How Do We Know How Far Away a Star Is?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Maddad, Jan 15, 2005.

  1. Maddad Time is a Weighty Problem Registered Senior Member

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    A paralax measurement, taking a star's position once now and once six months from now from the other side of Earth's orbit, will tell you how close the nearby stars are. The narrower the angle between the two sightings, the further away the star is.

    However, this only works if you know how far the Earth is from the Sun. How do we know that?
     
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  3. vslayer Registered Senior Member

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    we get 2 satellites a certian distance apart to record when light hits them, then triangulate
     
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  5. one_raven God is a Chinese Whisper Valued Senior Member

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    How do we know when the light left?
     
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  7. eburacum45 Valued Senior Member

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    This site gives a good description of the first steps on the ladder;
    http://www.astro.washington.edu/labs/eratosthenes/rung1.html

    these methods were sufficiently accurate to give a good approximation of the distance even in Greek times. Now that the distance to the Moon has been very accurately measured we can be pretty sure of distances out to the limits of accuracy of the Hipparchos satellite, which used the parallax method.

    Unfortunately there is a gap in accuracy between the most distant accurate Hipparchos measurements and the closest Cepheid variables, which means there is some inaccuracy in all measurements using those stars as standard candles. Still, the discrepancies are hopefully not too bad, and will decrease further when more accurate measurements are available.
     
  8. Maddad Time is a Weighty Problem Registered Senior Member

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    eburacum45
    I am a fan of Eratosthenes. That he came up with a reasonably accurate method of approximating the size of the Earth 2,200 years ago is nothing short of astonishing. However, this gives us the size of the Earth, not the distance from the Earth to the Sun. We had no method of determining this until the 20th century.
     
  9. eburacum45 Valued Senior Member

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    Sorry; I should have directed you to the next few steps in the ladder. Because they were great geometers, the Greeks could also estimate the distance of the Moon, and using Pythagoras' theorem, the Sun.
    http://www.astro.washington.edu/labs/eratosthenes/rung2.html
    http://www.astro.washington.edu/labs/eratosthenes/rung3.html
    Aristarchus was actually wrong by a factor of twenty; which is not really good enough, when all is said and done; but if better measurements had been available the value could have been much better.
     
    Last edited: Jan 15, 2005
  10. Nasor Valued Senior Member

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    You don't need to know when the light left, you just need to know the difference between the times it took light to hit the two satellites.
     
  11. Bowser Namaste Valued Senior Member

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    How do we account for the gravitational forces on light? Isn't it safe to assume the possibility that a distant point of light in the sky has been pulled, bent, and altered to such a degree by gravitational forces that its true position could never be determined by earthly observation?
     
  12. eburacum45 Valued Senior Member

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    Most light from nearby stars has not passed through any strong gravity wells, so is not detectably bent. However the light from the most distant galaxies in the Hubble deep field images has passed by many closer galaxies and is often affected by gravitational lensing.
     
  13. blobrana Registered Senior Member

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  14. Maddad Time is a Weighty Problem Registered Senior Member

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    eburacum45
    Hey, I like that use of trig to get an absolute (as opposed to realtive) measurement of the distance from the Earth to the Sun. It doesn't actually matter that Aristarchus was off by a factor of 20. The method was correct, so with better starting measurements the final answer would have been better.

    What we use today is radar. We bounce a beam off a nearby planet like Venus or Mars and measure the time it takes for the beam to return to us. It's like wrapping a blindfold around blobrana's eyes and sending him trundling off down the hallway. We know how fast he walks, so after he bounces off the far wall and returns, we look at our stopwatch to see how long it took for him to return with the goober on his forehead. That tells us how far away the wall was.

    In the same manner, we learn how far away objects are within about 100 astronomical units. When we learn how far away Venus is, we then apply Keplarian laws to figure out how far we are from the Sun.
     
  15. one_raven God is a Chinese Whisper Valued Senior Member

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    How do you know how long it took the light to reach the sattelites if you don't know what time it left?

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    To know how long it took youwould have to know what time it started and what time it left, no?
    If I arrived at your house at 5:16 PM, how long did it take for me to get there from my house?
    You don't know unless I told you what time I left my house.

    I know I must seem very thick right now, but what am I missing?
     
  16. Boris2 Valued Senior Member

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    >>>>I know I must seem very thick right now, but what am I missing?

    just a guess. they know the distance between the satellites. both satellites measure their angle to a feature on the Sun. so you have a base and two angles. maybe.

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  17. Nasor Valued Senior Member

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    Multiply the difference in times by the speed of light, and you know how much closer one satellite is than the other. Now you have a triangle where one side it the distance between the satellites, one side is the distance from your first satellite to the sun (distance D), and the other is the distance from the second satellite to the sun, which is D + some known distance. With that, you can compute the length of D and figure out how far away the sun is.
     

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