APOD: Sun and Moon Halo (2015 Apr 03)

Comments and questions about the APOD on the main view screen.
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APOD: Sun and Moon Halo (2015 Apr 03)

Post by APOD Robot » Fri Apr 03, 2015 4:11 am

Image Sun and Moon Halo

Explanation: Two pictures captured on April 1 are combined in this creative day and night composite. Separated in time by about 10 hours the images otherwise match, looking along the coast at Östersund Sweden. The relative times were chosen to show the Sun and a nearly full Moon at the same place in the cold, early springtime sky. In the night scene Jupiter also shines above the waterfront lights, while Sun and Moon are both surrounded by a beautiful circular ice halo. The Sun and Moon halos really do align, each with an angular radius of 22 degrees. That radius is a constant, not determined by the brightness of Sun or Moon but only by the hexagonal geometry of atmospheric ice crystals and the reflection and refraction of light. Of course tomorrow, April 4, will find the Sun and Moon on opposite sides of planet Earth for a total lunar eclipse.

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RocketRon

Re: APOD: Sun and Moon Halo (2015 Apr 03)

Post by RocketRon » Fri Apr 03, 2015 6:02 am

Great compostion.

How often would the moon be in the same exact spot in the sky 12 hours after the sun ?

P.S. This discussion was difficult to get to, something isn't linked correctly. ?
And there seem to be 2 posts for April 2nd.

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Re: APOD: Sun and Moon Halo (2015 Apr 03)

Post by Ann » Fri Apr 03, 2015 6:25 am

RocketRon wrote:Great compostion.

How often would the moon be in the same exact spot in the sky 12 hours after the sun ?
Actually, the pictures were taken 10 hours apart. But anyway, we know that the Sun and the Moon can be seen in exactly the same spot in the sky sometimes, otherwise we wouldn't get any solar eclipses.

To me, the most interesting thing about this composite is the relative colors, with the Moon picture looking much bluer than the Sun picture. Isn't that weird? Shouldn't moonlight be yellower than sunlight? After all, the Moon is a very dark reddish-brown in color, so that it ought to reflect sunlight in a yellower shade than the light coming to us directly from the Sun. But still the moonlight scene looks bluer. I'm baffled!

But this is a great shot, so congratulations, Göran Strand!

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Re: APOD: Sun and Moon Halo (2015 Apr 03)

Post by geckzilla » Fri Apr 03, 2015 6:44 am

RocketRon wrote:P.S. This discussion was difficult to get to, something isn't linked correctly. ?
And there seem to be 2 posts for April 2nd.
Yeah, there are a couple of mistakes on today's APOD. The editors were informed (thanks, bystander!) but they are presumably asleep.
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Re: APOD: Sun and Moon Halo (2015 Apr 03)

Post by Ann » Fri Apr 03, 2015 7:27 am

I have to say that I like the small cloud formation close to the horizon in the sunlit part of the image. It resembles Superman flying by, inspecting something on the ground, or maybe an old-fashioned springboard diver who missed the lake and is going to crash! :shock:

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RocketRon

Re: APOD: Sun and Moon Halo (2015 Apr 03)

Post by RocketRon » Fri Apr 03, 2015 8:58 am

My apologies, 10 hours.
Was a while between reading and posting.

Would the sun then moon being in the same spot always be 10 hours interval,
or would the moon take the same track as the sun - that day (only ?).
Not being totally familiar with celestial mechanics....

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Re: APOD: Sun and Moon Halo (2015 Apr 03)

Post by neufer » Fri Apr 03, 2015 11:27 am

Ann wrote:
RocketRon wrote:
How often would the moon be in the same exact spot in the sky 12 hours after the sun ?
Actually, the pictures were taken 10 hours apart.

But anyway, we know that the Sun and the Moon can be seen in exactly the same spot in the sky sometimes, otherwise we wouldn't get any solar eclipses.
Ann's analogy is apt.

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.)
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Re: APOD: Sun and Moon Halo (2015 Apr 03)

Post by alter-ego » Fri Apr 03, 2015 4:50 pm

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.
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Re: APOD: Sun and Moon Halo (2015 Apr 03)

Post by neufer » Fri Apr 03, 2015 6:34 pm

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.
Last edited by neufer on Sat Apr 04, 2015 3:16 am, edited 2 times in total.
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hoohaw

Re: APOD: Sun and Moon Halo (2015 Apr 03)

Post by hoohaw » Fri Apr 03, 2015 10:55 pm

Ann wrote:I have to say that I like the small cloud formation close to the horizon in the sunlit part of the image. It resembles Superman flying by, inspecting something on the ground, or maybe an old-fashioned springboard diver who missed the lake and is going to crash! :shock: Ann
Up, up, and away! Thanks, Ann, I'd not noticed Superman!

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Re: APOD: Sun and Moon Halo (2015 Apr 03)

Post 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.
A pessimist is nothing more than an experienced optimist