What did Immanuel Kant do for astronomy?

Answer 1

A lengthy answer:

Kant's most striking early contribution to knowledge, however, was his General Natural History and Theory of the Heavens (1755). Kant had two noteworthy theories in physics and astronomy. One was the "Nebular Hypothesis" of planetary formation. Kant reasoned that diffuse nebulae, dim clouds of dust and gas that were only first being well observed in his lifetime, would collapse under the force of gravity. As they did, they could begin spinning and would then spin out into a disk. From these spinning disks stars and planets would form. Unlike the greatest of earlier German philosophers, Leibniz, Kant was not himself much of a mathematician, so the theory was not given a mathematical form until the great French mathematician Laplace (1749-1827) in 1796. Although there was still argument in my childhood about the formation of planets, it now seems to be generally accepted that both stars and planets condense out of nebulae and collapsed, spinning disks of dust and gas. There are stellar "nurseries" that can be examined in places like the Orion Nebula. Kant was right. Unfortunately, some astronomy textbooks refer to the Nebular Hypothesis as the theory of Laplace alone, instead of the Kant-Laplace theory, and do not give Kant proper credit.

Kant's second theory was also about nebulae, of a different kind. Along with bright and dark diffuse nebulae and planetary nebulae (which have nothing to do with planets), there are spiral nebulae. Laplace believed that these were actually the spinning disks of the Nebular Hypothesis. Kant had a different idea. In 1750 Thomas Wright had suggested that the Milky Way, the Galaxy, was a vast spinning disk itself, consisting of stars and everything else, and that the earth was part of this system. An observational confirmation of this came from the great astronomer William Herschel in 1785. Kant's idea was that the tiny spiral nebulae were themselves external galaxies, "island universes" independent of the Milky Way. There was really no evidence for this. It was just a guess, and Kant may even have been confused about some issues. Nevertheless, it launched a great debate that lasted all the way until 1924. Astronomers were either Laplaceans or Kantians. Thus, as late as 26 April 1920, there was a formal debate over the issue between Harlow Shapley (the Laplacean) and H.D. Curtis (the Kantian) at the National Academy of Sciences. The matter was not settled by the debate but by Edwin Hubble (1889-1953) in 1923-1924, using the recently completed Mount Wilson 100 inch telescope, in the San Gabriel Mountains above Pasadena, California (the mirror was hauled up the mountain by mules) -- visible from the San Fernando Valley. Hubble was able to identify Cepheid variable stars in nearby spiral nebulae. With this kind of variable star, their period of variation is proportional to their absolute brightness. With their absolute brightness (M), compared to the apparent brightness (m), Hubble would know their absolute distance (m - M = 5 log (r/10), where r is in parsecs). They were far, far further away than the stars of the Milky Way Galaxy. They were in external galaxies. Kant was right. Hubble's sensational results were presented to a meeting of the American Astronomical Union in December 1924. Now, since it was just a guess, how much credit can we give to Kant? A fair amount, since many great ideas in science begin as guesses, sometimes with little or contrary evidence behind them (as was the case with Copernicus, come to think of it). More importantly, if we ask who the first person was to conceive the form of the universe as we now see it, filled with "billions and billions" of galaxies (as Carl Sagan liked to say), the answer is just: Immanuel Kant, a man who never left East Prussia and who never saw a mountain.

Answer 2

He changed the stars... if you know what I mean.

Answer 3

http://en.wikipedia.org/wiki/Immanuel_Kant