Harvard College Observatory: Stellar photometry
An exterior view of the 12-inch horizontal photometer at Harvard College Observatory, completed by 1890. The photometer was a large-scale adaptation of the meridian photometers the observatory had been using since the late 1870s and was used for measuring the magnitude of the very faintest stars.
Harvard University Archives, HUV 1210 (9-6) olvwork289693.
Early in his directorship, Edward Pickering took stock of research programs at other astronomy institutions and found the field of photometry to be lacking as a major undertaking. He therefore quickly launched what became an extended intiative to perfect methods of measuring stellar brightness and to deploy them systematically across the night sky.
Working with the nearby telescope maker Alvan Clark & Sons, Pickering initially devised polarizing photometers that would attach to Harvard’s Great Refractor telescope. Soon, though, they developed a series of free-standing meridian photometers that could simultaneously view a target star and Polaris, which was chosen as a standard, allowing a direct comparison of their magnitudes.
In 1882, the observatory published its first work devoted to these photometric efforts, known as the Harvard Photometry, which appeared in volume 14 of the Annals of the Astronomical Observatory of Harvard College. The work included analysis of the magnitudes of 4,260 stars brighter than 6th magnitude, covering every star visible from Cambridge, Massachusetts.1 The work received a stellar review, which appeared in 1885 in volume 8 of Observatory, a monthly review of astronomy:2
To say that the volume recently issued from the Observatory of Harvard College is, in this branch of astronomy, epoch making, is to do no more than justice to a work which must henceforth be regarded as at once the foundation and treasury of scientific stellar photometry.
Using a larger photometer, Pickering went on to extend the Harvard Photometry to about 60,000 stars of ninth magnitude or brighter by 1902, and by 1906 some 1.5 million magnitude measurements had been made of these stars.
Meanwhile, in the late 1880s, Pickering also began devising photographic methods of measuring magnitude. These measurements were difficult to standardize and were largely made by Williamina Fleming, encompassing 1,000 stars near the North Celestial Pole, 420 stars around the Pleiades, and 1,100 stars near the equator. In the early 1900s, further photographic measurements of magnitude were undertaken at the observatory by Edward King and Henrietta Leavitt, which included efforts to establish stellar magnitudes that were fundamental rather than relative to other stars.3
References
- See Solon I. Bailey, The History and Work of Harvard Observatory, 1839–1927 (McGraw-Hill, 1931), chapter 11; and Howard Plotkin, “Edward Charles Pickering,” Journal for the History of Astronomy, 21, 1990, 47–58, doi:10.1177/002182869002100106; Dava Sobel, The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars (Viking, 2016), pp. 12–13.
- T. Lewis, “Harvard Photometry,” Observatory, 8, 1885, 49–51, https://ui.adsabs.harvard.edu/abs/1885Obs.....8...49L/abstract.
- Bailey, History, and Plotkin, “Pickering,” op. cit. (1).
Cite this resource
Bretislav Friedrich and Maria McEachern, “Stellar photometry,” Harvard College Observatory history guide, American Institute of Physics, 2026, https://www.aip.org/history/harvard-observatory/stellar-photometry.
Note that this material was originally developed in concert with the Williamina Fleming history guide as part of the Women in the History of Quantum Physics collection.