Kapteyn, Jacobus Cornelius (1851-1922) - Dutch astronomer who studied and cataloged 454,000 stars of the Southern Hemisphere. He also studied the proper motion of stars and in so doing was able to measure the distances to them.
Kepler, Johann (1571-1630) - German astronomer who discovered the three primary laws of planetary motion. He was the first to recognize the elliptical, not circular orbits of the planets. He also published the Rudolphine Tables in 1627, a comprehensive compilation of astronomical data.
Kirkwood, Daniel (1814-1895) - American astronomer who specialized in the study of the asteroid belt. He described how the asteroids were not evenly distributed, gaps appeared because of the gravitational effects from Jupiter. These gaps are now called "Kirkwood Gaps".
Kuiper, Gerard (1905-1973) - his studies of the solar system led to the discovery of an atmosphere around Saturn's moon Titan, the discovery of Miranda, a moon of Uranus, and Nereid, and moon of Neptune. Also a belt of asteroids beyond Pluto is named in his honor.
LaCaille, Nicolas Louis de (1713-1762) - French astronomer who charted the skies of the Southern Hemisphere. He published catalogues of the stars which included fourteen new constellations, based upon scientific instruments and machines of the time.
LaGrange, Joseph Louis (1736-1813) - astronomer and mathematician who devised mathematical treatments for the motions of the objects in the solar system. His treatments included multi-body systems.
LaLande, Joseph Jerome Le Francais de (1732-1807) - French astronomer who published a catalog in 1801 of over 47,000 stars. He also recorded the position of Neptune a century before its discovery. Unfortunately, he thought it was a star, not a planet.
Langley, Samuel Pierpont (1834-1906) - American astronomer and inventor who in 1881 created the bolometer, a device used to measure tiny changes in heat. Using this device he was able to make careful measurements of solar radiation into the infrared.
LaPlace, Pierre Simon de (1749-1827) - French scientist and mathematician who used advanced mathematical techniques to study the gravitation interactions between Jupiter and Saturn. He used the mathematics to explain the observed perturbations in the planets orbits.
Lassell, William (1799-1880) - discovered two moons of Uranus in 1851, and also discovered the moon Triton in retrograde orbit around Neptune.
Leavitt, Henrietta (1868-1921) - in 1912 while studying the natures of Cepheid variables stars discovered the relationship between luminosity and period of the stars. The brighter they are, the slower their changes in brightness.
LeVerrier, Urbain Jean Joseph (1811-1877)- theoretically predicted the presence of the planet Neptune based upon the perturbations of the orbit of Uranus.
Liu Xin [Liu Hsin] (c. 50 A.D.) - studied and cataloged 1080 stars. He divided the stars into 6 magnitude categories. He also calculated an early estimate for the length of the year.
Lockyer, Sir Joseph Norman (1836-1920) - English astronomer who specialized in studying the Sun. He was the first to study the spectra of sunspots, and along with Janssen discovered the element helium in the Sun and studied prominences without a solar eclipse. In 1869 he founded the journal Nature.
Lomonosov, Mikhail Vasilievitch (1711-1765) - Russian astronomer who during the 1761 transit of Venus in front of the Sun was the first to detect the atmosphere of the planet. He was not credited with the discovery until 1910 when a translation of his work became known outside of Russia.
Lowell, Percival (1855-1915) - American astronomer who in 1894 founded an observatory in Flagstaff, Arizona in order to study Mars. He published maps of the planet in which he claimed included canals built by inhabitants of the planet.
Marius, Simon (1573-1624) - German astronomer who claimed to have spotted the Galilean moons of Jupiter prior to Galileo, a fact which has never been proven. He is credited with giving the four satellites their current names, and he is the first person to record the presence of the Andromeda Galaxy.
Maskelyne, Nevil (1732-1811) - the fifth Astronomer Royal of England who attempted to view the 1761 transit of Venus from the island of St. Helena in the South Atlantic. He also was the first person to determine the mass and mean density of the Earth.
Maunder, Edward Walter (1851-1928) - English astronomer who while studying the history of sunspots discovered a period from 1645 to 1715 in which there appeared a lack of sunspots. This has been since called the "Maunder minimum".
Maury, Antonia C. - Harvard astronomer who in studied the spectra of stars. Beginning in 1897 she categorized the different spectra of stars of the same color. She narrowed the classification sequences then in use.
Mayer, Johann Tobias (1723-1762) - German astronomer who published detailed observations of the Moon, including measurements of the positions of lunar mountains and other features. He study of the Moon helped to define the effect of lunar libration.
Messier, Charles (1730-1817) - French astronomer who in 1781 published a catalog listing over a hundred objects that can be found in the night sky. The list was to help astronomers by charting objects that they might encounter when searching for comets or other phenomena.
Meton (c. 440 BC) - discovered in 432 BC that 235 lunar months make up nearly 19 solar years (this is known as the Metonic Cycle). This allowed the Greeks to arrange their calendar to have 12 years of 12 lunar months followed by 7 years of 13 lunar months. This would keep their lunar calendars matched to the seasons.
Milne, Edward Arthur (1896-1950) - English scientist who developed ways to determine the temperature of the Sun and varying depths. His research also led to concept of a solar wind, and the theory of the "cosmological principle".
Mitchell, Maria (1818-1889) - the first professional woman astronomer in the United States. In 1847 she discovered her first comet and in 1865 was appointed a professor of astronomy at Vasser College.
Muller, Johann (1436-1476) - also known as Regiomontanus, early astronomer who took careful observations of astronomical events. He is most known for his book Ephemerides which plotted the positions of the Sun, Moon, and planets from 1475 to 1506.