Parallax

In the sixteenth century, Copernicus proposed that the earth and planets orbited the Sun. At the time one of the arguments proposed against the Copernican view was that there should be a heliocentric parallax if the Sun was indeed the center of the solar system. At the time no such parallaxes had been observed. Copernicus countered rather simply by stating that the stars were much farther away than anyone had ever imagined, so the parallax was too small to observe. When astronomers finally managed to measure a parallax, it turned out that Copernicus was right.

By measuring the parallax p, and knowing the size of 1 AU (Astronomical Unit), calculations of the approximate distance to a star are often posssible. Illustration by Argosy. The Gale Group.

Astronomers did not succeed in measuring a parallax angle until 1838, when three astronomers working independently measured the parallaxes of three different stars. Friedrich Bessel in Germany, Thomas Henderson in South Africa, and Friedrich Struve in Russia measured the parallaxes of the stars 61 Cygni, Alpha Centauri, and Vega, respectively.

From the ground, astronomers are now able to measure accurately parallaxes for only about 1,000 stars that are within 20 parsecs of the Sun. This ground based limit requires measuring parallax angles that are as small as 1/20th of a second of arc. The quarter mentioned above is now 62 mi (100 km) away. For greater distances astronomers must use even more indirect methods that build on the distances found by parallax measurements. Improving the accuracy of parallax measurements will also improve the accuracy of the indirect methods that depend on parallaxes.

The earth's atmosphere limits the accuracy of parallax measurements from the ground, by limiting the resolution (sharpness) of a stellar image. Sharper images, and therefore more accurate parallax measurements, require getting above the earth's atmosphere. In 1989, the European Space Agency launched the Hipparcos satellite, with the mission of measuring the parallaxes of roughly 120,000 stars. The Hubble Space Telescope is also capable of measuring parallaxes far more accurately than they can be measured from the ground. Accurate parallax measurements from these space missions enable accurate distance measurements to a much larger sample of stars. Accurate distances will both help us attain more accurate measurements of the fundamental properties of stars, therefore increasing our understanding of stellar structure, and improve the accuracy of our cosmic distance scales.