Can anyone explain why the edges of the Moon's shadow on the Earth are so blurry, while the outline of Io's shadow on Jupiter is so sharp?
APOD caption wrote:
The two bright spots that appear on the upper left are thought to be Jupiter and Saturn.
The top spot doesn't look like a planet to me. It looks rectangular.
Are you sure it isn't the ISS? The APOD image is from 1999, and the ISS was launched in 1998. So it could have been.
I don't know, but perhaps because the apparent size of the sun is smaller in Jupiter's sky, Jupiter's moons cast larger shadows on Jupiter?
Re: APOD: Looking Back at an Eclipsed Earth (2024 Mar 24)
Posted: Sun Mar 24, 2024 7:53 am
by Rauf
I tried recreating the moment in Space Engine, which is a space simulator (It has been accurate when I used it before to get the position of Jupiter and other planets in the sky)
I set the date and time to August 11,1999, 10:13 UTC, and the Moon's shadow fell on the UK.
But that's where Jupiter and Saturn were.
I tried to position Jupiter and Saturn near the horizon, but the shadow almost disappeared. I can't say Space Engine (or I) is accurate though!
Re: APOD: Looking Back at an Eclipsed Earth (2024 Mar 24)
Posted: Sun Mar 24, 2024 6:04 pm
by Fred the Cat
Looking back, how would a solar eclipse have appeared millions of years ago? Let’s say during the Eocene. Hard to say but that era had plenty of action here on Earth. Of our extinction events, many had relevance during the epoch.
Okay, that video is almost half an hour long, so I'll summarize it for you. We get a steady stream of low entropy from the Sun, which we can harness to extract useful energy to do all kinds of work for us and create waste products of higher entropy - only to have the Sun give us more low entropy every day, so that we can keep extracting useful energy and doing work and creating waste products of higher entropy every day!
Thank you for our low entropy, Sun!
And in tandem with the Moon, you give us pretty eclipses, too. But I'll leave that for others to discuss.
Ann
Re: APOD: Looking Back at an Eclipsed Earth (2024 Mar 24)
Posted: Sun Mar 24, 2024 6:20 pm
by rwlott
Ann wrote: ↑Sun Mar 24, 2024 5:29 am
Are you sure it isn't the ISS? The APOD image is from 1999, and the ISS was launched in 1998. So it could have been.
Today's APOD, like those on most Sundays, is a repeat, having been used on at least five occasions. It first appeared on 30 Aug 1999, just a few days after the image was snapped, with the statement in the accompanying text that it was taken from aboard the MIR, so I find it highly unlikely that our editors are mistaken.
Russ
Re: APOD: Looking Back at an Eclipsed Earth (2024 Mar 24)
Ann wrote: ↑Sun Mar 24, 2024 5:29 am
Are you sure it isn't the ISS? The APOD image is from 1999, and the ISS was launched in 1998. So it could have been.
Today's APOD, like those on most Sundays, is a repeat, having been used on at least five occasions. It first appeared on 30 Aug 1999, just a few days after the image was snapped, with the statement in the accompanying text that it was taken from aboard the MIR, so I find it highly unlikely that our editors are mistaken.
Russ
I think Ann meant that the bright spot mentioned in APOD could be ISS, and not a planet. It's clear the picture was taken from aboard the Mir
Re: APOD: Looking Back at an Eclipsed Earth (2024 Mar 24)
Posted: Sun Mar 24, 2024 9:30 pm
by Chris Peterson
Ann wrote: ↑Sun Mar 24, 2024 5:29 am
Can anyone explain why the edges of the Moon's shadow on the Earth are so blurry, while the outline of Io's shadow on Jupiter is so sharp?
For a shadow, the umbra is the area from which you cannot see any of the source, and the penumbra is the area where you can see part of the source. The smaller the (angular) size of the light source compared with the shadowing body, the more of the shadow is umbral. For a point source light, there is no penumbra at all... from the viewpoint of the shadow surface, you see the light and then you don't, without any transition. As the Sun appears much smaller from Jupiter, its satellites cast sharper shadows.
If you have good conditions, you can sometimes see this with Venus casting shadows. Stand with Venus at your back and a light colored wall in front of you, and your shadow will be absolutely sharp... quite unlike your shadow when cast by the Sun. And if you pay attention during a solar eclipse (it doesn't have to be total) you'll see your shadow get sharper along one axis.
Re: APOD: Looking Back at an Eclipsed Earth (2024 Mar 24)
Ann wrote: ↑Sun Mar 24, 2024 5:29 am
Can anyone explain why the edges of the Moon's shadow on the Earth are so blurry, while the outline of Io's shadow on Jupiter is so sharp?
For a shadow, the umbra is the area from which you cannot see any of the source, and the penumbra is the area where you can see part of the source. The smaller the (angular) size of the light source compared with the shadowing body, the more of the shadow is umbral. For a point source light, there is no penumbra at all... from the viewpoint of the shadow surface, you see the light and then you don't, without any transition. As the Sun appears much smaller from Jupiter, its satellites cast sharper shadows.
