Asteroid 109 Felicitas was discovered on October 9th 1869 by Christian Heinrich Freidrich Peters. It’s in the main belt, fairly large at about 90 km wide, and dark. It was named after the Roman goddess of luck, success, fertility, good fortune, and a host of related concepts.
On October 9th 1873 German physicist Karl Schwartzschild was born in Frankfurt. He was the man who came up with the formula for the event horizon known as the Schwartzschild Radius, which defines the size of a black hole. Very simply, it’s the size an object of a certain mass has to be so that nothing (not even light) can’t escape from its gravitational pull. So anything smaller than its own Schwartzschild radius can’t be seen. As an example, if the entire mass of the Earth were to be compressed into a sphere with a radius of nine millimetres, it would become a black hole. The Sun, however, would only need to be compressed to about 2 miles. So in this particular case, size would appear to be important.
August 9th, 1604, was a very important day for naked eye astronomy. It was the last time that a supernova in our galaxy was definitely observed by the naked eye. Kepler’s Supernova, as it is usually known, occurred in the constellation Ophiuchus, and was spotted by Johannes Kepler, who was in northern Italy at the time. There have almost certainly been at least two subsequent supernovae in the Milky Way, but there are no records in existence to show that either was seen with the naked eye.
One launch to mention quickly: HETE 2, (High Energy Transient Explorer) launched today in 2000 from a Pacific atoll, to detect gamma ray bursts and perform an X-ray sky survey.