In 1887 observatories worldwide embarked on an ambitious project to photograph the entire sky, cataloguing the positions of millions of stars to produce a document known as the Astrographic Catalogue. Australia played an important role in this effort, with Sydney, Melbourne and Perth Observatories all involved in the project. Each observatory was allocated part of the sky. Melbourne Observatory was given a region around the south celestial pole (the part of the sky directly overhead the South Pole), and Sydney Observatory a section of the sky which included the Southern Cross and a large part of the Milky Way.
Prior to this, the positions of stars were measured by eye by an astronomer peering through a telescope – a slow and tedious process. But with the advent of photography came the ability to detect stars too faint to be seen through the telescope, and to observe thousands of stars at a time and record them for later measurement. Using a special kind of telescope known as an astrograph, photographic plates were exposed to starlight to produce images of the sky from which the catalogue was eventually compiled.
But how did we get the positions of stars from these photographic plates? Sydney Observatory used optical plate measuring machines, which allowed the positions of the stars to be measured in Right Ascension and Declination (equivalent to latitude and longitude on Earth). This instrument improved on earlier designs by introducing an eyepiece scale which drastically reduced the time required to measure the stars on each plate. The painstaking measurements were made by hand, by groups of women known as ‘computers’ and recorded in what became known as the ‘Ladies’ Logs’. The contribution of these women to Australian astronomy has recently been studied in detail.
Following the closure of Melbourne Observatory in 1944, due to encroaching light pollution, Sydney Observatory took responsibility for processing the remaining photographic plates from Melbourne Observatory, as well as their own. In total Melbourne Observatory measured the positions of 218 000 stars, and Sydney Observatory 430 000. The Melbourne Astrographic telescope was also relocated to Sydney Observatory and in 2014 a major conservation project was completed to restore the telescope, which you can read about here and here. It is now on display in Sydney Observatory’s East Dome, along with one of the plate measuring machines, photographs of the work in progress, and excerpts from the Ladies’ Logs.
As an astronomer I have always been fascinated by the story of the Astrographic Catalogue, and the enormous amount of work that went into realising this project (which continued well into the 20th century). So I was thrilled on a recent trip down into MAAS’ basement collection store to come across a second one of these instruments. It has rekindled my interest in this part of astronomical history and I’m now eagerly tearing through The Glass Universe, by Dava Sobel, which details the incredible astronomical discoveries made by the women who measured another set of photographic plates taken at Harvard Observatory.
Written by Sarah Reeves.