Starship Asterisk*

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.

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?

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

)

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.

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.

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.)

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.

Click to view full size image

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.

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.

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.

: Click to view full size image

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!

by **heehaw** » Sat Nov 05, 2022 11:32 am

Fun, fun, fun: blow up the picture, and you will spot the two people!

by **APOD Robot** » Sat Nov 05, 2022 4:08 am

Lunar Eclipse at the South Pole

Explanation: Last May 16 the Moon slid through Earth's shadow, completely immersed in the planet's dark umbra for about 1 hour and 25 minutes during a total lunar eclipse. In this composited timelapse view, the partial and total phases of the eclipse were captured as the Moon tracked above the horizon from Amundsen-Scott South Pole Station. There it shared a cold and starry south polar night with a surging display of the aurora australis and central Milky Way. In the foreground are the BICEP (right) and South Pole telescopes at the southernmost station's Dark Sector Laboratory. But while polar skies can be spectacular, you won't want to go to the South Pole to view the total lunar eclipse coming up on November 8. Instead, that eclipse can be seen from locations in Asia, Australia, the Pacific, the Americas and Northern Europe. It will be your last chance to watch a total lunar eclipse until 2025.

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