Vital step were taken towards the detection of stellar parallax during the eighteenth century through the collection of extensive and precise stellar catalogues. In addition, more precise instruments, impossible for the crude technologies of earlier times, were being constructed. Both these factors contributed to many false alarms until the actual minuteness of stellar parallaxes was ascertained.

Flamsteed’s catalogue, the ‘Historia Coelestis Britannica’. Led the way in the new science of exact astronomy. Published post¬humously in its revised form in 1725 (after a pirated edition in 1712) it included positions of about 3000 stars down to an accuracy close to 10 seconds of arc. The previous extensive catalogue, that of Tycho Brahe, could only claim accuracies down to about 1 minute of arc. The second Astronomer Royal, Edmond Halley 1742), did not improve on this observational limit but he did con¬tribute to the problem in his discovery of the first actual motion of stars, known as their proper motion (or motion across the line of sight). Using proper motions astronomers could determine the relative distances of those stars which had detectable proper motions and thus select the best candidates for a measurable parallax.

Halley’s successor. James Bradley, (1693-1762), added many refinements to both instrumental construction and observing techniques. With respect to the latter, he produced an excellent table of the refraction of light through the atmosphere, thus allowing the effects of temperature and barometric pressure on all measurements to be removed. Bradley attempted to measure the parallax of the star ? Draconis but instead discovered a phenomenon called aberration, an equally strong proof of the Copernican theory and a confirmation of Roemer’s disc-oven’ of the finite velocity of light. Upon further reduction of his results, he noticed another small perturbation that he later concluded to be nutation, a wobbling of the Earth’s axis due to the gravitational attraction of the Moon. From these results Bradley showed that any parallactie displacement must be less than 2 seconds of arc and probably less than 0.5. In addition to these discoveries, he compiled a catalogue of about 3000 stars of unrivalled accuracy which was published in 1798 and 1805, lone after his death.

Other countries were also producing extensive catalogues. Nicolas de Lacaille, a French astronomer, published a catalogue of about 10000 southern stars while at the Cape, South Africa. Lalande. as director of the Paris Observatory, published his ‘Histoire Celeste Francaise’, which included recorded observations of nearly 50 000 stars down to 10th magnitude. At the same time the list of stars of known proper motion was expanding. Halley had measured four (Sirius. Aldebaran, Betelgeuse, Arcturus) in 1718.

One of the great astronomers of this period, Sir William Herschel (1738-1822). often called the father of sidereal astronomy, also examined the problem of parallax. He hoped to measure it by observing double stare and then using the method of differential parallax suggested by Galileo. With such a pair, the remote one could be used as a fixed reference point with which to compare any displacement of the nearer one. Herschel was very successful after an examination of many double stars, not in the determination of parallax, but in the discovery that double stars existed as actual physical pairs (binary stars). Previous to this, such pairs had been considered to be optical, or coincidental, doubles. This discovery meant that such pairs could not be used since they were equidistant from the Sun.

The final discovery of parallax came from three very different quarters in fairly close succession: Bessel in Germany, Henderson at the Cape and Wilhelm von Struve in Russia. Bessel and Struve had both equipped themselves with the best optics then available, developed by a young Bavarian optician, Joseph Fraunhofer, famous for his work on the solar spectrum. The final detection in 1838 ended several debates over previous illusory discoveries, since the small quantities actually involved could not have been ascertained by earlier instruments.

Bessel’s discovery came after many years of assiduous labour. In 1818 he published Bradley’s catalogue of 3000 positions reduced to the equinox of 1760; for the first time a catalogue contained not raw data, but positions corrected for the effects of precession, nutation and aberration. He then proceeded to extend the catalogue and was able to compile a catalogue with positions for over 63 000 stars. With the completion of his catalogue, Bessel acquired a HELIOMETER, developed by Fraunhofer at Konigsberg (the principle of this instrument is the separation of two distinct images of the same object by a measurable amount). With the exquisite de¬fining powers of this instrument, he attacked the problem of stellar distances; for this he used observations of the star 61 Cygni, which has an unusually large proper motion. A year later, at the end of 1838, he published a result of 0.3136 second of arc for 61 Cygni.

This result was followed closely by two others. In January, 1839, Thomas Henderson announced a parallax of about 1 second of arc for a Centauri, the result of observations made earlier while he was director at the Cape. Struve’s measurements extended over roughly the same period as Bessel’s. His telescope, mounted with an equatorial drive, used an object glass developed by Fraunhofer and contained the finest micrometer then available. His result, 0.2613 second of arc for a Lyrae, was announced in 1840.

The results signified a new age of instrumental precision and exploration beyond the Solar System. The original purpose behind the search — the proof of a moving Earth — had long been a moot point. As can be expected of any search which spans such a long period, the ultimate results may lie in an area completely alien to the originators of the search. Proof of a moving Earth was no longer necessary, since it had become an accepted part of theory. The results were more interesting in their own right – as distance determiners – and were important to the nineteenth century as a method of ascertaining the intrinsic brightness of stars. Both these interests, one lying in exact astronomy the other in astrophysics, represented an entirely new outlook on astronomy, an outlook completely different from that of the original pioneers of a moving Earth.

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