by alter-ego » Sat Apr 04, 2015 5:31 am
neufer wrote:alter-ego wrote:neufer wrote:
...
The probability that the moon be in the "same exact spot" in the sky
N hours after the sun should be independent of whether N = 0, 10 or 12.
However, the orbits of the Sun & Moon around the Earth cross twice in the full celestial sky so that there is almost always one position in the sky where the Sun can be found during the day followed by the Moon in the "same exact spot" X hours later. (That doesn't guarantee, however, that both will have nice cirrus halos.)
The positions are only coincident when the declinations are the same. At most this declination crossing happens 2 times per lunar orbit, or ~7% of the time. However, for a good fraction of time, the lunar declination never reaches the Sun's declination extremes due its orbital precession. This year is a good example. The Moon attains the Sun's declination only around the equinoxes, or about half the time. So for now, coincident positions X hours apart occur about 12 days per year.
Very good. I was (and still am) in the process of rethinking this thing.
Declination/ecliptic crossings (for any given spot on Earth) happen 2 times per lunar orbit/month (~25 times a year) as you say but, of course, the Moon could be at any phase at the time. For the Moon to cross the ecliptic within 3 days of full will happen only ~5 days a year (often around lunar eclipse times). In any event, the overexposed Sun & Moon shown here are probably no closer than ~ 2º in declination anyway.
The scaling to ~5 days makes sense, and this year, the number is 3 which is consistent with an expected ~50% reduction because the number of declination crossings are reduced by about the same fraction.
It turns out the timing for
exact coincidence of the APOD pictures was about 5 hours earlier than actually taken (~4.5° altitude instead of ~30° altitude here). I.e both component images were taken about 5 hours later which was necessary to see the halo too. For the later timings, the altitude error between the Sun and Moon should be ~0.8° (Sun higher). This is very close to a measured error of 1° from the separate component images.
The take-away is a real nice composition was possible without exact position overlap. The combination of FoV and overexposed Sun image helped achieve this.
[quote="neufer"][quote="alter-ego"][quote="neufer"]
...
The probability that the moon be in the "same [u]exact[/u] spot" in the sky
N hours after the sun should be independent of whether N = 0, 10 or 12.
However, the orbits of the Sun & Moon around the Earth cross twice in the full celestial sky so that there is [u]almost always[/u] one position in the sky where the Sun can be found during the day followed by the Moon in the "same [u]exact[/u] spot" X hours later. (That doesn't guarantee, however, that both will have nice cirrus halos.)[/quote]
The positions are only coincident when the declinations are the same. At most this declination crossing happens 2 times per lunar orbit, or ~7% of the time. However, for a good fraction of time, the lunar declination never reaches the Sun's declination extremes due its orbital precession. This year is a good example. The Moon attains the Sun's declination only around the equinoxes, or about half the time. So for now, coincident positions X hours apart occur about 12 days per year.[/quote]
Very good. I was (and still am) in the process of rethinking this thing.
Declination/ecliptic crossings (for any given spot on Earth) happen 2 times per lunar orbit/month (~25 times a year) as you say but, of course, the Moon could be at any phase at the time. For the Moon to cross the ecliptic within 3 days of full will happen only ~5 days a year (often around lunar eclipse times). In any event, the overexposed Sun & Moon shown here are probably no closer than ~ 2º in declination anyway.[/quote]
The scaling to ~5 days makes sense, and this year, the number is 3 which is consistent with an expected ~50% reduction because the number of declination crossings are reduced by about the same fraction.
It turns out the timing for [i]exact[/i] coincidence of the APOD pictures was about 5 hours earlier than actually taken (~4.5° altitude instead of ~30° altitude here). I.e both component images were taken about 5 hours later which was necessary to see the halo too. For the later timings, the altitude error between the Sun and Moon should be ~0.8° (Sun higher). This is very close to a measured error of 1° from the separate component images.
The take-away is a real nice composition was possible without exact position overlap. The combination of FoV and overexposed Sun image helped achieve this.