- Greek philosopher Aristotle (384 - 322 b.c. - https://en.wikipedia.org/wiki/Aristotle) believed that the Earth is a motionless sphere based on three observations:

> In Moon eclipses the Earth's shadow was always round > The Polar star has been seen at different elevation above the the horizon. Its different relative position in the sky in Greece and Egypt allowed to estimate the Earth's circumference > From a distance the sails of a tall ship have always been seen before the hull (according to S. Weinberg, Aristotle did not mention this fact) > According to S. Weinberg, Aristotle thought that the Earth did not move because astronomers were not able to observe the parallax of the stars and the fact that an object thrown up from one point fell to the same point

- Heraclides Ponticus (390 - 310 b.c.) proposed that the Earth rotated on its axis and perhaps, even around the Sun. See https://en.wikipedia.org/wiki/Heraclides_Ponticus
- Aristarchus of Samos (310 – 230 b.c.) in “On the Sizes and Distances (of the Sun and Moon)” estimated relative to the Earth sizes of the Sun and the Moon and the distances between the Sun, the Earth and the Moon in Earth diameters using phases of the Moon, as well as solar and lunar eclipses. See https://en.wikipedia.org/wiki/On_the_Sizes_and_Distances_(Aristarchus). Moreover, almost two millenniums before Copernicus he suggested that the Earth rotated around the Sun and that the distances to the stars were much larger than to the Sun.
- Eratosthenes of Cyrene (276 - 194 b.c.) calculated the circumference of the Earth using knowledge of the angle of elevation of the Sun at noon on the summer solstice in Alexandria and on Elephantine Island near Syene. He also calculated the tilt of the Earth’s axis, the distance from the Earth to the Sun and invented the leap day. See https://en.wikipedia.org/wiki/Eratosthenes
- Hipparchus of Nicaea (190 - 120 b.c.) developed a reliable method to predict solar eclipses, measured Earth’s precession, compiled the first star catalog with 800 stars and possibly invented astrolabe. See https://en.wikipedia.org/wiki/Hipparchus
- Claudius Ptolemy (100 - 170 a.c.) created a complete cosmological model. The Earth was in the center, surrounded by eight spheres: Moon, Mercury, Venus, Sun, Mars, Juniper and Saturn. The planets were assumed to move in a small circle called an epicycle, which in turn moved along a larger circle called a deferent. Both circles rotated clockwise and were roughly parallel to the plane of the Sun’s orbit (ecliptic). Despite the fact that the system was considered geocentric, each planet’s motion was not centered on the Earth but at a point slightly away from the Earth called the eccentric. See https://en.wikipedia.org/wiki/Deferent_and_epicycle
- 1514. Polish priest Nicholas Copernicus (initially anonymously) suggested the Sun-centered model, the absence of the celestial sphere as well as the universe’s boundary. The stars could be thought of as our Sun but father away. He was the first one who correctly described the order of the planets. Based on their observed period around the Sun, he estimated the radius of their orbits, which implicitly established the dependency of the speed of a planet and its distance to the Sun.
- 1609. Italian Galileo Galilei publicly supported Copernican theory despite the fact that it did not describe well the observed orbits. He observed in just invented first telescope the moons of Jupiter, orbiting around it, disproving Ptolemy’s model. The same year German Johannes Kepler almost by accident discovered an elliptical form of the orbits (by studying observations of the 16th-century Danish astronomer Tycho Brahe, so called Kepler’s first law of planetary motion. His second law states that the radius vector of any planet to the Sun sweeps out equal areas in equal time, the angular momentum conservation law, which proved crucial for the Newton’s law of gravitation.
- 1618. The third Kepler’s law: the squares of the sidereal periods of the planets are directly proportional to the cubes of their mean distances from the Sun (Encyclopedia Britannica).
- 1676. Danish astronomer Ole Christensen Roemer discovered by observing the moons of Jupiter, that the speed of light is very high but finite. His estimate of the speed of light was not too far from what we know today.
- 1687. Isaac Newton in his Philosophiae Naturalis Principia Mathematica postulated the law of universal gravitation (the second law) that explained the elliptical orbits and showed that the universe could not be static (the first law, the law of inertia). He got rid of the Aristotle’s idea of absolute rest, another words, the space despite Newton’s belief became relative.
- 1865. British physicist James Clerk Maxwell unified electricity and magnetism. According to his equations the electromagnetic field would travel at a constant speed. According to Newton, the speed (a relative distance divided by the absolute time) could only be relative, so relative to what? Ether theory was born. Another words, for the observers moving through out the ether towards the source of light or in the opposite direction the speed of light would be different.
- 1887. Albert Michelson (the first American to receive the Nobel prize in physics) and Edward Morley found that the speed of light is the same in both directions: along the Earth motion and perpendicular to it! It meant that not only the space was relative but the time as well.
- 1905. Einstein created the special theory of relativity (Lorentz transformations). It was an extension of Newton’s law of motion to an electromagnetic field. Basically, it postulated that the laws of science should be the same in all freely moving (except rotating) observation systems regardless of their speed. Such simple idea had great consequences: E = mc2 and that nothing else could move faster than the light. The energy of a moving object will add to its mass, another words, it will make it harder to increase its speed! At 10% of speed of light, the mass would be 0.5% higher than the mass of the object at rest, at 90% - more than double and an infinity at 100%, which would not be possible, because it would require infinite energy to achieve this speed. Hence, photons have zero mass. This gives up the concept of absolute time and establishes the space-time four-dimensional continuum (x,y,z,ct). Hence, the observers must agree on the constant speed of light. Therefore, the light is used to measure distances (the time is measured by atomic clocks).
- 1915. Einstein’s general theory of relativity (effect of gravity on Newtonian system) that explained the planet Mercury small motion deviations (43” per century) from Newton’s law predictions as well as predicted starlight curvature near Sun (which was confirmed in 1919 by Eddington and others) and gravitational (not Doppler!) red-shift in spectrum of starlight (confirmed in 1924 by Adams). The last phenomenon comes from the fact that time is slower in a stronger gravitational field. If it was ignored, the GPS would give position errors of few miles! Here it is: “All Gaussian coordinate systems are essentially equivalent for the formulation of the general laws of nature.” Another words, the equations of the general laws of nature must have the same form when moved from one Gaussian coordinate system to another. The space-time continuum in gravitational field becomes non-Euclidean.
- 1922. Russian Alexander Friedmann constructed the mathematical model of the universe based on the assumption that the universe was isotropic from any point of view. Einstein’s model was a special case when the curvature of space was independent of time, another words, when the universe was static. On the other hand, if such dependence exists, then the universe is expanding.
- 1929. Edwin Hubble proved Friedmann’s theory of expanding universe by measuring Doppler’s red-shift of the distant galaxies. Moreover, their speed was directly proportional to the distance from the Earth! 71km/s for every megaparsecs (Mpc = 3,261 lightyears). This lead to big bang theory, when everything was at the same point in space with an infinite density.

Last edited 2018-09-23 01:04 UTC by 6630 (diff)