h a l f b a k e r yExpensive, difficult, slightly dangerous, not particularly effective... I'm on a roll.
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Jim has a problem with the a near star parallax angle taken 6 months apart. The problem is the velocity of the sun (and everything else but me for that matter)...
Jim still reckons it is better to take an exact measure of the baseline from two distant points, at the same time, because this is a less
questionable measurement.
...
So the state of the art is a chord of an earth orbit or perhaps a lunar baseline unless we put a satellite into a solar orbit.
Parallax Angle
http://en.wikipedia.org/wiki/Parallax
The distance to the stars if nothing moves but me... [madness, Feb 16 2010]
The Parallax View
http://www.imdb.com/title/tt0071970/
Nothing to do with this idea at all. It's merely here by coincidence. [DrBob, Feb 16 2010]
AU
http://en.wikipedia...i/Astronomical_unit
[madness, Feb 16 2010]
Eratosthenes measures the circumference of the earth
http://www.juliantr...n/eratosthenes.html
This method provides the same answers whether you measure the lengths of your shadows simultaneously at two different locations, or if you take independent measurements at the two locations one year apart - it is over 2000 years old. [zen_tom, Feb 16 2010]
[link]
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What happened to my anno? |
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sp. "distant", "chord", "state" |
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You really need to give Jim a bit of an education. |
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You can correct completely for the velocity of the sun by
taking three sightings at 6-month intervals. Then you can
null out the parallax shift which is due to the movement of
the sun. |
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Ok what about the parallax shift due to movement of the near star and the thing you are trying to measure the distance too... |
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(This sounds like a methodology developed a couple of centuries ago... (I deleted the idea but then added it back because it still bugs me (along with red shift measurements))) |
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You mean "what if the star whose distance you want to
know is moving relative to the distant background of stars?
" |
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Again, three measurements at 6 month intervals can be
used to null-out all apparent parallaxes, except for the
true parallax caused by the earth's orbital motion. (Unless
you assume that large parts of the galaxy are moving to
and fro in synch with the earth). |
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Redshifts are a whole other kettle of worms, but work for
more distant stars than can be measured with parallax. |
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Nah --- it seems that AU is just an estimate. |
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Compensation needs to be made for changes in mass and velocity of the sun, near star, target star and any other body that was used in this calculation. |
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I would rather just depend on a single value --- the speed of light. (Assuming only that there is nothing between the two baseline points that will interfere with it.) |
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You decrease your separation, and thus precision by about 5 orders of magnitude if you use geostationary satelites relative to 6 month orbital difference. Considerably more if you use LEO satelites or terrestrial surface (even ignoring atmospheric issues). |
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Yip --- so the state of the art is not good enough... |
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Nope, you're proposing something that is considerably worse than state of the art, since state of the art does use the orbital motion. |
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There's also the fact that a two point measurement can miss other factors such as gravitational lensing, something that multiple shots over time have a chance of correcting for. |
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There is no reason not to take multiple measurements using a line of sight baseline (or line of sight baseplane)... |
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//Jim still reckons it is better to take an exact measure of the baseline from two distant points, at the same time, because this is a less questionable measurement.// |
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On one hand, that's a kind of obvious statement - if you want to triangulate the position of something, and your angle measuring device has an implicit level of reliability say ±1%, then to increase your accuracy, you need to increase the distances of your triangulation points - Eratosthenes measured the earth's circumference in 236 BC by triangulating the position of the sun from two locations a known distance apart knew that then, so I don't see how this is a new idea unless we determine "newness" in geological terms. |
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On the other hand, as we increase the distances involved and the non-uniform, non Euclidean nature of space and time comes into play, the statement becomes less obvious because triangulation works under the assumption that you are operating in Euclidean space. If Jim's worried that our measurements aren't to the nearest nanometre, he might try applying his brain to that problem, rather than rehashing things that we've known for more than 2000 years. [marked-for-deletion] not a new idea. |
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If we take the idea further, then it becomes even harder to implement as we try to determine a simultaneous time-point at distant locations - at distances of 1AU or more, the word "simultaneous" starts getting relativistic. |
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Of course, if there were some cosmic phenomenomom that
resulted in a hologram-like effect, it might turn out that the
nearest star is only a long bus ride away. |
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And further more lets say I want to know the position and velocity of a distant star now --- the orbital method cannot provide me the answer but a line of sight baseline can... |
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Even further a line of sight base line with a solar satellite will have half the baseline of the orbital method and no requirement for any compensation what so ever. |
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Further still a line of sight base line to an outer planet exceeds the accuracy of 1AU and still does not require compensation of any kind --- given the assumption mentioned earlier. |
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// it might turn out that the nearest star is only a long bus ride away // Which is disheartening for those of us who ride the short bus. |
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