Slide Rules

In 1620 Edmund Gunter (1581 - 1626), Professor of Astronomy at Gresham College in London, was the first to plot a logarithmic scale on a straight edge; it became known as Gunter's Line and became the basis of the slide rule. Over the next three hundred years a huge variety of slide rules were developed to help carry out a myriad of calculations. Slide rules disappeared from use in the mid 1970s, with the invention of pocket calculators, and are commonly thought of as one of the precursors to computers.

Astronomers often use long and complex equations in calculating the position of celestial objects. Slide rules were used to assist astronomers in shortening the time taken to complete these calculations and lessen the chance of human error.

These slide rules allowed observers to quickly make the corrections to a star's transit time based on the three standard adjustments for a transit telescope: azimuth, collimation and level errors. The way they used the slide rules was that the 'error for the day being set at zero of the scale, the quantity to apply is found opposite the star's declination'.

These slide rules were created specifically for use at Sydney Observatory. The azimuth and level error equations have the observer's latitude as one parameter. H. C. Russell, Government Astronomer at the time, would have supplied Tornaghi with the exact scale to be etched on the slide rules, using the specific location details of Sydney Observatory in the equations. These slide rules are significant in their uniqueness, due to the specific formula used; they are valueless in any other geographical location except Sydney Observatory.

Tornaghi handcrafted the slide rules out of brass and etched them with a scale. He was one of the most significant Australian scientific instrument makers of the 19th century, and these slide rules are an example of his excellent craftsmanship.

These slide rules, in conjunction with the 6 inch transit telescope (H9899), were used as support instruments in the mapping the stars project, to acquire accurate positions of reference stars. The project was initiated in 1887 at a meeting of the International Congress of Astronomers in Paris. Henry Russell from Sydney Observatory attended and was one of the many who accepted the proposal put forward by Dr. David Gill to map the stars by photography. The importance placed on this project can be gauged by the fact that it dominated the activities of Sydney Observatory into the 1960s.

These slide rules remain of national significance due to their pioneering role in Australian science and their association with Australia's earliest astronomers. They are also significant for their association with nineteenth century instruments and instrument makers.