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Calculating distance to object?

  1. Mar 15, 2007 #1
    First I would like to say sorry for registering just to ask this question. I tried searching but its kind of a long search term. If there is another thread you know of just linking it is fine.

    How do astronomers calculate the distance to an object? You can use the sun as the object. It can be a simple explanation, I just want to know the basics.
  2. jcsd
  3. Mar 15, 2007 #2
    There are several ways.

    First, for close objects stick your finger close to your face and blink your eyes, one on and one off and then the other on and the first one off. Note how far your finger moves. Now move your finger out as far as your arm will go and blink your eyes again. Note how far your finger moves. It didn't move as far did it? This same technique can be used in a grand scale with stars. When the Earth is on one side of its orbit, the location of the target star is noted against the background. Then on the other side of the orbit, it is noted again. Some simple trig and wola! The answer pops out. This is good for a few thousand light years.

    Out further, they use Cepheid variable stars. These stars vary in a cycle and the length of the cycle is related to the brightness of the star. This can be used out to the near by galaxies.

    Finally, how much the objects are red-shifted (which is dependent on how fast the objects are moving away from us) shows very distant distance. The older the object the greater the red-shift because the universe is expanding.
  4. Mar 15, 2007 #3
    If general relativity is correct, then this is a difficult and subtle problem in theory.

    In practice, distances are often calculated by comparing the amount of detected starlight with the total quantity of light which that type of star is thought to emit (same as guessing the distance of a car by the brightness of its headlights), but you're relying on the models for the different types of star.

    Another important method is to look at the object from different positions, just like the parrallax between your two eyes lets you judge distances.

    In the case of the moon (or separations on earth), these days we can simply time laser signals to measure its distance.

    I think the distance to the sun was found using the length of the year and the weight of the earth.
  5. Mar 17, 2007 #4
    Alright, thanks.
  6. Mar 17, 2007 #5


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    There are a lot of different methods that astronomers use to measure distances. Ned Wright has a nice summary:

    Distance Ladder
  7. Mar 18, 2007 #6
    One word: Trignometry
    Two words: Trignometry, and Angular Size

    These topics are covered in most Trig textbooks. It is explained in the first few chapters of: Barons Guide to Trignometry, which to this date is one of my favorite books.

    The first one, Paralax, is used to determine the distance of an object if it is close to Earth, but at large distances, this technique does not work at large distances.


    Has some information on the topic you are seeking.
    Last edited: Mar 19, 2007
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