The Physics of the Universe - Difficult Topics Made Understandable

Important Dates and Discoveries

A brief chronological listing of some of the most important discoveries in cosmology, astronomy, and physics, from ancient Babylon, India and Greece, right up to the 20th Century. Learn how some of the essential concepts and laws of modern physics which are mentioned in this website (and the earlier ideas out of which they grew) developed in a historical context. For a slightly different perspective, also see the section on Cosmological Theories Through History.

For convenience I have split it into sections:

  • 20th -16th Century B.C. - Ancient Babylonian tablets show knowledge of the distinction between the moving planets and the “fixed” stars, and the recognition that the movement of planets is regular and periodic.
  • 15th - 12th Century B.C. - The Hindu Rigveda of ancient India describes the origin of the universe in which a “cosmic egg” or Brahmanda, containing the Sun, Moon, planets and the whole universe, expands out of a single concentrated point before subsequently collapsing again, reminiscent of the much later Big Bang and oscillating universe theories.
  • 5th Century B.C. - The Greek philosopher Anaxagoras becomes arguably the first to formulate a kind of molecular theory of matter, and to regard the physical universe as subject to the rule of rationality or reason.
  • 5th Century B.C. - The Greek philosophers Leucippus and Democritus found the school of Atomism, which holds that the universe is composed of very small, indivisible and indestructible building blocks known as atoms, which then form different combinations and shapes in an infinite void.
  • Geocentric universe of Aristotle and Ptolemy - click for larger version
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    Geocentric universe of Aristotle and Ptolemy
    (Original Source N/A:
  • 4th Century B.C. - The Greek philosopher Aristotle describes a geocentric universe in which the fixed, spherical Earth is at the center, surrounded by concentric celestial spheres of planets and stars. Although he portrays the universe as finite in size, he stresses that it exists unchanged and static throughout eternity.
  • 4th Century B.C. - The Greek philosopher Heraclides proposes that the apparent daily motion of the stars is created by the rotation of the Earth on its axis once a day, and that the Sun annually circles a central Earth, while the other planets orbit the Sun (a geocentric model with heliocentric aspects).
  • 3rd Century B.C. - The Stoic philosophers of ancient Greece assert a kind of “island universe” in which a finite cosmos is surrounded by an infinite void (similar in principle to a galaxy).
  • 3rd Century B.C. - The Greek mathematician and geographer Eratosthenes proved that the Earth was round, and made a remarkably accurate calculation of its circumference and its tilt (as well as devising a system of latitude and longitude, and, possibly, estimating the distance of the Earth from the Sun).
  • 3rd Century B.C. - The Greek astronomer and mathematician Aristarchus of Samos is the first to present an explicit argument for a heliocentric model of the Solar System, placing the Sun, not the Earth, at the center of the known universe. He describes the Earth as rotating daily on its axis and revolving annually about the Sun in a circular orbit, along with a sphere of fixed stars.
  • 2nd Century B.C. - The Greek astronomer Hipparchus of Nicea makes the first measurement of the precession of the equinoxes, and compiles the first star catalog (in which he proposes our modern system of apparent magnitudes). He also improves on the Solar System model of Apollonius of Perga, in which an eccentric circle carries around a smaller circle (an epicycle), which in turn carries around a planet.
  • 2nd Century B.C. - The Hellenistic astronomer and philosopher Seleucus of Seleucia supports Aristarchus’ heliocentric theory, and links the tides to the influence of the Moon.
  • 2nd Century A.D. - The Roman-Egyptian mathematician and astronomer Ptolemy (Claudius Ptolemaeus) describes a geocentric model, largely based on Aristotelian ideas, in which the planets and the rest of the universe orbit about a stationary Earth in circular epicycles, which becomes the scientific orthodoxy for nearly two millennia (essentially until Copernicus in the 16th Century). He also details the complex motions of the stars and planetary paths using equants, allowing astronomers to predict the positions of the planets.
  • 5th Century A.D. - The Indian astronomer and mathematician Aryabhata proposes that the Earth turns on its own axis, and describes elliptical orbits around the Sun, which some have interpreted as heliocentrism.
  • 6th Century A.D. - The Christian philosopher John Philoponus of Alexandria argues against the ancient Greek notion of an infinite past, and is perhaps the first commentator to argue that the universe is finite in time and therefore had a beginning.
