Eliminating errors, automating observation: The Photographic Zenith Tube of the Neuchâtel Observatory in Switzerland, by Julien Gressot

Figure 1. The PZT at the Neuchâtel Observatory. Copyright image – Bibliothèque de la Ville de La Chaux-de-Fonds, Département audiovisuel, Fonds Fernand Perret, used with permission.

In the mid-twentieth century, the Neuchâtel Observatory decided to modify its time determination operating chain – the set of scientific instruments, mathematical operations and methods that provide the exact time at the end of the process. The Photographic Zenith Tube (PZT) and quartz clocks were the central elements of this transformation, which began in the 1940s. These technologies had a profound impact on the spatial organisation, the status of the human factor and the practice of time determination at the Observatory.

Edmond Guyot (1900-1963), the third director of the Observatory, was the protagonist of this transformation. In 1946, he sought to implement the recommendations of the International Astronomical Union (IAU) meeting held in Copenhagen in September 1946, underlining “the great interest in equipping observatories with PZTs”.

Thereafter, he contacted the Washington Naval Observatory (USNO) and the company Grubb & Parsons, then located in Newcastle upon Tyne, to obtain a PZT. The company had been in discussion since 1943 with the Royal Observatory at Greenwich (ROG) to develop an instrument, which had its origins in developments at the USNO to combine photography with a zenith instrument, the objective being to determine the time to a thousandth of a second by eliminating many of the errors involved in the process (personal equation, collimation, azimuth, flexion errors, …).

Grubb & Parsons developed three models in parallel, for the Neuchâtel Observatory (1954, Figure 1), for Greenwich Observatory (1955), and for the Mount Stromlo Observatory in Australia (1956). Research visits funded by a SIS grant to the Tyne & Wear Archives, the Cambridge Library, the Archives de l’État de Neuchâtel (AEN), and the Musée international d’horlogerie de La Chaux-de-Fonds (MIH) allowed me to learn more about the process of design, manufacture and the circulation of knowledge leading to the creation of these scientific instruments.

The Neuchâtel PZT became fully automated in 1959. The work at the Observatory was modified; the human observer was replaced by an operator in charge of preparing the observation cycle of the instrument, which took place at night without human intervention. However, the photographic output led to an increase in the work of reduction and comparison of the resulting photographic plates; the Observatory – with its limited staff – had to outsource these calculations.

Despite its initial hopes, it was soon found that the PZT also contained sources of error (the gelatin of the plates, refraction of the atmosphere, etc.). Moreover, the PZT came at a moment of paradigmatic breakthrough in time measurement as atomic clocks were developed, which quickly surpassed the precision of astronomical time determination. The use of the PZT for obtaining accurate time therefore lasted only a few years before becoming obsolete.

The material and archival study of the PZT allows us to better understand the place of an instrument that represents the culmination of the automation of time determination, shortly before astronomy lost its expertise in the field to physics.

Julien’s research was funded by a 2021 SIS Grant. A longer piece on this subject will feature in a future SIS Bulletin.

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