The Measurement Of Distance 1Presentation Transcript
The Measurement of Distance 1.7
Astronomers track and record the positions of the stars. They must know direction and distance .
Determining the distance to a star is difficult because we cannot actually travel to the star and measure the distance.
Instead, astronomers must be very clever and measure the distance indirectly.
One distance measurement method is called triangulation.
Based on the principles of geometry.
Surveyors use this to measure the distance to faraway objects.
Triangulation makes up the cosmic distance scale .
Parallax measurements take advantage of the fact that, as the Earth orbits around the Sun, relatively near-by stars appear to move with respect to the fixed, very distant stars.
This is the same thing that happens when you look at a close object with first one eye and then the other.
For example, hold your thumb at the tip of your nose. Look at your thumb with first your right eye and then your left. Your thumb appears to move because your eyes are not at exactly the same place, so each eye views the thumb from a different angle.
Now hold your thumb at arm's length and repeat the experiment. Your thumb will still appear to shift, but will not appear to move as much as it did when it was closer.
The same thing happens to stars.
The closer stars appear to shift more than the farther stars. The "fixed" background stars are not really fixed; they are just so far away that we cannot distinguish their apparent shift. The apparent shift of the stars is called their parallax.
Parallax is simply the apparent change in the position of an object due to a change in the location of the observer. In order to measure the parallax of stars which are very far away, we must use the largest baseline possible.
( The baseline is the distance between the two points where we take the measurements. For the experiment above with your thumb, the baseline is the distance between your eyes.) A larger baseline results in a larger shift, which means that we can measure the parallax of stars which are farther away.
The largest baseline we can use for ground based observations is the diameter of the Earth's orbit. Using the Earth's orbit, we make one measurement of the position of a star in, say, June, and the second measurement in December (6 months later).