His work De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres) set out his arguments for a heliocentric (sun-centered) system instead of a geocentric (Earth-centered) system.
. On this day in 1965 – February 20th – NASA's Ranger 8 ended its mission by a crash landing on the Moon.
Ranger 8 was the first NASA spacecraft to take high resolution close-up images of the Moon's surface. Before its own mission ended, it transmitted over 7000 images, intended to help choose landing sites for Apollo missions.
. On this day in 1945 – February 23rd – Ukranian astronomer Svetlana Gerasimenko was born.
Gerasimenko's name is well known because she was the co-discoverer of comet 67P/Churyumov-Gerasimenko. This was the comet explored by the Rosetta mission . Both the Rosetta spacecraft and the lander Philae are on the comet as it continues to orbit the Sun.
. On this day in 1987 – February 23rd – the light from supernova SN 1987A reached Earth.
The supernova occurred in the Tarantula Nebula in our neighboring galaxy the Large Magellanic Cloud. It was the closest supernova seen in nearly four hundred years, visible to the unaided eye in the southern hemisphere. It has greatly increased our understanding of this spectacular phenomenon.
. On this day in 1990 – February 23rd – Pioneer 11 crossed the orbit of Neptune on its way out of the Solar System.
Although on that date the spacecraft went beyond the most distant planet, since there are dwarf planets and many other objects beyond Neptune, the spacecraft still hasn't left the Solar System. It's now nearly 128 times farther from the Sun than the Earth is.
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