# Friedrich Bessel

## Bessel, Friedrich (1784-1846) (World of Earth Science)

German astronomer

Friedrich Bessel was a self-taught astronomer. Born in Minden, Germany in 1784, he became an accountant in Bremen, but his true interests were astronomy and mathematics. In fact, in 1806, at the age of 20, he recalculated the orbit of Halley's comet, which was due to reappear in 1835. This so impressed astronomer Heinrich Olbers (1758840) that Olbers helped Bessel obtain a post at the observatory.

Bessel worked laboriously. He produced a new star catalogue of over 50,000 stars and introduced improvements to astronomical calculations, developing a method of mathematical analysis along the way that can be applied to many problems not related to astronomy. He oversaw construction of the first large German observatory and served as its director from 1813 until his death in 1846.

Bessel's greatest achievement was in determining the parallax of a star. As the earth orbits the sun, our position relative to any star shifts by a maximum of 186 million miles (299,274,000 km, the diameter of the earth's orbit). Thus, the apparent position of any star in the sky will change slightly through the year. The amount of observed shift is the parallax. Knowing an object's parallax, it is possible to calculate the distance to it.

In 1838, Bessel announced he had obtained the parallax for a star called 61 Cygni. He had chosen this star because it had shown the largest proper motion of any known star. The large degree of movement, he assumed, was because the star was relatively close, and the closer an object, the greater its parallax would be.

Bessel's calculations showed that 61 Cygni was about 10 light-years from Earth. (This was the introduction of the term light-year.) Although that is actually very close for a star, the distance was mind boggling in 1838. The earlier astronomer Johannes Kepler had believed the stars were 0.1 light-year away, and Isaac Newton had risked enlarging that to two light-years.

Discovering the parallax of a star also put another nail in the coffin of the "Earth-centered" concept of the universe. Since parallax could be obtained from a moving Earth, Nicholas Copernicus's assertion that the Earth orbited around the sun was further strengthened.

In 1841, Bessel drew a remarkable conclusion about the stars Sirius and Procyon. He had noticed displacements in their motion that could not be attributed to parallax. A parallax shift shows smooth motion; these two stars seemed to be wobbling. He concluded that there had to be invisible companions in orbit around each star. The gravitational tug of the companion would account for the observed wobble. This theory later turned out to be correct.

Bessel was responsible for encouraging astronomers to direct their attention to the stars beyond the solar system.