Originally posted by xoggoth
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The Great Global Warming Swindle
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Originally posted by Marina View PostIt wouldn't have to reflect anything, only fan it out so more goes past Earth without hitting it.Comment
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Originally posted by TimberWolf View PostPresumably positioned at a langrange point? I've not seen the proposal, what's to stop sunlight pushing it out of position, very quickly?
Arthur C Clarke, and more recently Gregory Benford, proposed a Fresnel mirror hovering at the L1 Lagrange Point between the Earth and the Sun.
It would be a large spinning circular disk of thin plastic, several hundred miles across (or lots of smaller ones) with an etched pattern of concentric circles that cause it to act like a concave lens and slightly diverge sunlight in a small range of wavelengths. But as you say, the proportion of sunlight it reflected back would cause a reaction tending to push it in the opposite direction, away from the Sun.
Maybe that could be counteracted by positioning it slightly closer to the Sun so a small net gravitational attraction toward the Sun would counteract the radiation pressure pushing it away. But being closer to the Sun might (?) give it a slight tendency to run ahead of the Earth's orbit. Also, the Moon would be slightly pulling it this way and that all the time. So you'd need thrusters to adjust its position.
Maybe rather than thrusters it would be simpler and cheaper to have several extra "steering mirrors" orbiting closer to the Earth and controlling the main Fresnel lens position by reflecting light onto reverse-facing mirrors round its circumference.Work in the public sector? Read the IR35 FAQ hereComment
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Originally posted by Groover73 View PostYou might think its some left wing conspiracy - but im a Tory voter, ...
Man made globe warming is a load of bollocks; it is a device used by the political class to justify taxes by frighten people out of money. This technique is used over and over again e.g. look at the nasty terrorist threatening your existence, gives us honourable politicians more powers and we'll sort'em. Knobs like you just fall for it hook, line, sinker, fishing rod, angler, river bank ...
See linky for an honest discussion of the science.Comment
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I think you posted the wrong link - that site isn't an "honest discussion of the science", it's just some bozo's blog where he slags off anything that doesn't fit in with his political views, and praises anything that does. No discussion, no intellectual honesty, and definitely no science.
It's a bit like describing this as an honest discussion of modern English literary theory.Comment
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Originally posted by OwlHoot View PostA couple of Earth sunshade proposals are discussed here.
Arthur C Clarke, and more recently Gregory Benford, proposed a Fresnel mirror hovering at the L1 Lagrange Point between the Earth and the Sun.
It would be a large spinning circular disk of thin plastic, several hundred miles across (or lots of smaller ones) with an etched pattern of concentric circles that cause it to act like a concave lens and slightly diverge sunlight in a small range of wavelengths. But as you say, the proportion of sunlight it reflected back would cause a reaction tending to push it in the opposite direction, away from the Sun.
Maybe that could be counteracted by positioning it slightly closer to the Sun so a small net gravitational attraction toward the Sun would counteract the radiation pressure pushing it away. But being closer to the Sun might (?) give it a slight tendency to run ahead of the Earth's orbit. Also, the Moon would be slightly pulling it this way and that all the time. So you'd need thrusters to adjust its position.
Maybe rather than thrusters it would be simpler and cheaper to have several extra "steering mirrors" orbiting closer to the Earth and controlling the main Fresnel lens position by reflecting light onto reverse-facing mirrors round its circumference.
Perhaps a new L1 just has to be calculated to take into account the (in affect) weakened force from the Sun, that would be closer to the Sun than would be the case, if not for the extra forces it is subjected to from sunlight.Last edited by TimberWolf; 7 June 2008, 21:15.Comment
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Originally posted by TimberWolf View Post.. In addition to radiative pressure on the lens from partial reflection, wouldn't the divergent lens be subject to a second force, again radially directed away from the Sun, as it gains momentum lost by light rays bent off the radial (Sun-Earth) axis? ..
But as the transmitted light is slightly divergent, its reaction would be a very slight tendency to "crumple" the lens towards its centre. and over time this would need counteracting by thrusters (or light reflected from the steering lenses I mentioned) to pull the lens back out in its own plane.Last edited by OwlHoot; 7 June 2008, 21:58.Work in the public sector? Read the IR35 FAQ hereComment
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Originally posted by OwlHoot View PostI don't think the transmitted light would have any effect in moving the lens relative to the Sun. Any light absorbed by the lens and re-emitted at longer wavelengths, as heat in effect, would presumably radiate equally in all directions.
But as the transmitted light is slightly divergent, its reaction would be a very slight tendency to "crumple" the lens towards its centre. and over time this would need counteracting by thrusters (or light reflected from the steering lenses I mentioned) to pull the lens back out in its own plane.
I appreciate that crumpling could result as a reaction to photons being refracted in a direction perpendicular to the ecliptic, but I think momentum changes in a radial direction still need to be accounted for.
To clarify let's me try to explain what I mean in 2 dimensions, where say the Sun-Earth axis is called X and photons passing through the lens on this axis are refracted by some perpendicular angle, in a Y direction.
The net force on the lens in the Y direction is zero since photons are homogeneously refracted through the lens. This could potentially crumple the lens if it is weak or not spun though.
In the X direction photons come in on some axis parallel to the X-axis, and are refracted by some amount into the Y-axis. For example lets say they enter parallel to the X-axis and emerge 45 degrees to the X-axis. Momentum of the photons moving at 45 degrees remains as before (unless the photon changes energy?), but along X (and Y) this is now srqt(0.5) respectively. Momentum in the Y direction has been explained without needing to balance any books. To balance momentum lost in the X direction, the lens must experiences a force (sqrt0.5) away from the SunComment
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Originally posted by TimberWolf View PostI appreciate that crumpling could result as a reaction to photons being refracted in a direction perpendicular to the ecliptic, but I think momentum changes in a radial direction still need to be accounted for.
To clarify let's me try to explain what I mean in 2 dimensions, where say the Sun-Earth axis is called X and photons passing through the lens on this axis are refracted by some perpendicular angle, in a Y direction.
The net force on the lens in the Y direction is zero since photons are homogeneously refracted through the lens. This could potentially crumple the lens if it is weak or not spun though.
In the X direction photons come in on some axis parallel to the X-axis, and are refracted by some amount into the Y-axis. For example lets say they enter parallel to the X-axis and emerge 45 degrees to the X-axis. Momentum of the photons moving at 45 degrees remains as before (unless the photon changes energy?), but along X (and Y) this is now srqt(0.5) respectively. Momentum in the Y direction has been explained without needing to balance any books. To balance momentum lost in the X direction, the lens must experiences a force (sqrt0.5) away from the Sun
If that's how it works in two dimensions, I hate to think what the explanation in three dimensions is going to be likeComment
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Originally posted by TimberWolf View PostMomentum of the photons moving at 45 degrees remains as before (unless the photon changes energy
Momentum is a vector quantity, so of course the photon momentum is changed by being bent by the lens in being refracted, I should have said the energy of the photon is unchanged. This isn't precisely true either though, since in order that both energy and momentum are conserved, either a new low energy photon is created (which I find unlikely) or energy is lost by the photon (its wavelength increases) to account for the lens being pushed away from the Sun and gaining some momentum. The lens won't actually get pushed radially away from the Sun, it would tend to be pushed into an elliptical orbit, though by choosing the right (more sunward) L1 position forces could presumably be balanced without needing to supply extra external energy/forces. I do hope we don't need quantum theory to get this solar reflector working... Anyway I imagine the astrophysicist who came up with the idea does this kind of calculation before breakfast.Comment
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