Monday, August 20, 2012

ABC Wednesday: F is for Far, Far Away...

This post is for PhenoMenon, astronomy, "F."  How do we figure out how far something is in astronomy?...  What do your blinking eyes have in common with how NASA calculates interstellar distances?...

Hold out a finger or a thumb and close your right eye and then your left.  Your finger will appear to move.  This is very much how distances are calculated in space!
How do we know how far away a star is?  Have you ever wondered that?  No one has ever been to another star, so how do we know how far away they are?

If you focus on a distant object - such as a house or a car - the object will appear to move relative to your finger.  (I made this image for my course this semester.)
If you hold up a finger and look at a distant object, closing first your left and then your right eye, the object will appear to move.  Objects further away will appear to move less than objects that are close to you.  

For example, your finger appears to move a lot if it's very close to your nose, but if you extend your arm it only moves a little bit.

This is due to the fact that your eyes are a small distance apart from one another and so they see the object at slightly different angles.

But for objects very far away - like stars - our eyes aren't far enough apart to detect this angle.  If you try it with the moon, the moon will not appear to move.

In fact, in most cases the distances across the Earth aren't even far enough.  We have to use something else.

What we use is the same idea as your eyes, but at a large, large scale - the scale of the solar system!

From the NASA educational website 
We measure the angle of the star at one time of year, such as the summer, and then the angle when the Earth is on the other side of the sun, in the winter.  By measuring the angle we can use trigonometry to measure how far away the star is.


  1. "So far away, doesn't anybody stay in one place anymore?"
    Well, no, Carole, and they never did!

  2. Excellent post. Coincidently, I just reviewed all this a few weeks ago on Wikipedia, and I much prefer these simple measurement and formulae to the skull-cracking complexity of Doppler Shifts. Oh, and thanks for commenting on Found One!

    1. Thank you for dropping by!

      I don't mean to be rude, but you can't use Doppler Shifts to predict distance, only radial velocity (the rate at which something is moving towards or away from you.)

      While very, very distant objects are more redshifted than close objects due to the Doppler Effect and expansion of the universe, that's only averages.

      Very close objects can be significantly redshifted or blueshifted because they are moving towards Earth or away from Earth. For example, stars in our galaxy revolve around the central point at different rates.

      They will be blue-shifted (moving towards us) or red-shifted (moving away from us) due to their velocity relative to us in the rotation of the galaxy, not their absolute distance.

    2. A better way to phrase it:

      We know that on average very distant objects tend to be redshifted. But that tells us nothing about the distance of individual stars.

      This is similar to saying that just because an object is redshifted doesn't mean that it's particularly distant.

      Or, in analogy: If I know that the average weight of a student in my classroom is 180 lbs, that doesn't tell me how much Joe weighs.

  3. A beautifully explained, informative post! Very interesting!

  4. The heavens and all the stars above never stop amazing us.Very inyeresting! Have a great week.
    Wil, ABC Team.

  5. Fascinating, distance is something that I am not good at lol.

    Rose, ABC Wednesday Team

  6. Fascinating but my brain is just too tired tonight to wrap around the concept.
    Thanks for sharing and giving great examples.

  7. Fantastic science lesson! I've often wondered about this question, but my brain isn't really wired to comprehend much of the answer! ;-) But I still think it's fascinating.

  8. Fascinating how so many things can be determined in the blink of an eye!!! Fabulous "F" post. Thanks!

  9. you dumb-downed this science so that even I can understand! Good stuFF!