Need help with a head-scratcher

[W2ET]

A couple of days ago, a friend took an amazing picture of the sun precisely at sunset with its azimuth precisely aligned with his dock. The sun was located on a centerline between the two rows of pilings on either side.

I can't find a solar calculator online which will tell me the next time this will happen. In other words, what is the next date when the sun's azimuth at the astronomical sunset is exactly as it was when and where the original picture was taken?

Any thoughts?

Bill

[Dave Neptune]

Bill, a friend of mine uses an app for his photography. He can pick a location and request a celestial back ground for his observing site, it's really cool and he has some really nice shots. I will see if I can get the info from him. He is in Washington St and sells background photos for advertising. I think he needs an accurate position for his spot and a position of the background. He has many moonrise shots that are incredible.

That view should appear twice a year albeit a couple of hours off each year due to leap year.

Dave Neptune

[W2ET]

Bill, a friend of mine uses an app for his photography. He can pick a location and request a celestial back ground for his observing site, it's really cool and he has some really nice shots. I will see if I can get the info from him. He is in Washington St and sells background photos for advertising. I think he needs an accurate position for his spot and a position of the background. He has many moonrise shots that are incredible.

That view should appear twice a year albeit a couple of hours off each year due to leap year.

Dave Neptune

Hi, Dave:

That makes me wonder if my friend could do something similar with the moon setting. Frankly, I've never really paid attention to moonsets (if that's a word), but we do get some spectacular sunsets here.

Apparently, the timing of my friend's shot was pure luck. A couple of days either way and I think he might have missed it.

Oh, let's add another variable, requiring that it be a full moon. Years out? The Aztecs probably had this down cold.

Bill

[Dave Neptune]

Bill, the Greeks had a mechanical mechanism built from bronze that would do those calculations it is Antikythera mechanism. A fascinating piece of engineering even for today.

I put an E-scud out to my buddy and will let you know when I hear from him.

Dave Neptune

[Dave Neptune]

Bill, this is the link he gave me. There should be some others that pop up too.

https://apps.apple.com/us/app/the-photographers-ephemeris/id366195670

Give this a try, may or may not help but at least it is a start.

Good Luck

Dave Neptune

[Peter]

Bill,

A less elegant solution is this from the NOAA

https://www.esrl.noaa.gov/gmd/grad/solcalc/

It will give you the azimuth of the Sun at any time/date you specify.

For your friend, this will involve a little hunt and peck. However, presumably he/she knows the date and time of the original picture. From that you can predict when it will occur again.

If it happened on March 21 it will happen again very close to Sept 21 (give or take according to leap years).

If it occurred two months prior to March 21 it will occur two months after Sept 21.

For example, I selected Bismarck, ND as the city and May 21 as the date. It tells me that sunset that day will occur at 21:19 and the azimuth of the Sun will be 301.74 degrees.

May 21 is two months after March 21, so I then went to July 21, which is two month before Sept 21. On that day, in that location, sunset will occur at 21:28 and at that time the azimuth of the Sun will be 301. 5 degrees

Not a perfect match so I went to July 20 - sunset at 21:29 and the sun's azimuth will be 301.81 degrees. Probably close enough?

Hope that helps,

Peter

[W2ET]

Dave, the website you recommended was amazing, and it quickly provided the answer regarding sunset. I then used Peter’s suggestion to check for pilot error, and all was well. Thanks to both for the prompt response. The pool of accumulated knowledge and experience on this forum is mind-boggling.

It turns out the answer is six months minus one day. At this location and time of year, the azimuth at sunset indexes by about half a degree daily, so a one-to-one match between the two dates would be fortuitous but not likely. In this instance, the difference between the two dates was 0.13 degrees (271.48 versus 271.61). Close enough for government work, as they say.

The determination of a moonset match remains a to-do.

Bill

[W2ET]

Well, what are the odds of this? The target I had calculated for sunset at the desired azimuth for the location of interest is September 21st. On that date, it turns out that sunrise is at 6:52 am and the moonset of a full moon is 7:17 am at the desired azimuth for that location is within 3 degrees of the desired azimuth.

It’s theoretically possible to shoot moonset in the morning and sunset in the evening of the same day at the desired azimuth or nearly so, essentially 12 hours apart?

Bill

[scratchee]

I know a tool for this but you have to be on Salisbury Plain to use it.

[Peter]

"It’s theoretically possible to shoot moonset in the morning and sunset in the evening of the same day at the desired azimuth or nearly so, essentially 12 hours apart?"

The full moon always rises when the sun sets and vice versa. The moons orbit is tilted about 5 degrees to the ecliptic so it does not always rise/set at the same azimuth as the sun but sometimes it does - when it is full and its orbit intersects the ecliptic.

It would seem to be a fairly rare thing for this to occur at the particular azimuth of interest so tell your friend to grab that photo.

Peter