If you have good conditions, you can sometimes see this with Venus casting shadows. Stand with Venus at your back and a light colored wall in front of you, and your shadow will be absolutely sharp... quite unlike your shadow when cast by the Sun. And if you pay attention during a solar eclipse (it doesn't have to be total) you'll see your shadow get sharper along one axis.
Thanks, Chris!
Ann
Re: APOD: Looking Back at an Eclipsed Earth (2024 Mar 24)
Ann wrote: ↑Sun Mar 24, 2024 5:29 am
Are you sure it isn't the ISS? The APOD image is from 1999, and the ISS was launched in 1998. So it could have been.
Today's APOD, like those on most Sundays, is a repeat, having been used on at least five occasions. It first appeared on 30 Aug 1999, just a few days after the image was snapped, with the statement in the accompanying text that it was taken from aboard the MIR, so I find it highly unlikely that our editors are mistaken.
Russ
I think Ann meant that the bright spot mentioned in APOD could be ISS, and not a planet. It's clear the picture was taken from aboard the Mir
Thanks, Rauf!
Ann
Re: APOD: Looking Back at an Eclipsed Earth (2024 Mar 24)
Ann wrote: ↑Sun Mar 24, 2024 5:29 am
Can anyone explain why the edges of the Moon's shadow on the Earth are so blurry, while the outline of Io's shadow on Jupiter is so sharp?
For a shadow, the umbra is the area from which you cannot see any of the source, and the penumbra is the area where you can see part of the source. The smaller the (angular) size of the light source compared with the shadowing body, the more of the shadow is umbral. For a point source light, there is no penumbra at all... from the viewpoint of the shadow surface, you see the light and then you don't, without any transition. As the Sun appears much smaller from Jupiter, its satellites cast sharper shadows.
If you have good conditions, you can sometimes see this with Venus casting shadows. Stand with Venus at your back and a light colored wall in front of you, and your shadow will be absolutely sharp... quite unlike your shadow when cast by the Sun. And if you pay attention during a solar eclipse (it doesn't have to be total) you'll see your shadow get sharper along one axis.
Is that simply because the area of the Sun casting the shadow is decreasing (making it essentially equivalent to a smaller - though now oblong - Sun)?
Re: APOD: Looking Back at an Eclipsed Earth (2024 Mar 24)
Ann wrote: ↑Sun Mar 24, 2024 5:29 am
Can anyone explain why the edges of the Moon's shadow on the Earth are so blurry, while the outline of Io's shadow on Jupiter is so sharp?
For a shadow, the umbra is the area from which you cannot see any of the source, and the penumbra is the area where you can see part of the source. The smaller the (angular) size of the light source compared with the shadowing body, the more of the shadow is umbral. For a point source light, there is no penumbra at all... from the viewpoint of the shadow surface, you see the light and then you don't, without any transition. As the Sun appears much smaller from Jupiter, its satellites cast sharper shadows.
If you have good conditions, you can sometimes see this with Venus casting shadows. Stand with Venus at your back and a light colored wall in front of you, and your shadow will be absolutely sharp... quite unlike your shadow when cast by the Sun. And if you pay attention during a solar eclipse (it doesn't have to be total) you'll see your shadow get sharper along one axis.
Is that simply because the area of the Sun casting the shadow is decreasing (making it essentially equivalent to a smaller - though now oblong - Sun)?
If your light source is a line segment, as opposed to a circle, a shadow cast by an intervening object will have a much broader penumbra on the axis of the line than perpendicular to it. I think that's what you're saying here.
Re: APOD: Looking Back at an Eclipsed Earth (2024 Mar 24)
For a shadow, the umbra is the area from which you cannot see any of the source, and the penumbra is the area where you can see part of the source. The smaller the (angular) size of the light source compared with the shadowing body, the more of the shadow is umbral. For a point source light, there is no penumbra at all... from the viewpoint of the shadow surface, you see the light and then you don't, without any transition. As the Sun appears much smaller from Jupiter, its satellites cast sharper shadows.
If you have good conditions, you can sometimes see this with Venus casting shadows. Stand with Venus at your back and a light colored wall in front of you, and your shadow will be absolutely sharp... quite unlike your shadow when cast by the Sun. And if you pay attention during a solar eclipse (it doesn't have to be total) you'll see your shadow get sharper along one axis.
Is that simply because the area of the Sun casting the shadow is decreasing (making it essentially equivalent to a smaller - though now oblong - Sun)?
If your light source is a line segment, as opposed to a circle, a shadow cast by an intervening object will have a much broader penumbra on the axis of the line than perpendicular to it. I think that's what you're saying here.
Yes, and that the result is as if the Sun were overall smaller (along one axis), making it appear overall "sharper."
Looking Back at an Eclipsed Earth
(Or at least from orbit.) 
But I'll leave that for others to discuss.
