How astronomers measure a star's distance by looking from both sides of the Earth's orbit: parallax
30seconds
Hold a thumb at arm's length and blink one eye, then the other: the thumb jumps against the background. Astronomers do the same with the whole orbit of the Earth as their pair of eyes. A nearby star seems to wobble against the far stars over a year, and the size of the wobble is its parallax.
Distance in parsecs is simply 1 divided by the parallax in arcseconds. Bessel measured it for 61 Cygni in 1838, and Gaia now does it to millionths of an arcsecond. Drag the Earth, change the distance, and watch the wobble shrink.
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