Comments and questions about the
APOD on the main view screen.
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heehaw
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by heehaw » Sat Nov 05, 2022 11:32 am
Fun, fun, fun: blow up the picture, and you will spot the two people!
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orin stepanek
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by orin stepanek » Sat Nov 05, 2022 11:44 am
Too cold for me!
IMO! From this map; it looks like Hawaii may have a great view point!
Orin
Smile today; tomorrow's another day!
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johnnydeep
- Commodore
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by johnnydeep » Sat Nov 05, 2022 12:10 pm
The extreme linearity of the path of the moon across the sky perplexes me. And it's parallel to the horizon to boot. Is this something special due to this being at the south pole, or would the path be so linear everywhere on earth? Sadly, I expect my 3D sense is failing me yet again.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
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Chris Peterson
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by Chris Peterson » Sat Nov 05, 2022 2:02 pm
johnnydeep wrote: ↑Sat Nov 05, 2022 12:10 pm
The extreme linearity of the path of the moon across the sky perplexes me. And it's parallel to the horizon to boot. Is this something special due to this being at the south pole, or would the path be so linear everywhere on earth? Sadly, I expect my 3D sense is failing me yet again.
How could the path of the Moon over a few hours be anything other than linear? It travels on a plane, and that plane doesn't substantially change its angle with respect to an observer on Earth over a short time.
I think you're confusing the actual path with the apparent path on an image.
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Sa Ji Tario
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by Sa Ji Tario » Sat Nov 05, 2022 2:18 pm
johnnydeep wrote: ↑Sat Nov 05, 2022 12:10 pm
The extreme linearity of the path of the moon across the sky perplexes me. And it's parallel to the horizon to boot. Is this something special due to this being at the south pole, or would the path be so linear everywhere on earth? Sadly, I expect my 3D sense is failing me yet again.
We are used to the Sun rising over the horizon, culminating and disappearing at sunset, at the poles things are different. Since spring began in the southern hemisphere, the Sun begins to illuminate the area of the Antarctic maximum circle, these days it only rises a little and sets for a few hours, when the beginning of summer arrives it will illuminate the south pole 24 hours a day, turning above the horizon 360º without setting, as the Moon is doing now, observe that the Moon does not rise or fall, it moves horizontally as the Sun would. Due to the luminance, the leafy vegetables that are planted in the south also they develop in half the time as at the equator, there are typically 20 hours of sunlight during the southern spring and summer, algae and phytoplankton in the sea do.
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johnnydeep
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by johnnydeep » Sat Nov 05, 2022 7:43 pm
Chris Peterson wrote: ↑Sat Nov 05, 2022 2:02 pm
johnnydeep wrote: ↑Sat Nov 05, 2022 12:10 pm
The extreme linearity of the path of the moon across the sky perplexes me. And it's parallel to the horizon to boot. Is this something special due to this being at the south pole, or would the path be so linear everywhere on earth? Sadly, I expect my 3D sense is failing me yet again.
How could the path of the Moon over a few hours be anything other than linear? It travels on a plane, and that plane doesn't substantially change its angle with respect to an observer on Earth over a short time.
I think you're confusing the actual path with the apparent path on an image.
I guess I was thinking that at the poles, the stars paths are circles around the poles, so why wouldn't the moon's path look similar? Plus, this pic apparently shows the moon's progress over 1.5 hours, which is 16th of the total 24 hour path, so why doesn't it show a slight curve? (Of course, the moon is also orbiting the earth at 12.5 degrees per day, but over 16th of a day that wouldn't affect the apparent path much.)
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"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
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Chris Peterson
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by Chris Peterson » Sat Nov 05, 2022 7:54 pm
johnnydeep wrote: ↑Sat Nov 05, 2022 7:43 pm
Chris Peterson wrote: ↑Sat Nov 05, 2022 2:02 pm
johnnydeep wrote: ↑Sat Nov 05, 2022 12:10 pm
The extreme linearity of the path of the moon across the sky perplexes me. And it's parallel to the horizon to boot. Is this something special due to this being at the south pole, or would the path be so linear everywhere on earth? Sadly, I expect my 3D sense is failing me yet again.
