, the Moon’s orbital plane plane is ~5 degrees off and the Earth’s equator is about ~23 degrees off. Sometimes those two factors compound each other and sometimes they offset. Which gives a complicated wobbly cycle to the anglular difference across a range of +/- 28 = 56 total degrees.
With the result that when the Moon rises (full, crescent, half, whatever, doesn’t matter) where you see it appear along your local horizon is subject to large fluctuations north and south as these angles interact over time.
Folks upthread mentioned that the moonrise point changes, but not why (that I noticed).
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.
ETA I guess it depends on what you are calling “large” in this photographic context.
