Historic Williamstown Observatory telescope identified

An important historic telescope has been identified in the Surveying and Geomatics Engineering Collection in the Department of Infrastructure Engineering at the University of Melbourne. It is a portable transit telescope, 77 cm in length, marked ‘Potter, London’.

A transit telescope is used by astronomers to undertake accurate measurements of the passing of stars across the lines in the eyepiece, typically to determine local astronomical time or the longitude of the location. J. D. Potter was a London instrument maker, supplying sextants, charts and other navigational instruments to the Admiralty and mariners.

The size and maker of the telescope, and the rarity of the instrument, make it evident that this was the instrument of 30-inch focal length by Potter, purchased by the Victorian Government for the new Williamstown Observatory and received in April 1854. The observatory had commenced in July 1853 to provide time signals to mariners so that they could calibrate their chronometers; this was essential so that they could determine longitude on their return voyage from Australia. Until the arrival of this instrument, Government Astronomer Robert Ellery had relied on little more than a sextant to make his observations. The observatory was relocated in 1863 as the Melbourne Observatory, adjacent to the Botanic Gardens, with Ellery in charge.

The Potter transit instrument is a significant historic item, being the first telescope of Victoria’s government observatory. How and when the telescope was transferred to the University of Melbourne is unclear. It may have passed to Professor William Wilson, the University’s foundation professor of mathematics, who was closely associated with the establishment and operation of the Melbourne Observatory.

Image: J. D. Potter, London (manufacturer), Portable Transit Telescope, 30-inch focal length, c.1853