Feature

The Dearborn Observatory in 1889. University Archives, Northwestern University Libraries. (1889). Dearborn Observatory, Northwestern University, Evanston, Illinois. Retrieved from https://dc.library.northwestern.edu/ items/4257692e-8d56-4593-b316-1a57042e6716

A Brief History of the Dearborn Observatory

By Andrew Laeuger

A brief history of the Dearborn Observatory, from its historic rise to its quiet and unsuspecting presence on campus today.

Hidden in the cove formed by the Technological Institute, Silverman Hall, and the Garrett-Evangelical Theological Seminary, the Dearborn Observatory lies secluded from Northwestern’s skyline. Here, it serves as a reminder of the University’s historical role in observational astronomy.

The Observatory began not as a project funded by bureaucratic grant sources nor by University budget decisions, but instead out of pure public interest in the beauty of the heavens. Documents from the establishment of Dearborn tell how money was raised under the condition that donation size would correspond to a patron’s level of access to the telescope. For example, a donation of $100 in the 1860s would earn one lifetime access to the observation room.

In that same spirit of enthusiasm for astronomy, the Chicago Astronomical Society would not settle for anything less than the best. One of the founding members of the Society, Thomas Hoyne, traveled from Chicago to Boston in four days to purchase what was, at the time, the largest refracting lens ever produced. The polished piece of glass had a 1.5-foot diameter!

Equipped with one of the most powerful refracting telescopes in the world, the Observatory became effective at producing high-resolution images of the night sky on photographic plates. This precision allowed for two types of

astronomical measurements to be carried out with great accuracy: the determination of the distance from Earth to a star based on its parallax1 and the detection of double stars.2

Throughout its lifetime, Dearborn has accounted for the discovery of more than 100 binary stars and thousands of long-exposure observations of dim red stars.3 It has also improved precision in models of the positions of objects orbiting throughout the solar system.4

One of the most famous discoveries from the lens that was installed in Dearborn’s main telescope was the detection of Sirius B. While testing the lens prior to its sale to Hoyne, lensmaker Alvin Graham Clark became the first person to observe that Sirius A, the brightest star in the night sky, has a companion (Sirius B) that is about 10,000 times dimmer than itself.

Additionally, in the early 1930s, astronomers used photographs taken by the Dearborn telescope of the dwarf planet Eros as it completed a nearEarth pass to conduct one of the most complete measurements of the parallax of the sun to date.

However, as early as the late 1910s, ambient light pollution began affecting astronomical data collection at the Observatory. Then-Director Philip Fox began to grow wary of the proliferation of Chicago’s presence in the night sky, citing the inexorable spread of smoke and electric lights as the city continued to expand.

Even today, new sources of light pollution, like the clusters of StarLink satellites launched by SpaceX starting in 2019, make the work of ground-based astronomical observatories ever more difficult. One can only hope that we begin to appreciate the role of astronomy in both the ancient stories that have inspired our modern culture and the contemporary discoveries that drive us towards new understandings of the universe. Then, with steps to reduce light pollution and bring new sources of funding to this all-important science, observational astronomy may once again thrive in Chicago, and Dearborn may bring its guests the same wonder it brought 150 years ago. ■

1 A parallax is the slight change in a star’s location in the sky based on our own position in our orbit about the sun. 2 Double start are stars which when seen through our eyes appear as one bright spot, but are actually composed of a pair of stars orbiting about each other. 3 With enough searching, one can dig up the hundreds of pages of meticulously catalogued observations, dating back to the first years of the 20th century, which the observatory’s directors published in hopes of passing on all that they learned to the next generation of astronomers. 4 Today, the original Dearborn refracting telescope has a home in Chicago’s Adler Planetarium, where the public can continue to admire the skill and craftsmanship required to assemble such a powerful and elegant instrument.