FAS James Bradley

Flamsteed Astronomy Society

page 3 of 3

Did you know about...

James Bradley and the Dragon’s Head?

In 1742 Bradley was appointed 3rd Astronomer Royal, succeeding Edmund Halley. Bradley’s time saw the breakthrough in solving the longitude problem by lunar distances. Tobias Mayer had been working at Gottingen on a ’theory of the Moon’ and had conducted observations the equal of Bradley’s in precision. In 1754 Mayer submitted his tables to the Board of Longitude hoping for an award from the Longitude Prize of 1714. The tables were passed to Bradley who checked them against the Greenwich data and found them to be very accurate. In 1761 he despatched Nevil Maskelyne to St Helena to observe the Transit of Venus that year. Maskelyne used Mayer’s tables to find longitude by lunars on the voyage and returned convinced about the approach. In 1764 Maskelyne was sent to Barbados to check on the trial of Harrison’s fourth Marine Timekeeper... but that’s another story!

Back in 1749 Bradley got the Admiralty to buy the 12-foot zenith sector for £45 (a huge sum) and moved it to Greenwich. He also found the instruments left to him by Halley in a sorry state. He went cap-in-hand to the King and got funds for a new brass mural quadrant and transit instrument, both by John Bird, and up to his high standards of 1 arc-second precision. Bradley also got funds for a new building to house the instruments—now the Meridian Building at Greenwich and opened in 1750. He worked diligently with the new equipment for the 20 years of his time as Astronomer Royal, mainly re-observing the positions of 3200 stars in Flamsteed’s catalogue. Bradley left over 60,000 observations of unprecedented accuracy. Unfortunately, his heirs took possession of the data on his death and ownership disputes with the Board of Longitude dragged on. Bradley’s work was not fully published until 1805, 43 years after he died.

It was not to be until 1838 that the first reliable measurement of stellar parallax was announced by Friedrich Bessel in Germany. There was a direct link back to Bradley: in 1818 Bessel had published Bradley’s catalogue with positions corrected for precession, aberration, and nutation. This, together with an outstanding new instrument by Fraunhofer, formed the basis for his attack on the parallax of 61 Cygni. In 1838 the publication of his measurements just pipped announcements by Thomas Henderson from his work at the Cape, and Wilhelm Struve in Dorpat. Bessel found the parallax of 61 Cygni to be 0.314 arc-seconds — far beyond Bradley’s capability to detect. 61 Cygni is much closer than Gamma Draconis which had been chosen only because of its transit near the zenith at London. Today’s value for the parallax of Gamma Draconis is 0.022 arc-seconds (distant 148 light-years or 9.36 million AUs). Bradley needed a 100 times improvement in his instrumentation. But with Bradley the chase was on. Airy called Gamma Draconis “the birth star of modern astronomy”.

8-foot Transit Instrument by John Bird —marked Bradley’s Meridian © NMM

James Bradley was born in Gloucestershire in 1693 to aristocratic parents. He was partly raised by his uncle the Revd. James Pound (1669-1724) who nursed him though smallpox in 1717. Bradley often stayed with Pound in Wanstead where he received his introduction to astronomy. Uncle James was an expert amateur astronomer and friend of Newton and Halley. They often entrusted him with observing projects which Bradley assisted. Bradley attended Balliol College Oxford and followed his uncle into the church. He became vicar of Bridstow near Hereford in 1718, but astronomy had become his spare-time passion. In 1721 the Savilian Chair of Astronomy at Oxford came vacant and, with Newton’s recommendation, Bradley was appointed and left Bridstow to take up astronomy full-time.

In 1724 Pound died. In the same year Bradley was approached by Samuel Molyneux, an amateur astronomer of considerable means. He knew of Hooke’s attempt to measure parallax with a zenith telescope and decided to try himself with a better instrument. He’d commissioned George Graham to build the 24-foot zenith sector at Kew and wanted Bradley to help with the observing programme.

With the 24-foot sector Bradley and Molyneux could measure with a precision of 1 arc-second. This was 60 times better than Tycho’s capability at Uraniborg. Bradley ushered in a new era of high-precision in observation. He was extraordinarily careful to anticipate and measure all causes of error in his instruments. Although he failed to measure the parallax of Gamma Draconis, Bradley could safely say that he would have succeeded if it had been any bigger than one arc-second. Gamma Draconis was therefore at least 400,000 AUs distant. At about the same time (1728) Isaac Newton announced his estimate of the distance to Sirius. Based on the relative brightness of Sirius and the Sun, he calculated the distance at 1 million AUs (not too bad—the modern value is about 550,000 AUs; 8.6 light-years). The stars were vastly more distant than thought by previous generations.

Bradley continued with the 12-foot sector at Wanstead until 1747. The long programme of observations enabled him to verify yet another phenomenon which caused an apparent change in stellar positions by as much as 9 arc-seconds. He found that the Earth was wobbling on its axis because of the gravitational effect of the Moon on its oblate form, an effect predicted by Newton. Bradley called this nutation.

No wonder stellar parallax was so elusive. The tiny apparent shift of position was camouflaged in a welter of overlapping effects: precession, as the Earth’s axis slowly rotates on a 26,000-year cycle; refraction, as the incoming starlight is bent by the Earth’s atmosphere depending on the angle of incidence; aberration, caused by the Earth’s motion through space; and nutation, as the Earth was made to wobble by the Moon.