Herschel discovers Uranus: from a comet report to a planet

On March 13, 1781, William Herschel noticed an unusual object while observing stars through a telescope in Bath, England. Changing the magnification left the stars looking like points, while this object appeared as a small disk. He recorded the difference.

Over the following nights it moved against the background stars. Herschel measured its position and initially reported the object as a comet.

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Herschel’s observations were submitted to the Royal Society as Account of a comet, read on April 26, 1781, and published in Philosophical Transactions. The record preserved positions from the discovery night and subsequent observations, with diagrams tracing the object among the stars.

The title retained his initial interpretation. Continued measurements revealed the object’s identity through the orbit it followed.

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Astronomers combined observations from several locations to calculate an orbit. The object moved around the Sun on a nearly circular path beyond Saturn. It was established as a planet, extending the known reach of the Solar System.

The object was Uranus. Its accumulated position measurements were later used to examine irregularities in its motion, providing clues in the search for another outer planet.

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Mercury, Venus, Mars, Jupiter and Saturn were familiar planets visible to the unaided eye. Herschel’s systematic telescopic examination of the sky revealed a new member whose identification required repeated observation and measurement.

The Royal Society’s surviving manuscript and diagrams preserve that work. Their dates and star charts trace how an unusual point of light, followed over several nights, entered the list of Solar System planets.

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