This timeline illustrates the changing rates of progress in our understanding of gravity and planetary motion.

(Click on the timeline to expand it.)

The annotations are roughly separated into "experimental" (above the line) and "theoretical" (below it). Newton is exceptional for appearing on both sides. The long "dry spell" between Ptolemy and Copernicus is noteworthy: Aristotle's emphasis on the circle as a "perfect" geometric figure seems to have blinded thinkers for a sizable part of recorded history.

- Pythagoras is credited not only with proving the distance theorem, but with realizing that the "evening star" and the "morning star" are one and the same: Venus.
- Aristarchus was the first person to figure out the correct structure of the Solar System.
- Ptolemy wrote
*Almagest*, an astronomical treatise based on circular motions of the Sun, planets and stars about the Earth, which codified astronomical thinking for almost one and a half millenia. - Nicolaus Copernicus (1473-1543) re-invented the
**heliocentric**model of the Solar System, in which the planets revolve around the Sun. - Tycho Brahe (1546-1601) made the most accurate astronomical observations up to his time, without the use of a telescope. Kepler used them to derive his laws of planetary motion.
- Galileo Galilei (1564-1642) performed experiments which led to his development of
**kinematics**: the quantitative description of motion. He built the first astronomical telescopes, using them to discover the**Galilean Moons of Jupiter**: Io, Europa, Ganymede and Callisto. He observed Saturn's rings, although he did not understand their true nature, and he observed the phases of Venus, providing evidence for the validity of Copernicus' model. His*Dialogue Concerning the Two Chief World Systems*features a series of illuminating and entertaining discussion between three characters: Salviati, a proponent of the Copernican view; Simplicio, a proponent of the Ptolemaic view, and Sagredo, who has not yet made up his mind. - Johannes Kepler (1571-1630) developed his three laws of planetary motion:
- planetary orbits are ellipses, with the Sun at one focus;
- orbits sweep out sectors of equal area in equal time, and
- orbital period
^{2}is proportional to semi-major axis^{3}

- Isaac Newton (1643-1727) developed his law of gravitation, designed the reflecting telescope, and used prisms to observe spectra. He also invented calculus, but we won't be using it much.
- Ernest Mach (1838-1916) hypothesized that
**inertia**(what makes it hard to change velocity) is related to the contents of the universe as a whole, an idea which later influenced Einstein. - Roland Eotvos (1848-1919) performed the first experiments to test the equivalence of
**gravitational mass**(source of the force of gravity) and inertial mass. This was called the "Equivalence Principle" by Einstein, and is one of the assumptions behind General Relativity. - Albert Einstein (1879-1955) developed the Special (1905) and General (1915) Theories of Relativity. He won his Nobel prize in 1921, however, for explaining the photoelectric effect, in which photons eject electrons from metals; it took many years for his theories of relativity to gain widespread understanding and acceptance.
- Karl Schwarzschild (1873-1916) discovered the first exact solution to Einstein's equations, later understood to describe a static black hole.
- In 1974, Russell Hulse and Joseph Taylor discovered a binary pulsar; after two decades of observations, they determined that its changing rate was consistent with the gravitational waves predicted by General Relativity. They won the Nobel prize in 1993 for this work.
- String Theory, which began in the 1960s as a theory of strong interactions, is an elegant and complex theory which posits that particles are actually strings. To date it has no experimental verification. Could this be another of Aristotle's circles?
- GPS (Global Positioning System) is probably the first example in human experience of the commercialization of so esoteric a theoretical framework as General Relativity.

(View Cosmos DVD 2, episode 3, Kepler's life.)Some particularly interesting dates:

These dates represent first successes; for a more complete picture of the history of space exploration, see NASA's Chronology of Lunar and Planetary Exploration.

©2010, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

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