How could the path of the Moon over a few hours be anything other than linear? It travels on a plane, and that plane doesn't substantially change its angle with respect to an observer on Earth over a short time.
I think you're confusing the actual path with the apparent path on an image.
I guess I was thinking that at the poles, the stars paths are circles around the poles, so why wouldn't the moon's path look similar? Plus, this pic apparently shows the moon's progress over 1.5 hours, which is 16th of the total 24 hour path, so why doesn't it show a slight curve? (Of course, the moon is also orbiting the earth at 12.5 degrees per day, but over 16th of a day that wouldn't affect the apparent path much.)
The stars appear to rotate around the poles because of Earth's rotation. The closer a star is to a pole, the smaller the circle it traces out over a day. If you are at a pole, stars at 0° declination hug the horizon, and will not show any circular motion at all, just travel parallel to the horizon. Which is what we see the Moon doing here. When the eclipse occurred, the Moon was at a declination of -18°, and thus an altitude of +18°. So quite close to the horizon.
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johnnydeep
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by johnnydeep » Sat Nov 05, 2022 8:16 pm
Chris Peterson wrote: ↑Sat Nov 05, 2022 7:54 pm
johnnydeep wrote: ↑Sat Nov 05, 2022 7:43 pm
Chris Peterson wrote: ↑Sat Nov 05, 2022 2:02 pm
How could the path of the Moon over a few hours be anything other than linear? It travels on a plane, and that plane doesn't substantially change its angle with respect to an observer on Earth over a short time.
I think you're confusing the actual path with the apparent path on an image.
I guess I was thinking that at the poles, the stars paths are circles around the poles, so why wouldn't the moon's path look similar? Plus, this pic apparently shows the moon's progress over 1.5 hours, which is 16th of the total 24 hour path, so why doesn't it show a slight curve? (Of course, the moon is also orbiting the earth at 12.5 degrees per day, but over 16th of a day that wouldn't affect the apparent path much.)
The stars appear to rotate around the poles because of Earth's rotation. The closer a star is to a pole, the smaller the circle it traces out over a day. If you are at a pole, stars at 0° declination hug the horizon, and will not show any circular motion at all, just travel parallel to the horizon. Which is what we see the Moon doing here. When the eclipse occurred, the Moon was at a declination of -18°, and thus an altitude of +18°. So quite close to the horizon.
But shouldn't it still be slightly curved and shown in the red path for an observer at the south pole (the original pic is from
http://hildaandtrojanasteroids.net/stars3regions.jpg, which doesn't work in the img tags for some reason):
Oh wait, I think I now see what you meant in your first reply: that the path is indeed curved on the celestial sphere, but since the observer is embedded at the center of that circular path, s/he would only see a straight line. My perpetual problem seems to be that I always want to take the universal observer-independent viewpoint which makes it extra hard to appreciate actually
being an observer at a particular location. Definitely
not good for understanding practical astronomy and astrophotography.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
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alter-ego
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by alter-ego » Sun Nov 06, 2022 4:18 am
johnnydeep wrote: ↑Sat Nov 05, 2022 7:43 pm
Chris Peterson wrote: ↑Sat Nov 05, 2022 2:02 pm
johnnydeep wrote: ↑Sat Nov 05, 2022 12:10 pm
The extreme linearity of the path of the moon across the sky perplexes me. And it's parallel to the horizon to boot. Is this something special due to this being at the south pole, or would the path be so linear everywhere on earth? Sadly, I expect my 3D sense is failing me yet again.
How could the path of the Moon over a few hours be anything other than linear? It travels on a plane, and that plane doesn't substantially change its angle with respect to an observer on Earth over a short time.
I think you're confusing the actual path with the apparent path on an image.
I guess I was thinking that at the poles, the stars paths are circles around the poles, so why wouldn't the moon's path look similar? Plus, this pic apparently shows the moon's progress over 1.5 hours, which is 16th of the total 24 hour path, so why doesn't it show a slight curve? (Of course, the moon is also orbiting the earth at 12.5 degrees per day, but over 16th of a day that wouldn't affect the apparent path much.)
