Kepler, Johannes Johannes Kepler, oil painting by an unknown artist, 1627; in the cathedral of Strasbourg, France. Erich Lessing/Art Resource, New York

It may seem morbid to herald the death of a famous astronomer as an important date. Undoubtedly, Tycho Brahe was extremely valuable to science while he lived. He created the most precise observational instruments of his time, the best until the invention of the telescope, and with them conducted meticulous observations of the sky. However, Tycho jealously guarded his data, especially from his assistant, Johannes Kepler, whom he set to the task of fitting Mars’s orbit into his celestial model (in which Earth was the center of the universe). After Tycho’s death, Kepler was able to obtain that data (although he did not use the most legal means). Using Tycho’s observations, Kepler discovered that the orbit of Mars—and those of all the other planets—was an ellipse, not a circle. From there Kepler crafted his laws of planetary motion, which describe how the planets orbit the Sun in the solar system and set the stage for Newton’s description of gravity.