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cantonic t1_j4mb9mn wrote

Reply to comment by chechomsky in How do we know that we are in a certain place inside our galaxy? and how do we know how big it is just by looking at the cross section we are in? by friday_panda

They will look at a star when the earth is on one side of the sun, then look at the same star when the earth is on the other side of the sun, 6 months later. The change in position is about 180 million miles. How the position of the star has changed in that 6 months gives them enough information to calculate how far away the star is.

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