Gill Reversible Transit Circle

Reversible Transit Circle

6  inch Telescope (15 cm)



Summary; History; Current; Technical; Sources; Links; Gallery:

In brief

Important  Contributions:
This  telescope was for a long time the only active transit instrument in the Southern Hemisphere.


  • Designed by David Gill. Became the model for most transit circles made since.
  • This telescope was in continuous use for observing the planets and selected  stars to provide a fundamental system of star positions which is the observational basis of dynamic astronomy, accurate geodesy and of the determination of stellar  motions.
  • Instrument was the main source for the southern portion of the Fundamental  Katalog (FK4)

Historical Background

The telescope was designed by Sir David Gill and installed in 1905. It was a phenomenal instrument and became the example of how to design a transit circle. The telescope was modernised in the post World War Two period by installing cameras to photograph the declination circles, as well as a host of electronic instrumentation. The instrument remained in use until satelites such as Hipparcos and Gaia came into use rendering the instrument obsolete. [conversation with Ian Glass, Nov 2020, cdc]

Cape Observatory

Where  Located:
Cape Observatory

Current Information

Present  Location:
S.A.A.O. (used to be the Cape Observatory)


Not Operational

The building is in serious need of repair. The telescope itself requires a service. The supporting instruments such as cameras to measure the declination will be a great problem to overhaul.

Technical Details

Type: Refractor
Aperture: 6 inch (15.24 cm)
Focal Length:
Manufacturer: Troughton and Simms. [Laing, p.13.]

  • The instrument as designed by Gill was completed in 1905 [Laing, p.29.]
  • The  instrument was modernised with electronics and cameras in the 1950’s and 60’s. [Laing, p.17; p. 9.]



Pictorial Sources:

  • Laing,  J.D. (ed.), The Royal Observatory at the Cape of Good Hope 1820 – 1970 Sesquicentennial Offerings.


  • Collins,  P. & Moore, P.; Astronomy in Southern Africa, p79.
  • Laing,  J.D. (ed.), The Royal Observatory at the Cape of Good Hope 1820 – 1970 Sesquicentennial Offerings, p.13; p.17; p.29.
  • Smits, P., A Brief History of Astronomy in Southern Africa. (Unpublished)



Reversible Transit Circle.
Photo Credit: Charles Field, Wynberg. Source: Laing.

RevTCircle_15c6i_-02r   RevTCircle_15c6i_-03r
Alexander Menzies at the eyepiece.
Source: A.S.S.A. Archives: Peter Smits Collection.

The Dome of the Reversible Transit Circle.

Source: A.S.S.A. Archives: Peter Smits Collection.

Part of a compilation of sketches concerning the Cape Observatory.
Published by “The Cape Times”, 19 September 1908. Source:Warner – Astronomers.