What happens to constellations that aren't needed anymore?

Although the stars are real, constellations are products of human imagination. All that you need to make changes is to convince other people to accept them. But today astronomers accept the 88 official constellations agreed by the International Astronomical Union (IAU). Since there are no gaps or overlaps, every part of the sky is uniquely defined.

Before the IAU definitions, constellations came and went. Here are half a dozen of my favorite extinct constellations.

Joseph Jérôme de Lalande (1732-1807)
Three of the obsolete constellations I've chosen were invented by Lalande, an 18th century French astronomer. He was prominent enough to be one of the 72 scientists honored by Gustave Eiffel on his famous tower. Lalande supplied a number of star positions to Johann Bode (1747-1826) for his star atlas Uranographia. Bode also accepted some of Lalande's ideas for new constellations, but none survived into the 20th century.

Felis
Lalande wanted to add a cat to sky since there were several constellation dogs, but no kitties. Felis was his answer to this. Lalande said, "I love cats very much. I will have this picture engraved on the star map. The starry sky has made me tired enough all my life to allow me to have a little fun now." He put Felis in between Antlia (the vacuum pump) and Hydra (the water snake) after taking some of their dim stars to make his little cat.

Globus Aerostaticus
Lalande told Bode that the Montgolfier brothers' invention of the hot air balloon was a scientific triumph that should be recognized. So Bode created Globus Aerostaticus. I've never ridden in a hot air balloon, but they're beautiful to see in the sky. I don't think Lalande ever rode in one either, though in 1798 he was part of the public spectacle of a balloon trip made by André-Jacques Garnerin, the Official Aeronaut of France. Lalande assisted Garnerin's passenger, an attractive young woman, into the basket.

The balloon constellation was made out of dim stars between Capricornus (the sea-goat) and Microscopium (the microscope). It looks very pretty in the old atlases. A pity it's gone.

Custos Messium
Astronomers tended to name celestial objects after influential people, in hopes of patronage. This was sensible for anyone without private means. But I liked Custos Messium because Lalande honored the work of another astronomer. Charles Messier (1730-1817) was the great comet hunter. Custos Messium means “guardian of the harvest”, but it's also a pun on Messier's name.

Custos Messium was in what's now northern Cassiopeia between Cepheus and Camelopardalis, next to another now-obsolete constellation, Rangifer (the reindeer). When Lalande put it on his celestial globe in 1775, he made it out of what we might call “spare stars”, i.e., too dim to be part of an older constellation. He chose the part of the sky where the comet of 1774 was first seen. Even though Messier hadn't discovered it, he had made extensive observations of it.

Rangifer
Rangifer is also known as Tarandus, and I quite like the idea of having a reindeer in the sky. In the skylore of northern Scandinavia, the Sami people have a reindeer. However the animal doesn't figure in classical myths.

Pierre Charles Le Monnier (1715-1799) invented the constellation. He was a prodigy, having begun serious observing before he was sixteen, and been admitted to the French Academy of Sciences when he was twenty. He was twenty-one when he was sent with Maupertuis and Clairaut to the Arctic. They went to measure a meridian arc to help determine the shape of the Earth. The reindeer commemorates that expedition.

Telescopium Herschelii
Since the Herschels are my astronomy heroes, I've included a constellation invented in honor of William Herschel. Abbé Maximilian Hell (1720 - 1792), director of the Vienna Observatory, was also a Herschel admirer. Hell devised two constellations, one for Herschel's large telescope and one for a small telescope. Between the two telescopes - around Gemini and Auriga - was the area of sky where Herschel had discovered the planet Uranus.

Bode put only one telescope in his atlas. It was where Hell had located the larger Herschel telescope. Since Bode knew Herschel's telescopes, his drawing of Telescopium Herschelii was a good likeness of the telescope Herschel had used to discover Uranus.

Argo Navis
Argo Navis is the only obsolete constellation that was one of Ptolemy's original 48. It represented Jason's ship Argo as he searched for the Golden Fleece. I love the idea of this glorious ship, low in the heavens, sailing westward through the southern sky. However the constellation had over eight hundred stars visible to the unaided eye. It was so big that it was quite unwieldy for astronomers. Abbé Nicolas-Louis de Lacaille (1713-1762) is known for his invention of over a dozen constellations to fill in the gaps in the southern hemisphere skies. But he also dealt decisively with Argo Navis, dividing it into three smaller constellations: Carina (the keel), Puppis (the poop deck) and Vela (the sails).