The "ancients" handed down to us a collection of instruments which were handheld, but capable nevertheless of surprising accuracy. During the Renaissance, these astrolabes and amillary spheres were replaced with more elaborate devices, reflecting the capabilities of our increased capacity to manufacture precision instruments. Mural quadrants and equatorially mounted circles replaced their ancestors. The golden era of positional astronomy was ushered in by the coming of the Industrial Revolution in England.

The culmination of this science was presided over by George Biddell Airy, the Astronomer Royal. When Airy assumed his position in 1836, he found the Royal Observatory at Greenwich filled with antiquated instrumentation, some of it dating back to Halley's day. He set about designing and supervising the constructing of new equipment that would bring a degree of accuracy to the observatory's work which had not been imagined to be possible.

His greatest achievement was the 8" transit circle that he designed in the years 1840 - 1848. The design incorporated three new principles, which were all aimed at reducing error. First, as many parts as possible were to be included in single castings. Second, screws were to be avoided at all costs. And third, the necessity for adjustments to the completed parts was to be avoided as much as possible. The 8" lens was produced by Simms, the most respected lens maker of the day, for £300. This was an unheard of aperture for a transit circle, and it made possible the measurement of faint stars and asteroids. The mount was supplied (to Airy's design) by Ransomes and May.

The giant declination circle is graduated on a silver strip that can be read by six equidistant microscopes. Using all six, and taking their mean, the observer can read to 0.01"! In practice, however, instrumental and personal errors reduce this figure somewhat.

Such was the accuracy of this instrument, and the volume of excellent measurements that it produced, that Greenwich was chosen as the basis of our longitude system.

The circle, along with its accompanying chronograph, was in service for over a century. When the Royal Observatory relocated to Herstmonceaux in the 1960's the circle's work was assigned to more modern equipment there. But Airy's transit circle remained in place, because the old observatory had become an astronomical museum. Visitors can see it today, sitting proudly astride the prime meridian which it defines.