Historical Interlude: Variable Star Observing in the 18th Century

Paper title: John Goodricke, Edward Pigott, and Their Study of Variable Stars (arXiv: 1204.6241)
Author: Linda M. French
First Author’s Affiliation: Illinois Wesleyan University

Journal: American Association of Variable Star Observers (accepted)


One of the things I like about astronomy is its rich history. There are records of astronomical studies being done right back to the earliest civilizations in India, Mesopotamia, and Egypt. It’s kind of cool to realize that as an astronomer, you’re a member of a millenia-old intellectual tradition!

Of course, early astronomy was more astrology and calendar-keeping than the rigorous cousin of physics it is today. Today’s paper presents a snapshot taken just as astronomy was transitioning to the modern scientific era. The author studies the lives and works of two of the first variable star observers, using primary source material to explore how they lived, worked, and thought.


The author spent a sabbatical at the university of York, studying the journals and papers of the English astronomers John Goodricke and Edward Pigott related to their discovery and characterization of variable stars. While the bulk of the paper is biographical in nature and best read as part of a whole, there are a few things that jumped out at me.

First, England was a surprisingly progressive place by the late 1700s. A common misconception about premodern Europe is that it was always a challenging environment for scientists (think Galileo, Darwin, and Bruno).  In this model, the scientist is a “lone, heroic figure” (French 2012), who must struggle to enlighten backward society. While this may have been true during the Renaissance, by the late 1700s a surprisingly intellectually open society had started to emerge. John Goodricke, for example, attended Warrington, a school affiliated with the Non-Conformist movement that emphasized science and freethinking in its curriculum. When Goodricke and Pigott announced their discovery of stellar variability, astronomers around England leapt to test their claim with observations. Astronomy was clearly well into its transition to the modern, data-driven science it is today.

Second, I was astonished at the precision these scientists were able to eke out with their techniques. Two centuries removed from CCDs and one century from photographic plates, they completely lacked any mechanical method of measuring the starlight quantitatively. However, they were nonetheless able to identify stars that varied from the brightnesses cited in stellar catalogs, as well extract their periods to the precision of an hour, all measuring solely by eye! Their observing notebooks are so detailed that an observation can be dated to within a week, just by comparing their starmaps to positions calculated using modern methods. It makes me realize how spoiled I am with my ubiquitous digital storage and electronic data collection!

Figure 1: drawing from Goodricke’s journal. The detail and precision of the celestial bodies plotted is sufficient to date the image to November 1779.

The sophistication of the analysis also jumped out at me. Goodricke and Pigott distinguished two classes of objects: the first consisted of objects like Algol, which exhibited a single sharp change in brightness on a regular basis. The second included objects like delta Cephei, whose brightnesses changed continuously and whose peak brightnesses were not necessarily identical from period to period. In the case of Algol, the duo correctly surmised that the changes in brightness could be explained by transits of some dimmer object across the star, and even postulated that it might be a planet! Thus, they anticipated the transit method of planet detection by over two centuries. In fact, a transit is responsible for the variability in the Algol system, but it is a fainter companion star – not a planet – that periodically passes in front of the primary star. While Goodricke and Pigott eventually settled on an explanation based on starspots instead of the eclipsing hypothesis, it was still a prodigious feat of insight to consider it in the first place.

All in all, they were doing some good astronomy back in the 1700s! Delta Cephei and Algol have variability evident to the naked eye, so determining their period is the kind of project you can do on your own time or for a public outreach effort. If you do try this, drop me a line — let’s see how we compare to Goodricke and Pigott!

About Sukrit Ranjan

I am a graduate student at Harvard University studying extrasolar planets and astrobiology. In my free time, I like books, board games, and flying over Boston. http://abstrusegoose.com/308


  1. Marvelous post for the historian in me. It’s hard to imagine what the night sky looked like then even when you go to areas reasonable dark.

  2. AAVSO continues the tradition, particularly with the recent successful campaign that resulted in thousands of visual estimates for the infrequent eclipsing binary, epsilon Aurigae (P=27 years).



  1. Астрономия за неделю. 30 апреля — 6 мая 2012 — Большая Вселенная - [...] in the 18th Century, впечатлениями о которой поделился в блоге astrobites гарвардский аспирант Сукрит Ранджан (Sukrit [...]

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