Some History

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:
  1. planetary orbits are ellipses, with the Sun at one focus;
  2. orbits sweep out sectors of equal area in equal time, and
  3. orbital period² is proportional to semi-major axis³

• 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.)

Here, and throughout the text, you can click on any of the images to see a larger version.

	October 22, 2136 B.C. - first record of a solar eclipse.	source
	July 4, 1054 - The Crab Nebula Supernova occurred.	source
	January 7, 13, 1610 - Galileo discovers Callisto.	source
	January 7, 13, 1610 - Galileo discovers Europa.	source
	January 7, 13, 1610 - Galileo discovers Io.	source
	January 7, 13, 1610 - Galileo discovers Ganymede.	source
	March 25, 1655 - Christiaan Huygens discovers Titan.	source
	March 13, 1781 - William Herschel discovers Uranus.	source
	September 23, 1846 - G. J. Galle discovers Neptune.	source
	October 10, 1846 - William Lassell discovers Triton.	source
	August 18, 1868 - Norman Lockyer discovers Helium in the Sun.	source
	August 11, 1877 - Asaph Hall discovers Deimos.	source
	August 17, 1877 - Asaph Hall discovers Phobos.	source
	June 30, 1908 - The Tunguska Event occurred.	source
	May 29, 1919 - General Relativity is tested during solar eclipse.	source
	February 18, 1930 - Clyde Tombaugh discovers Pluto.	source
	February 16, 1948 - Gerard Kuiper discovers Miranda.	source
	October 4, 1957 - Sputnik 1 orbits Earth.	source
	January 2, 1959 - Luna 1 arrives at the Moon.	source
	December 14, 1962 - Mariner 2 arrives at Venus.	source
	July 14, 1965 - Mariner 4 arrives at Mars.	source
	January 31, 1966 - Luna 9 lands on the Moon.	source
	October 18, 1967 - Venera 4 enters Venus' atmosphere.	source
	February 24, 1968 - Jocelyn Bell discovers pulsar in the Crab Nebula.	source
	July 20, 1969 - Apollo 11 lands on the Moon.	source
	December 15, 1970 - Venera 7 lands on Venus.	source
	February 15, 1973 - Pioneer 10 passes through the asteroid belt.	source
	December 3, 1973 - Pioneer 10 arrives at Jupiter.	source
	March 29, 1974 - Mariner 10 arrives at Mercury.	source
	July 20, 1976 - Viking 1 lands on Mars.	source
	March 10, 1977 - Uranus' rings discovered.	source
	March 4, 1979 - Jupiter's ring discovered by Voyager 1.	source
	September 1, 1979 - Pioneer 11 arrives at Saturn.	source
	June 13, 1983 - Pioneer 10 becomes first spacecraft to leave the Solar System.	source
	September 11, 1985 - International Cometary Explorer arrives at Giacobini-Zinner.	source
	January 24, 1986 - Voyager 2 arrives at Uranus.	source
	August 25, 1989 - Voyager 2 arrives at Neptune.	source
	December 7, 1995 - Galileo enters Jupiter's atmosphere.	source
	February 22, 2000 - Stardust collects coma material from P/Wild 2.	source
	February 12, 2001 - NEAR lands on Eros.	source
	December 3, 2001 - Genesis collects solar wind particles.	source
	January 14, 2005 - Huygens probe lands on Titan.	source
	July 15, 2015 - New Horizons probe arrives at Pluto	source

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.

• Table of Contents
• References
• Index

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