Newcomb, Simon (1835-1909) - in 1899 published a comprehensive table of the motion of the Moon. He was a prolific writer who in an early paper, objected to the theory that the asteroid belt was formed by the breakup of a planet between Mars and Jupiter.
Nicholson, Seth Barnes (1891-1963) - American astronomer who took careful measurements of temperatures of the Moon. The results led to the theory that the Moon was covered with a thin layer of dust, acting as an insulator. He also discovered four new moons of Jupiter.
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.
Parsons, William (Lord Rosse) (1800-1867) - using a reflecting telescope with a 6 inch mirror, he was able to discern the spiral structure of certain galaxies, including the Whirlpool Galaxy found near the constellation of Ursa Major.
Payne-Gaposchkin Cecilia Helena (1900-1979) - first female professor at Harvard. She discovered the basic chemical compositions and their abudances in the atmospheres of stars.
Philolaus (c. 480 BC) - a follower of Pythagoras, he believed that the Earth, Moon, and planets traveled in spheres which circled around a central fire that was partially visible in the Sun.
Piazzi, Giuseppe (1746-1826) - Italian astronomer who in 1801 discovered the first asteroid, Ceres.
Picard, Jean (1620-1682) - French astronomer who helped to found the Paris Observatory and is most known for determining the the circumference of the Earth, the first scientist to accomplish it since Eratosthenes of ancient Greece.
Pickering, Edward (1846-1919) - American astronomer who made advances in the study of stellar spectra. He was the first to take large-scale spectra from photographic plates, thereby allowing many the spectra of many stars to be observed, instead of a single star at a time.
Plato (427-347 BC) - Greek philosopher whose real name was Aristocles. He believed the Sun, Moon, planets, and stars moved around the earth in perfect spheres, and the Moon shown by reflected sunlight.
Proctor, Richard Anthony (1837-1888) - English astronomer who was the first to theorize that craters on the Moon were the result of meteor bombardment. He also created maps of Mars, naming features after English astronomers, this was eventually changed by Schiaparelli.
Ptolemy, Cladius (c.120- c.151 A.D.) - Roman astronomer whose work influenced astronomy until the end of the Dark Ages. Proponent of the geocentric model of universe (the Sun, Moon, and planets orbit the earth), he wrote the Megale Syntaxix, better known as the Almagest in which he describes the details of the geocentric theory. Included in the work are detailed charts for predicted future locations of the objects in the solar system.
Pythagoras (580- c.500 BC) - Greek mathematician who proposed that the Earth was a globe surrounded by 8 concentric spheres which had the Moon, Sun, and known planets attached to them.
Ramsden, Jesse - English telescope maker of the late 18th and early 19th centuries. His telescopes were in high demand by astronomers of the time.
Riccioli, Giovanni Battista (1598-1671) - Italian astronomer who is credited with naming many features of the Moon, including naming the craters after famous scientists. He was also the first to observe a double star through a telescope.
Rittenhouse, David (1732-1796) - American clock and instrument maker and astronomer who observed the transit of Venus in 1769 from his Pennsylvania farm. He is considered by many to be the father of American astronomy.
Roche, Edouard (1820-1883) - demonstrated that a moon is limited in its orbit around a planet. If the moon passes within a certain distance (Roche Limit) it will be torn apart by the gravitational interactions between itself and the planet.
Romer, Ole (1644-1710) - Danish astronomer who is credited with the invention of the transit telescope in 1689. By observing the Galilean satellites of Jupiter he came to the conclusion that light travels at a finite speed, and was one of the first scientists to attempt to calculate it.
Russel, Henry Norris (1877-1957) - in 1913 plotted a two-dimensional diagram of the absolute magnitude versus spectral type for stars. In 1911 it was also independently created by Ejnar Hertzsprung. The diagram today is called the Hertzsprung-Russell H-R) Diagram and is an invaluable tool to astronomers.