The stars are really far away, but how exactly do astronomers measure their distance? There are many different ways to measure the distance to an astronomical object, but for "nearby" stars, the easiest method is to use parallax. A stars parallax is a measurement of it's apparent position on the sky, related to very distant background stars, at two different positions in the Earth's orbit around the sun.
In the image above we have the Earth orbiting counterclockwise around the sun, a nearby star which we want to measure the distance to, and distant stars which appear stationary during the Earth's movement around the sun. When the Earth is to the left of the sun, the nearby star appears to be amongst distant stars on the right hand side. Six months later, the Earth is to the right of the sun, and the star appears amongst distant stars on the left. Half the apparent angular distance on the sky that the nearby star appeared to move is called parallax and is measured in arcseconds. Arcseconds are an angular measurement on the sky, defined such that a star that in one parsec (~19 trillion miles) away from Earth will have a parallax of 1 arcsecond. Using basic geometry, we find that the distance to any nearby star (in parsecs) is equivalent to 1/parallax (in arcseconds), or d=1/p. Just by using simple math that you probably learned in high school, you can calculate the distance to the stars. No calculus or crazy physics required! Now finding the distance to much more distant objects is a bit trickier, and will be the topic of future posts.
Image Credit: astroprofspage.com
