Discovery of Neptune

On this day (or the 23rd), the autumn equinox of the planet Neptune was discovered.

How this planet was discovered may seem quite strange to the casual person. That's because Neptune was discovered mainly through predictive calculation before it was really formally observed. The naked eye can't pick up Neptune in the night sky because it's far too dim. With the advent of the telescope, observation of distant bodies, including Neptune was possible. 

Neptune was first discovered in observation during the autumn equinox of 1846, by German astronomers Johann Gottfried Galle and Heinrich Louis d'Arrest. Yes, d'Arrest was his actual name. Thing is, others had spotted the planet in the skies before, like Galileo Galilei in 1612 and 1613. However, those astronomers didn't realize that it was another planet they were observing. There is no clear evidence he thought it was a planet, though. It's why he's not credited as the discoverer. 

English astronomer John Herschel (son of William Hershel, who discovered Uranus) said he had observed Neptune in July 1830. His telescope was more powerful and advanced than Galileo's, but he thought it was a star.