Star Distances

by Jeanette Cain

More articles in Stars

Ptolemy showed a parallax method for calculating the distance to the Moon in 140 AD. In 1669, Robert Hooke, an English physicist, tried to measure the parallax of a star, but failed. Up until 1838, astronomers had no idea of the size of the universe.

In 1838, Friedrich Bessel measured the distance to the star 61 Cygni using the parallax method. Thomas Henderson, a Scottish astronomer gave the distance to alpha Centauri after Bessel. Stellar distance measuring was advanced through the Hipparcos survey satellite. Hipparcos used the parallax method to pinpoint the distance of thousands of stars.

As Earth orbits around the Sun, the closer stars will seem to move from side to side when against the background of stars at a greater distance. The parallax refers to the angle through which a star will move over a six-month period. When astronomers know this angle, they will be able to use geometry (simple geometry) to calculate the distance of the star. When the parallax is small, the star will seem to be farther away. If astronomers encounter a parallax angle that is too small to be measured, they know that those stars are over a few hundred light-years away.

Distance measures: one light-year is equal to 9.5 trillion kilometers, which is the distance that it takes light to travel in one year; and, one par sec is equal to 3.26 light-years, which is the distance a star will show a parallax angle of 1/3,600 of a degree, or one arc second. Proxima Centauri is one of three stars making up the Alpha Centauri system. Our Sun's closest neighbor is Proxima Centauri, which is 4.2 light-years away. Consider this: if the Sun was about the size of a soccer ball, then the distance of Proxima Centauri would be as similar a distance as Paris, France is from New York City.

Conduct your own parallax experiment: With your two index fingers in front, place one at arm's length, but place the other finger halfway between. Now rock you head from side to side, BUT keep one eye closed. When looking at the finger closest to you, it will seem to move farther than your distant finger. It will also seem to move faster. The movement that you have experienced is the measure of the parallax. The smaller the parallax, the farther away your finger will seem.

In 1989, when the ESA (European Space Agency) launched the Hipparcos satellite, it provided scientists with the disturbing effects going on in Earth's atmosphere. For 3 and 1/2 years it measured, with precision, the distances of star positions. With this information, scientists have been able to calculate the parallaxes of 118,000 stars, and have accurate measures for stars up to 500 light-years from our Sun.


1. Couper, Heather and Nigel Henbest. Space Encyclopedia DK Publishing, Inc.: NY 1999

2. Editors. Secrets of the Universe. International Master Publishing: US. 1999