How Far are the Stars

Until 1838, Astronomers had little idea of the true size of the Universe. But in that year, Friedrich Bessel used a technique called the parallax method to make the first successful measurement of the distance to a star.

Modern astronomers have many different ways of figuring out how far away an object is, but they all depend ultimately on the parallax method.

As the Earth orbits the Sun, the nearer stars seem to move from side to side against the background of more distant stars. The angle through which the star moves over a period of six months is called its parallax.

Knowing this angle, astronomers can use simple geometry to calculate the star’s distance. The smaller the parallax, the further away the star.

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