Author Topic: how would zero-radiation pressure sunshades work?  (Read 10087 times)

Offline edzieba

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Re: how would zero-radiation pressure sunshades work?
« Reply #40 on: 01/28/2023 08:18 am »
You posted a graph that correctly showed that the sun blocking dropped as you got closer to the sun from L1. But you need to graph the other side as you get closer to earth.
No. The graph is distance from Earth, not L1.
Quote
A one meter diameter disk blocks more sunlight at 1000 meter than it does at one meter.
Still incorrect, for the reasons I've previously posted. The assumption that light intercepted by an object is proportional to light blocked on a target is invalid for extended sources like the Sun.

Offline ppnl

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Re: how would zero-radiation pressure sunshades work?
« Reply #41 on: 01/28/2023 09:41 pm »
You posted 
A one meter diameter disk blocks more sunlight at 1000 meter than it does at one meter.
Still incorrect, for the reasons I've previously posted. The assumption that light intercepted by an object is proportional to light blocked on a target is invalid for extended sources like the Sun.

No, as long as the penumbra is smaller than the earth this is correct. Every photon that hits the disk would have hit the earth. If you don't believe me then draw the path where the photon would have hit the disk but missed the earth. There is no such path. Simple geometry.



Only when the penumbra is larger than the earth do you have such paths. It is the region of the penumbra that exceeds the radius of the earth that defines those paths. It is geometry, just draw it.

Offline Bizgec

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Re: how would zero-radiation pressure sunshades work?
« Reply #42 on: 01/28/2023 10:11 pm »
You posted 
A one meter diameter disk blocks more sunlight at 1000 meter than it does at one meter.
Still incorrect, for the reasons I've previously posted. The assumption that light intercepted by an object is proportional to light blocked on a target is invalid for extended sources like the Sun.

No, as long as the penumbra is smaller than the earth this is correct. Every photon that hits the disk would have hit the earth. If you don't believe me then draw the path where the photon would have hit the disk but missed the earth. There is no such path. Simple geometry.



Only when the penumbra is larger than the earth do you have such paths. It is the region of the penumbra that exceeds the radius of the earth that defines those paths. It is geometry, just draw it.
Please just stop hand-waving and playing with words and do the maths. Or just read and understand the calculation already posted, which is correct. Your assumptions are wrong, period.

Offline ppnl

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Re: how would zero-radiation pressure sunshades work?
« Reply #43 on: 01/29/2023 01:18 pm »
You posted a graph that correctly showed that the sun blocking dropped as you got closer to the sun from L1. But you need to graph the other side as you get closer to earth.
No. The graph is distance from Earth, not L1.
Quote
A one meter diameter disk blocks more sunlight at 1000 meter than it does at one meter.
Still incorrect, for the reasons I've previously posted. The assumption that light intercepted by an object is proportional to light blocked on a target is invalid for extended sources like the Sun.

It is invalid for extended objects like the sun only at distances large enough  that the penumbra is larger than the earth. A one meter diameter disk blocks more sunlight when it is one meter in the air than it does when on the ground.

How many photons hit the disk on the ground in one second? Lets say a gazillion. How many of those would have hit the earth if not for the disk? All gazillion of them. It blocked a gazillion photons.

How many photons hit the disk one meter up in one seconds? Say a gazillion and three because it is ever so slightly closer to the sun. How many of those photons would have hit the earth if not for the disk? All gazillion and three. It blocked three more photons than the one on the ground.

Until the penumbra is larger than the earth all the photons that hit either disk is blocked from the earth. And one of those disks is hit by more photons because it is closer to the sun. It blocks more photons.

you need to visualize the geometry.


Offline edzieba

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Re: how would zero-radiation pressure sunshades work?
« Reply #44 on: 01/29/2023 11:13 pm »
you need to visualize the geometry.
See the geometry calculated above.

Calculating light hitting the occluder, penumbra sizes, etc, are all calculating the wrong thing: as the goal is to block light from hitting the Earth, then directly calculate light blocked from hitting the Earth. Only then will useful answers on the most effective method of blocking light from hitting the Earth be found.

Offline ppnl

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Re: how would zero-radiation pressure sunshades work?
« Reply #45 on: 01/30/2023 01:35 am »
you need to visualize the geometry.
See the geometry calculated above.

Calculating light hitting the occluder, penumbra sizes, etc, are all calculating the wrong thing: as the goal is to block light from hitting the Earth, then directly calculate light blocked from hitting the Earth. Only then will useful answers on the most effective method of blocking light from hitting the Earth be found.

Do you agree that for a one meter disk on the ground that every photon that hits the disk is a photon shaded from the earth? And if so calculating the number if photons hitting the disk is calculating the shading of the earth?

Offline Proponent

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Re: how would zero-radiation pressure sunshades work?
« Reply #46 on: 03/17/2023 08:03 pm »
Bow before the penumbra, master of us all.

Finally, I see the light, or, rather, the darkness.

It was when I tried to derive McInness's equation that I realized he implicitly assumes that the entirety of the shade is contained within the sun's apparent disk as seen from all points on the sunward side of the earth.  That's equivalent to the entire earth being within the penumbra.

Thank you, ppnl and eriblo, for persisting.  I repent and now too bow before the penumbra, master of us all.
« Last Edit: 03/18/2023 11:55 am by Proponent »

Offline Twark_Main

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Re: how would zero-radiation pressure sunshades work?
« Reply #47 on: 04/13/2023 08:10 am »
However, the point where the apparent diameters of the Sun and the Earth are the same is 1.36•106 km which is closer than L1 (1.5•106 km) so this will not be the case for passive sun shades.
A complete non-sequitur. We do not care at what point in space the Earth and the Sun have the same apparent size, as we are not attempting to block light at an arbitrary point in space. We care about the total coverage of the solar disc from the point of view of the Earth's surface. And in that case, the further an object of fixed size is from the Earth the smaller the region of the solar disc it can cover.

Here's one way to think of it:

The optimum point must occur where the penumbra of the shadow exactly covers the Earth.  No more, no less. This way no shadow is "wasted," but the sunshade is located the closest it can possibly be to the Sun (which increases the photon flux as 1/r2).

In this location, it is pretty easy to show (using similar triangles) that the apparent size of the Sun and Earth must be the same*. :D


* assuming the starshade is much smaller in diameter than the Sun and Earth



« Last Edit: 04/13/2023 09:13 am by Twark_Main »
"The search for a universal design which suits all sites, people, and situations is obviously impossible. What is possible is well designed examples of the application of universal principles." ~~ David Holmgren

 

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