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03/08/2006 07:44:00 PM · #26 |
Originally posted by American_Horse:
Axis 'A' is winter time on Earth furthest from the Sun, axis 'B' is summer time on the Earth when it is closest to the Sun.
Now, the Sun is a huge light source even from axis 'A'.
Here is the question.
If weather did not exist. If I set up a light meter and everything being even, and metered the axis of 'A' and 'B', will there be a differance in the f stop of light. |
Actually, in the Northern hemisphere, the earth is farthest from the sun in summer and closest in winter. So your initial statement is only true south of the equator. It's the wobbling of the earth's axis that gives us the seasons.
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03/08/2006 07:49:44 PM · #27 |
| The inverse square law applies only to a point source of light. The Solar disc has an apparent intercept angle of approximately 0.5 degrees when viewed from Earth. However, the function is close enough that inverse square calculations will be very close! |
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03/08/2006 08:04:26 PM · #28 |
Originally posted by ElGordo:
The inverse square law applies only to a point source of light. The Solar disc has an apparent intercept angle of approximately 0.5 degrees when viewed from Earth. However, the function is close enough that inverse square calculations will be very close! |
Hee hee, true dat! I dare you to compute & post the error in the inverse square approximation for this example!
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03/08/2006 08:09:21 PM · #29 |
Originally posted by joebok:
No - you can't just square the %. Say the distance at closest is 1. The amount of light reaching the earth depends on the square of the distance (actually the inverse square) - say k/1^2. The amount of light at a further distance, 1.033 (a 3.3% increase) is k/(1.033)^2.
Ratio, the k's cancel and you have 1^2/(1.033)^2 = .937..., or about a 6.287% decrease. |
Absolutely right Joe, sloppy math on my part. |
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03/08/2006 08:12:55 PM · #30 |
Originally posted by strangeghost:
Originally posted by joebok:
No - you can't just square the %. Say the distance at closest is 1. The amount of light reaching the earth depends on the square of the distance (actually the inverse square) - say k/1^2. The amount of light at a further distance, 1.033 (a 3.3% increase) is k/(1.033)^2.
Ratio, the k's cancel and you have 1^2/(1.033)^2 = .937..., or about a 6.287% decrease. |
Absolutely right Joe, sloppy math on my part. |
Good to know I wasn't alone in that!
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03/08/2006 08:22:41 PM · #31 |
Originally posted by kirbic:
Originally posted by ElGordo:
The inverse square law applies only to a point source of light. The Solar disc has an apparent intercept angle of approximately 0.5 degrees when viewed from Earth. However, the function is close enough that inverse square calculations will be very close! |
Hee hee, true dat! I dare you to compute & post the error in the inverse square approximation for this example! |
The apparent size of the disk of the sun should also vary as the inverse square of distance from earth. The disk of the sun consists of a large number of point light sources that do conform to the inverse square law. At aphelion then there would be 3.3% fewer point light sources to also account for. This means that both the luminosity decrease based on distance and the decreased size of the solar disk based on distance should both be accounted for together. So instead of just 3.3% change in distance there is also a 3.3% difference in disk size. That results in an overall 6.6% decrease which is about a 43% decrease in total light reaching Earth at aphelion. That is a little under 1/2 of an f/stop difference... I think! LOL!!!
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03/08/2006 08:27:05 PM · #32 |
So everyone can actually take a look at what we're talking about here, here's a link to a photographic project I did a few years ago with slide film.
Enjoy! |
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03/08/2006 08:42:24 PM · #33 |
Originally posted by stdavidson:
Originally posted by kirbic:
Originally posted by ElGordo:
The inverse square law applies only to a point source of light. The Solar disc has an apparent intercept angle of approximately 0.5 degrees when viewed from Earth. However, the function is close enough that inverse square calculations will be very close! |
Hee hee, true dat! I dare you to compute & post the error in the inverse square approximation for this example! |
The apparent size of the disk of the sun should also vary as the inverse square of distance from earth. The disk of the sun consists of a large number of point light sources that do conform to the inverse square law. At aphelion then there would be 3.3% fewer point light sources to also account for. This means that both the luminosity decrease based on distance and the decreased size of the solar disk based on distance should both be accounted for together. So instead of just 3.3% change in distance there is also a 3.3% difference in disk size. That results in an overall 6.6% decrease which is about a 43% decrease in total light reaching Earth at aphelion. That is a little under 1/2 of an f/stop difference... I think! LOL!!! |
Nope, you don't need to "double up" the effect. The actual error in the point source approximation is incredibly small at the Sun's angular size, but the math can be either simple or pretty involved, depending on which way you attack it ;-)
Here's a related problem: The Sun's surface radiates a given amount of power per unit area. If a viewer is some distance from an infinitely large rectangular light source that also radiates a given amount of power per unit area, then moves twice as far away, how much does the amount of light he sees decrease? Hint: no calculator needed.
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03/08/2006 09:11:48 PM · #34 |
If the source has an infinitely large surface of uniform luminosity, then there will be no attenuation of the light striking any point at any finite distance from the object. Difficult to visualize!! (This presumes a perfect transmission media)
Message edited by author 2006-03-08 21:17:31. |
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03/08/2006 09:34:15 PM · #35 |
Originally posted by ElGordo:
If the source has an infinitely large surface of uniform luminosity, then there will be no attenuation of the light striking any point at any finite distance from the object. Difficult to visualize!! (This presumes a perfect transmission media) |
Wow, didn't take long to get that one answered!
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03/09/2006 10:41:01 PM · #36 |
Originally posted by kirbic:
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Originally posted by ElGordo:
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The inverse square law applies only to a point source of light. The Solar disc has an apparent intercept angle of approximately 0.5 degrees when viewed from Earth. However, the function is close enough that inverse square calculations will be very close!
Hee hee, true dat! I dare you to compute & post the error in the inverse square approximation for this example!
The apparent size of the disk of the sun should also vary as the inverse square of distance from earth. The disk of the sun consists of a large number of point light sources that do conform to the inverse square law. At aphelion then there would be 3.3% fewer point light sources to also account for. This means that both the luminosity decrease based on distance and the decreased size of the solar disk based on distance should both be accounted for together. So instead of just 3.3% change in distance there is also a 3.3% difference in disk size. That results in an overall 6.6% decrease which is about a 43% decrease in total light reaching Earth at aphelion. That is a little under 1/2 of an f/stop difference... I think! LOL!!!
So with this scientific explanation, does this mean that the earth is in some sort of Zonal Range? Some sort of xyz in time and space with differant sources compared to each other to come up with some figures, one of which is yours?
I need a drink.
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03/09/2006 10:52:03 PM · #37 |
Originally posted by American_Horse:
So with this scientific explanation, does this mean that the earth is in some sort of Zonal Range? Some sort of xyz in time and space with differant sources compared to each other to come up with some figures, one of which is yours?
I need a drink. |
Are you planning some sort of take over while you keep thier minds occupied? LOL
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