Johannes Kepler (27.12.1571-15.02.1630) was a German scientist and one of the creators of celestial mechanics. He was born in Weil der Stadt (Württemberg). He graduated from the University of Tübingen (1593). In 1594-1600 he worked at the Higher School in Graz. In 1600 he moved to Prague to join the Danish astronomer Tycho Brahe, and soon after Brahe's death he became mathematician at the court of the Emperor Rudolf II. In 1612 he moved to Linz and in 1626 to Ulm. He spent the last years of his life in poverty and wandering.
His work was in the fields of astronomy, mechanics, optics and mathematics. Using the observations of Tycho Brahe and his own, he discovered the laws of planetary motion (Kepler's three laws). He set out the first two laws in the treatise “New Astronomy” (1609) and the third in the treatise “The Harmony of the World” (1619). He was an active supporter of the teaching of N. Copernicus and his work helped to establish and develop it. Kepler's treatise “Epitome of Copernican Astronomy”, in which he showed that the first two laws he had discovered for Mars hold for all the other planets and for the Moon, and the third also for the motion of the four satellites of Jupiter then known, was placed by the Vatican on its list of forbidden books. Kepler's laws became the basis for I. Newton's discovery of the law of universal gravitation. He also held that the Sun is one of a great number of stars, and that the other stars, scattered through space, are likewise surrounded by planets. In 1627 he completed his last major work, “The Rudolphine Tables”, from which the positions of the planets could be calculated for any moment of time to a fairly high accuracy. Several generations of astronomers used these tables.
Kepler's contribution to optics was great as well. The treatise “Supplements to Witelo”, which appeared in 1604, set out the foundations of the new optics and gave the mechanism of vision. He developed a theory of the image that took account of its geometrical construction, demolishing the ingenious theory of his predecessors, and proposed his own scheme of vision, in which the rays of light emitted by bodies are refracted in the refracting medium of the eye and form an image on the retina, which leads to a physiological and psychological process ending in the subjective perception of the external world. In 1604 he formulated the law of the inverse proportionality between illuminance and the square of the distance to the light source.
He is known as the designer of a telescope (the so-called Keplerian telescope, made up of two biconvex lenses) described in the treatise “Dioptrics” (1611). His treatises also describe an approximate law of refraction and the phenomenon of total internal reflection, and give the lens formula and the conditions of vision. In Kepler we also find the idea of gravitation (1609) and the thought that the cause of the tides in the oceans is the Moon. He proposed the concept of force as the cause of acceleration.