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Name: Sean
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A


Question:
How can you tell how far away stars are from earth?


Replies:
The nearby stars are done using parallax, taking images of the star relative to other stars while Earth is on either side on its orbit of the Sun. There are lots of basic astronomy books that explain this.

For farther objects, new yardsticks are required, like Cepheid variables.

All best

David Levy


Sean,

On a basic level, we can do triangulation (much in the same way that surveyors or cartographers used to measure distances of objects on Earth). In this process, two spots (spot A and spot B) of known distance from each other are chosen, the target object is sighted from these spots, a triangle is thus formed between the target object, spot A and spot B. If we know the angle made between target-spot A-spot B, the distance between spot A and spot B, and the angle formed by target-spot B-spot A, then we can calculate the distance of the target from the midpoint between spot A and spot B by simply calculating the height of the triangle formed (we can also calculate spot A to target, or spot B to target distances).

To make more accurate measurements of the angles formed, you want to make the distance between spot A and spot B as great as possible, so often, the angle of the telescope looking at the target object will be measured at some time of the year. We then wait for the Earth to move around the Sun for a bit, so that at a later time of the year, when some known distance has been covered in the Earth's orbit around the Sun, the angle to the object is again measured, and the triangulation can again be done.

Greg (Roberto Gregorius)


Sean,

The basic concept of measuring these very large distances is called Parallax. The easiest way to understand parallax is to put one finger up a few inches in front of your face. Now close one eye. Switch back and forth between opening your left eye and closing your right and closing your left and opening your right. When you do this, do not look at your finger, but at the background--the wall, trees, whatever. When you do this, you will see your finger apparently move. You know in fact that your finger is not actually moving, but since your eyes are a couple inches away from each other, each eye sees a slightly different position of your finger when compared to the background. In fact, you can create a triangle from your left eye, to your right eye, to your finger, back to your left eye. Since you can measure the distances between your eyes, you already know one of the distances of the triangle. Using geometry you can solve for the internal angles of the triangle and determine the distance to your finger.

You can do the same thing with stars! Now look at something very far away and alternate blinking your eyes, just as before. You should notice that you do not observe parallax anymore. The distance between your eyes is so small compared to the distance to very far objects that you would now have to use a second person many yards or miles away to observe the parallax. The same thing applies to stars! So how do we measure the parallax of stars? We use the Earth's orbit to create the distance "between your eyes". Every 6 months, the Earth is approximately on the other side of its orbit around the Sun--186,000,000 miles away from where you were 6 months ago! This is quite a bit bigger of a distance than between your eyes. Since we know this distance, we can use the same math as before to determine the distance to stars.

Below is a link to a much more detailed explanation of parallax, though it might be a little too complicated for you to understand without an extensive math background. There are still a few paragraphs and pictures that would be useful for you to look at though.

http://en.wikipedia.org/wiki/Parallax

Matt Voss



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