How Was Friedrich Bessel the First Astronomer To Use Parallax To Calculate the Distance To a Star?

After William Herschel proposed his theory of “island universes”, or galaxies lying far outside our own Milky Way, astronomers became curious about measuring the distance to the stars.

In 1838, the German astronomer Friedrich Bessel used improved telescopes and a technique called parallax to prove that the universe was much more vast than even Herschel had imagined.

Parallax is the apparent shift in the position of an object when compared to something more distant.

If you hold up one finger and look at it through one eye, and then the other, you will see the effect of parallax. In determining a star’s parallax, Bessel needed to measure its shift over at least one year.

Calculations using Earth’s distance from the Sun would then reveal the star’s distance from Earth.

In 1792, Giuseppe Piazzi, an Italian monk, discovered a star moving so fast that he called it the “flying star.”

Its official name became 61 Cygni because it was located in the constellation of Cygnus, the Swan. Bessel decided that since all stars must travel at a similar speed, 61 Cygni’s movement must have meant that it was one of the nearest stars to Earth.

61 Cygni also stayed above the horizon almost year-round, so it made an excellent choice for his first parallax experiment.

Bessel chose two stars from the background to serve as the reference points to judge 61 Cygni’s parallax.

He then measured the angular distance between the stars with his new Fraunhofer telescope, which was so accurate that it could measure the width of a pinhead from a distance of 2 miles.

The changes in these two measurements would provide Bessel with 61 Cygni’s parallax after a year of viewing.

One complication in calculating 61 Cygni’s parallax, or apparent movement, was the star’s real movement.

Bessel used past observations taken of the star from 1755 and 1830 to find 61 Cygni’s rate of travel.

At the end of the year, Bessel had his parallax: its apparent movement was 0.3 second of arc. That meant that it was 657,000 times farther away from Earth than the Sun, or 60 trillion miles away.

It was a startling discovery to the astronomy world especially considering they knew it was one of the nearest stars.

The nearest star to Earth is Proxima Centauri. It’s only about 25 trillion miles, or 4.3 light-years, away.

When Bessel was taking his nightly parallax readings, he repeated each measurement up to 16 times to make certain it was accurate.