October 13
Charles Messier Identifies First Known Spiral Galaxy
On a clear October night in 1773, French comet hunter Charles Messier recorded a faint nebula in Canes Venatici whose spiral form would only be revealed decades later.
Summary
By the late eighteenth century, astronomers were cataloging nebulae and star clusters to aid comet hunting, with French observer Charles Messier compiling a famous list of such objects. On October 13, 1773, Messier recorded the object now known as Messier 51, later called the Whirlpool Galaxy, noting its unusual appearance in the constellation Canes Venatici. This marked the first identification of a spiral galaxy, though its true nature as a distant stellar system would not be confirmed until the twentieth century with improved telescopes. Messier's methodical observations advanced systematic astronomy and provided a foundation for later catalogs used by William Herschel and others. The discovery occurred amid growing interest in the structure of the universe beyond the solar system.
Context
By the mid-eighteenth century, European astronomers were systematically charting the night sky to support comet searches and refine stellar positions. Improved reflecting telescopes allowed observers to distinguish fixed nebulae and star clusters from transient comets, prompting the creation of dedicated catalogs. Charles Messier, working from Paris under royal patronage, became one of the most diligent such observers, motivated by personal comet discoveries and the need to avoid false alarms.
What Happened
On October 13, 1773, Messier turned his telescope toward the constellation Canes Venatici while monitoring a comet then visible. He noted a very faint nebula without stars near the eye of the Northern Greyhound, below the star Eta Ursae Majoris. Messier logged the position and appearance in his observing records, later assigning it the number 51 in his growing list. He did not remark on any unusual structure at the time, describing it simply as a diffuse patch.
Aftermath
Messier included the object as M51 in the 1781 edition of his catalogue. His colleague Pierre Méchain independently observed a nearby companion (now NGC 5195) in March 1781 and noted the pair’s double appearance. The entry helped standardize references for later observers using larger instruments.
Legacy
Lord Rosse’s 72-inch reflector at Birr Castle first revealed M51’s spiral arms in 1845, establishing it as the prototype “spiral nebula.” Recognition that such objects were distant galaxies comparable to the Milky Way came only in the 1920s through Edwin Hubble’s Cepheid measurements. Messier’s catalog, begun as a practical aid for comet hunters, became a foundational reference for extragalactic astronomy and remains in use today.
Why It Matters
Messier's find initiated the recognition of spiral galaxies as distinct cosmic structures, contributing to the eventual understanding of the Milky Way as one among billions and spurring developments in extragalactic astronomy that shaped modern cosmology.
Related Questions
Why was Messier cataloging nebulae in the first place?
He needed a reliable list of fixed objects that could be mistaken for comets during his searches.
Did Messier know M51 was a galaxy?
No; he described it only as a faint nebula, and its extragalactic nature was proven more than 150 years later.
When was the spiral shape of M51 first noticed?
In 1845 by Lord Rosse using a large reflecting telescope in Ireland.
Where is the Whirlpool Galaxy located in the sky?
In the constellation Canes Venatici, near the handle of the Big Dipper.
How far away is M51?
Approximately 27 million light-years, making it one of the nearer bright spiral galaxies.
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Sources
- What Happened on October 13, A&E Television Networks. Accessed 2026-07-06.
- October 13 - Wikipedia, Wikimedia Foundation. Accessed 2026-07-06.