Olbers, Heinrich Wilhelm Matthius (1758-1840) - German physician and amateur astronomer who developed a mathematical method to plot the orbit of comets. He also discovered the second known asteroid, Pallas. Oort, Jan (1900-1992) - astronomer who studied both comets and the galaxy. He is credited with the idea of the "Oort Cloud" an area 50,000 AU from the Sun thought to contain billions of cometary bodies. He also made detailed studies and calculations of the differential rotation of the galaxy.