The apparent shift in a star’s position caused by our travelling in Earth’s orbit is called its annual parallax, because when we return to the position of our first observation the star also returns to its original position.
Galileo suggested a way in which the annual parallax of some stars could be detected. Suppose two stars lie in almost the same direction from Earth but one of them, say, is five times further away than the other. As the Earth-based observer is carried around the Sun, the two stars would appear to move in orbits similar in shape but the orbit of the nearer star would be five times bigger than the more distant one.
If the angular position of the nearer star is measured relative to the more distant – the more distant star being treated as a fixed reference point – then the difference between biannual measurements will be four-fifths of the true parallax of the nearer star. This error would be offset by the greater accuracy obtained by measuring the difference between two relatively small angles rather than between two relatively large ones.
In 1767, however, John Michell (c.1724-93) was able to show that double stars are so numerous that it was statistically impossible for them to all result from the chance of two stars being in the same direction from Earth. Most pairs of stars must therefore be physically connected (binary stars), i.e. at the same distance from Earth and therefore unsuitable for Galileo’s method.
The first serious attempt to measure the annual parallax of a star was made by Hooke. The observed position of a celestial body is the direction in which light from this body is travelling when it reaches the observer. Except when it is directly overhead, light from a celestial body enters the atmosphere obliquely and its path is bent by refraction. Hooke was aware of this and it happened that Gamma Draconis was passing directly over his lodgings at Gresham College in London.
He built a telescope into the fabric of the house but was only able to make a few measurements before illness and an accident to the telescope lens put an end to his observations.
James Bradley and Samuel Molyneux (1689-1728) decided on a new attempt to measure the parallax of Gamma Draconis. In 1725 a telescope was mounted in Molyneux’s house. As the star passed overhead, the telescope was tilted with the movement of the star and the angle of tilt from the vertical was then measured. The results were puzzling: the movement was so large that it was unlikely to be caused by parallax.
Bradley eventually worked it out. The speed of light is ≈300,000 km/sec and the speed of Earth in its orbit around the Sun is ≈30 km/sec, or about 1/10,000th of the velocity of light. An observer’s motion around the Sun causes his direction to the stars to be displaced much in the same way as rain appears to slant on a moving observer. The displacement is about 1/10,000 of a radian or 20.5 arc seconds in the direction of Earth’s velocity, precisely what was observed.
Bradley named the effect he had found aberration and announced the discovery in 1729. The aberration of starlight is over twenty times greater than the largest parallax. Although not the long-sought annual parallax it did in fact prove the Copernican hypothesis that Earth is moving, although few doubters remained in 1729. Bradley also discovered nutation.
In 1750 Bradley began to record the information necessary to modify the raw data so as to obtain more accurate measurements of star positions. He died before he could complete this task but Friedrich Wilhelm Bessel (1784-1846) after publishing his Fundamenta astronomiae (1818), a catalogue of over three thousand stars, began an extensive programme of measuring star positions and proper motions. By 1833 he had highly accurate positions for 50,000 stars.
Stars nearest Earth have the greatest parallax. The previous criterion for this had been brightness but it was becoming apparent that stars varied enormously in brightness. A more reliable guide seemed to be a large proper motion.
Bessel chose 61 Cygni, a dim star with a large proper motion (5.2 arc seconds per year). In 1838 he announced its parallax to be 0.314 arc seconds over the 150×106 km baseline, giving the distance from Earth to the star as 1/0.314=3.2 parsecs (small angle formula) or 10.2 light years (the modern value is ≈11.4 light years). A few weeks later Thomas Henderson (1798-1844) announced a parallax of just over one arc second for the southern star Alpha Centauri AB.
In 1837 Friedrich Georg Wilhelm von Struve (1793-1864) announced the results of seventeen observations from which he had derived a parallax for Alpha Lyrae (Vega) of 1/8th of an arc second (close to the modern value). Three years later he gave the results from one hundred observations from which he inferred a parallax that was twice as great. Other astronomers, however, were sceptical of Struve’s results.
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