THE STUDY OF MOTION: A TIMELINE
Aristotle (384 BCE – 322 BCE)
 Developed a philosophical theory of motion, in which a body moves to its
“natural place”, and then stops
Aristarchus of Samos (c. 310 BCE – c. 230 BCE)
 Measured the distances to the Sun and Moon, and their diameters, in units of the diameter of the Earth
 Possibly believed the Sun was at the centre of the universe (and the Earth therefore moved around the Sun)
Eratosthenes of Cyrene (c. 276 BCE – 195 BCE)
 Measured the diameter of the Earth
Hipparchus (c. 190 BCE – c. 120 BCE)
 Developed the first accurate, quantitative models for the motion of the Sun and the Moon
 Discovered the precession of the equinoxes: a motion of the celestial poles (the extensions of the Earth’s north-south axis into the celestial sphere) in which the poles travel round a circular path approximately once every 26,000 years
Ptolemy (c. 90 CE – 168 CE)
 Devised a mathematical model of an Earth-centred (geocentric) universe, based on circular motions, which survived (with changes and corrections) for approximately 1500 years
Nicolaus Copernicus (1473 – 1543)
 Devised a mathematical model of a Sun-centred (heliocentric) universe, based on circular motions, which he claimed was superior to Ptolemy’s in terms of calculations
 May have believed his model was not just a mathematical convenience, and that the Earth really did move around the Sun, but this is not absolutely certain
Tycho Brahe (1546 –1601)
 Greatest naked-eye astronomer in history
 Recorded positions of planets to an accuracy of 1-2 minutes of arc (1/60 –
1/30 of a degree)
 Devised a “Hybrid model of the universe, in which the Sun and Moon moved around the Earth, but all other planets moved around the Sun
Galileo Galilei (1564 – 1642)
 Believed in and publicized Copernicus’ s heliocentric theory

 First to use a telescope to make astronomical observations (in 1609); observed mountains on the moon, phases of Venus, and moons in orbit around Jupiter
 Carried out quantitative experiments to investigate laws of motion of terrestrial objects
 Rolled balls down ramps and discovered that the distance traveled was proportional to the time taken
 May or may not have dropped objects from the leaning tower of Pisa.
Johannes Kepler (1571 – 1630)
 Determined to solve the problem of describing the motion of the planets, he arranged to work with Tycho Brahe in order to have access to Brahe’s data
 After many years of effort, devised a model for planetary motion, using circular motions, which was far more accurate than any previous attempt: only the predicted positions of Mars disagreed with Brahe’s observations— and never by more than 8 minutes of arc (approximately 1/8 of a degree)
 Rejected this first model, knowing that Brahe’s observations were accurate to at least 2 minutes of arc
 Published his first 2 laws of planetary motion in 1609: o Each planet moves in an ellipse, with the Sun at one focus of that ellipse o The line segment joining the planet to the Sun sweeps out equal areas in equal times
 Published his third law, which relates the motions of different planets, in
1619: o The square of a planet’s period is proportional to the cube of the semimajor axis of its orbit
Isaac Newton (1642 – 1727)
 One of two independent discoverers of calculus (the other was Gottfried
Leibniz)
 In 1687, published Philosophiae Naturalis Principia Mathematica
(“Mathematical Principles Of Natural Philosophy"); usually called the
Principia), in which he stated his 3 laws of motion: o First law: Every body remains in a state of rest or uniform motion
(“constant velocity”), unless it is acted on by a force. o Second law: A body of mass m subject to a force F undergoes an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass, i.e., F = ma. o Third law: The mutual forces of action and reaction between two bodies are equal and opposite.
 Showed that Kepler’s three laws of planetary motion followed from these general laws, as do Galileo’s law for falling bodies near the Earth, and descriptions of motion in many other settings.