Mr. Herschel's Millstone
Today, an early Milky Way model.
The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.
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William Herschel lived in late-1700s Bath, England. A very talented musician, he composed music, played the organ, and taught. But in his spare time, he looked at the stars.
Herschel, originally not a professional astronomer, was a serious amateur. To see the stars, he built his own telescopes in his back garden.
This painting of William Herschel (1738 - 1822), was done in 1785 by the famous English portraitist, Lemuel Francis Abbott (1760/61 - 1803). This was around the same time this super-amateur, known as “The King’s Astronomer” following his discovery of Uranus, was doing his “star gauging” counts. It was in this way he tried to determine the shape of the Milky Way Galaxy with the aid of his younger sister, Caroline (1750 – 1848). (Public domain)
In 1781, he made a huge discovery with one of his modest telescopes; a new planet—Uranus. No one had ever found one in recorded history, doubling the solar system’s size, a great leap for science.
But Herschel didn’t stop there. He observed other astronomical things, like foggy patches called nebulae. Some of these looked like planets, so he named them "planetary nebulae," which is still used today.
Eighteenth century astronomers didn't know the Milky Way was a galaxy, or that others even existed. They thought everything they saw in the night sky was only inside our own giant group of stars.
Herschel decided to map its shape. He used another large homemade telescope with a mirror three times bigger than the one used to find Uranus. Without special tools to measure how far away stars were, he invented his own method called "star gauging."
Herschel assumed all stars were about the same brightness. If a star looked bright, it was close; if it looked faint, it was far away. Though not quite right, it was a clever first try.
Pointing his telescope to over 600 different directions in the sky seen near London, Herschel tallied the stars observed at the eyepiece. If he counted many stars, he thought the galaxy was broader in that direction. If he counted fewer, he was looking at the edge.
It was painstaking work, often assisted by his sister, Caroline, who transcribed his findings.
Counting and calculations completed, Herschel drew the very first picture of our Milky Way. It looked like an “amoeba,” which he interpreted to be like a gristmill’s millstone. Our Sun lay somewhere near the middle.
The results of Herschel’s “star gauging” counting and measurements across over 600 regions of the night sky he observed with his homemade “20-foot” reflecting telescope at Observatory House, Slough west of London. Telescopes at this time were referred to by their focal length (20-foot) as opposed to the diameter of their objective lens (in a refractor) or its mirror (in a reflector). Our Sun is the slightly larger black dot just to the right of center. Sometimes described as looking like an “amoeba,” this is what led the German-British astronomer to conclude that our Milky Way Galaxy could possibly be shaped like a gristmill’s millstone. The original image is from “On the Construction of the Heavens. By William Herschel, Esq. F. R. S. Philosophical Transactions of the Royal Society of London, Vol. 75. (1785), pp. 213-266.” (Public domain)
We now know the Milky Way is spiral-shaped, with the Sun far from the center. And, over a century after his work, stars have different brightnesses, which didn’t match Herschel's assumption.
A modern interpretation (not to scale or proportion) of Herschel’s millstone model of the Milky Way Galaxy. This great stellar assembly, roughly 87,400 light-years ±3,600 light-years across and 1,000 light-years thick, would not have a hole in its center like this in reality. Full of stars, the large white dot in the disk’s center represents where Herschel believed where the Sun was located. (Original millstone drawing from Wikipedia by Madboy74, CC BY-SA 4.0, with the starfield and Sun added by the author)
Does that make his work less important? No, as Herschel’s observations are an example of human ingenuity. He showed us that with a homemade telescope, some simple math, and a lot of patience, one person can map our Galaxy.
I’m Tom Callen on behalf of the University of Houston, and also interested in the way inventive minds work.
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Some references
The Milky Way Galaxy - Wikipedia
Edwin Powell Hubble - Wikipedia
This Episode first aired on June 17, 2026.