Without any disagreement wrt all the posts here, I want to comment on this composite.
FYI, for all practical purposes the correct 50° nominally parallel path (in azimuth) is displayed with maximum eclipse correctly positioned against the stars. However, there's one minor fault in the lunar path detail. Over the course of the composite image, the true lunar altitude changes by ≈ +0.7°(higher altitude,) because of the (more southerly) change in declination (~-12arcmin/hour). Interestingly, with close inspection, the composite image path shows a decrease (not increase) in altitude by close to the same factor. It could be there was a minus sign error in generating the lunar path, or just a coincidence (pretty common in my experience
)
A pessimist is nothing more than an experienced optimist
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Chris Peterson
- Abominable Snowman
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by Chris Peterson » Sun Nov 06, 2022 4:22 am
alter-ego wrote: ↑Sun Nov 06, 2022 4:18 am
johnnydeep wrote: ↑Sat Nov 05, 2022 7:43 pm
Chris Peterson wrote: ↑Sat Nov 05, 2022 2:02 pm
How could the path of the Moon over a few hours be anything other than linear? It travels on a plane, and that plane doesn't substantially change its angle with respect to an observer on Earth over a short time.
I think you're confusing the actual path with the apparent path on an image.
I guess I was thinking that at the poles, the stars paths are circles around the poles, so why wouldn't the moon's path look similar? Plus, this pic apparently shows the moon's progress over 1.5 hours, which is 16th of the total 24 hour path, so why doesn't it show a slight curve? (Of course, the moon is also orbiting the earth at 12.5 degrees per day, but over 16th of a day that wouldn't affect the apparent path much.)
Without any disagreement wrt all the posts here, I want to comment on this composite.
FYI, for all practical purposes the correct 50° nominally parallel path (in azimuth) is displayed with maximum eclipse correctly positioned against the stars. However, there's one minor fault in the lunar path detail. Over the course of the composite image, the true lunar altitude changes by ≈ +0.7°(higher altitude,) because of the (more southerly) change in declination (~-12arcmin/hour). Interestingly, with close inspection, the composite image path shows a decrease (not increase) in altitude by close to the same factor. It could be there was a minus sign error in generating the lunar path, or just a coincidence (pretty common in my experience :))
How are you measuring that?
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alter-ego
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by alter-ego » Sun Nov 06, 2022 5:22 am
Chris Peterson wrote: ↑Sun Nov 06, 2022 4:22 am
alter-ego wrote: ↑Sun Nov 06, 2022 4:18 am
johnnydeep wrote: ↑Sat Nov 05, 2022 7:43 pm
I guess I was thinking that at the poles, the stars paths are circles around the poles, so why wouldn't the moon's path look similar? Plus, this pic apparently shows the moon's progress over 1.5 hours, which is 16th of the total 24 hour path, so why doesn't it show a slight curve? (Of course, the moon is also orbiting the earth at 12.5 degrees per day, but over 16th of a day that wouldn't affect the apparent path much.)
Without any disagreement wrt all the posts here, I want to comment on this composite.
FYI, for all practical purposes the correct 50° nominally parallel path (in azimuth) is displayed with maximum eclipse correctly positioned against the stars. However, there's one minor fault in the lunar path detail. Over the course of the composite image, the true lunar altitude changes by ≈ +0.7°(higher altitude,) because of the (more southerly) change in declination (~-12arcmin/hour). Interestingly, with close inspection, the composite image path shows a decrease (not increase) in altitude by close to the same factor. It could be there was a minus sign error in generating the lunar path, or just a coincidence (pretty common in my experience
)
How are you measuring that?
Yeah, good question. You just beat me to a correction edit I was going to make.
I made the assumption that the horizon is parallel to the long axis of the image, which is not necessarily correct, so I cannot claim that path error. In fact, the visible ground is sloped such that the lunar path yields a (more correct) positive altitude. However, I believe the local horizon can be deduced from the image, and I won't be surprised if there is no conclusive lunar path error. I enjoy these kinds of problems, so will pursue this to see where it goes.
A pessimist is nothing more than an experienced optimist