Astronomy for Kids

Online Astronomy Classes for Children

Astronomy Packet:  7
Assignment # 5



The nearest star to the Earth besides the Sun is Proxima Centauri, also known as Alpha Centauri.  Scientists claim that this star is about 4 light years away.   We need to discuss an important question.  How do we know how far away a star is?  We cannot stretch a giant tape measure that far, so how can we be sure of how far they are?

To measure distances to stars, scientists use something called parallax.  To help you understand how parallax works, let's try an experiment.  Stick your arm straight out in front of you with your thumb pointing up.  Now close one eye, and then the other.  Keep changing which eye is open and which is closed.

Notice that as you change your eyes, your thumb seems to move back and forth compared to the background.  Now move your thumb so that it is closer to your face.  Do the same experiment over again.  This time your thumb will seem to move much more back and forth.  The closer your thumb is to you, the more it seems to move.

Our eyes are too close together to see the parallax of the distant stars, but if we take a picture of a star then wait six months until we are on the other side of the Earth's orbit, we can then see how far the star shifted in the sky back and forth.  The closer a star is, the more it will seem to shift. 

By measuring how much a star shifts, it is possible to figure out how far away that star is from us.

Assignment # 5

How do astronomers determine the distance to a star or other object in the Universe?

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