Derby and District Astronomical Society

The Journal of the Derby and District Astronomical Society
September - December 2005

How Far Away Are The Stars?
By Maurice Batchelor

James Bradley (1693-1762) In early days, mapping the Solar system had been difficult enough, but figuring out the distances of stars was to present much more of a problem. The first man to tackle this very seriously was the Rev James Bradley, who was in fact the third Astronomer Royal. He succeeded Halley in 1742 and he held the position until he died 20 years later. Bradley was born in 1693 and was educated at Oxford. He then proceeded to take up the position of vicar at Burstow in Monmouthshire (as it was then).

In the year 1721, he became Professor of Astronomy back at Oxford, remaining there until the death of Halley, when he moved to Greenwich. Bradley was a very careful observer and he decided to improve the existing star catalogues. He measured the positions of more than 6,000 stars and his results are still of great value, even today. In order to achieve this, one method he used was parallax. This is the apparent movement of one object relative to another when viewed from a different point. For example, suppose you hold a pencil at armís length and line it up with a distant object using just one eye, then without moving your head or the pencil, use the other eye. The pencil will no longer be lined up with the distant object. It will appear to have shifted. This method of assessing the distances of stars is only effective with stars of less than a few hundred light years away. For stars of greater distances, the shifts become so small that they are swamped by unavoidable errors in observation. Often in astronomy, 2 types of parallax exists as follows:

Diurnal Parallax

This is the apparent movement of one celestial body relative to another when viewed from different parts of the Earthís surface. Owing to the small size of the Earth, and the vast distances of the stars, the Diurnal Parallax of stars is absolutely insensible, but it is noticeable with closer objects.

Annual Parallax

Annual Parallax, or what is sometimes known as Trigonomical Parallax, is more noticeable but even so for the greater number of stars being so far away that not even the displacement of the Earth around the Sun, by 186,000,000 miles produces any reasonable effect. However, for a certain number of nearer stars, the effect is measurable. This is what Bradley was concerned with and he achieved a reasonable degree of accuracy.

The parallax method of measuring the distance of a star.
The parallax method of measuring the distance of a star.
Not to scale! Diagram by the author.

Bradley went on to measure the star Gamma Draconis in the region of the Great and Little Bears, knowing that the parallax shift would be very small, he was puzzled about its mysterious behaviour. There were shifts indeed, but not altogether due to parallax. It appeared to move in tiny circles. He had not taken into consideration the time it takes for light travelling at 186,000 miles per second to come from a star and also the motion of the Earth, moving at an average speed of some 18Ĺ miles per second around the Sun. He found that other stars behaved in the same way and this effect was due to what is now known as aberration.

To conclude the, it was found that the parallax method could only be used for relatively nearby stars. For more remote objects, it was necessary for astronomers to use instruments based on the principle of a spectroscope.

Read Maurice's other article in this edition of Aries, on the life and laws of Johannes Kepler, by clicking here.