On this day in 1571 – December 27th – Johannes Kepler was born near Stuttgart in what is now Germany.
Using Tycho Brahe's data, Kepler formulated his three laws which described the orbits of the planets mathematically. This only worked if the Sun - not the Earth - was at the center of the Solar System.
On this day in 1864 – December 31st – Robert Grant Aitken was born.
Aitken was an American astronomer of international repute. He was best known for his systematic study of double stars, and served for ten years as chair of the double star committee of the International Astronomical Union.