  • 7th Century - The Indian astronomer Brahmagupta, a follower of the heliocentric theory of the Solar System earlier developed by Aryabhata, recognizes gravity as a force of attraction in his "The Opening of the Universe" of 628, in which he describes a force of attraction between the Sun and the Earth.
  • 9th Century - The Muslim astronomer Ja'far ibn Muhammad Abu Ma'shar al-Balkhi develops a planetary model which some have interpreted as heliocentric model.
  • 9th - 11th Century - Early Muslim and Jewish theologians such as Al-Kindi, Saadia Gaon and Al-Ghazali offer logical arguments supporting a finite universe.
  • 11th Century - The Arab polymath Alhazen (also known as Ibn al-Haytham) becomes the first to apply the scientific method to astronomy.
  • 11th Century - The Persian astronomer Abu al-Rayhan al-Biruni describes the Earth's gravitation as the attraction of all things towards the center of the Earth, and hypothesizes that the Earth turns daily on its axis and annually around the Sun.
  • 11th Century - The Persian polymath Omar Khayyam demonstrates that the universe is not moving around Earth, but that the Earth revolves on its axis, bringing into view different star constellations throughout the night and day. He also calculated the solar year as 365.24219858156 days (correct to six decimal places).
  • 14th Century - The Arab astronomer and engineer Ibn al-Shatir (of the Iranian Maragha school of astronomy) refines and improves the accuracy of the geocentric Ptolemaic model and develops the first accurate model of lunar motion.
  • 15th Century - The Turkish/Persian astronomer and mathematician Ali Qushji rejects the Aristotelian notion of a stationary Earth in favor of a rotating Earth.
  • 15th Century - Somayaji Nilakantha of the Kerala school of astronomy and mathematics in southern India develops a computational system for a partially heliocentric planetary model in which Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits the Earth.
  • Heliocentric universe of Copernicus
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    Heliocentric universe of Copernicus
    (Original Source N/
  • 1543 - The Polish astronomer and polymath Nicolaus Copernicus (adapting the geocentric Maragha model of Ibn al-Shatir to meet the requirements of the ancient heliocentric universe of Aristarchus), proposes that the Earth rotates on its axis once daily and travels around the Sun once a year, and demonstrates that the motions of celestial objects can be explained without putting the Earth at rest in the center of the universe. His Copernican Principle (that the Earth is not in a central, specially favored position) and its implications (that celestial bodies obey physical laws identical to those on Earth) first establishes cosmology as a science rather than a branch of metaphysics, and marks a shift away from anthropocentrism.
  • 1576 - The English astronomer Thomas Digges popularizes Copernicus’ ideas and also extends them by positing the existence of a multitude of stars extending to infinity, rather than Copernicus’ narrow band of fixed stars.
  • 1584 - The Italian philosopher Giordano Bruno takes the Copernican Principle a stage further by suggesting that even the Solar System is not the center of the universe, but rather a relatively insignificant star system among an infinite multitude of others.
  • 1587 - The Danish nobleman and astronomer Tycho Brahe develops a kind of hybrid of the Ptolemaic and Copernican models, a geo-heliocentric system similar to that of Somayaji Nilakantha, now known as the Tychonic system. This involves a static Earth at the center of the universe, around which revolve the Sun and the Moon, with the other five planets revolving around the Sun.
  • 1605 - The German mathematician and astronomer Johannes Kepler establishes his three Laws of Planetary Motion, mathematical laws that describe the motion of planets in the Solar System, including the ground-breaking idea that the planets follow elliptical, not circular, paths around the Sun. Newton later used them to deduce his own Laws of Motion and his Law of Universal Gravitation.
  • 1610 - The Italian mathematician and physicist Galileo Galilei develops an astronomical telescope powerful enough to identify moons orbiting Jupiter, sunspots on the Sun and the different phases of Venus, all of which are instrumental in convincing the scientific community of the day that the heliocentric Copernican model of the Solar System is superior to the geocentric Ptolemaic model.
  • 1632 - Galileo Galilei first describes the Principle of Relativity, the idea that the fundamental laws of physics are the same in all inertial frames and that, purely by observing the outcome of mechanical experiments, one cannot distinguish a state of rest from a state of constant velocity.
  • 1633 - The French philosopher René Descartes outlines a model of a static, infinite universe made up of tiny “corpuscles” of matter, a viewpoint not dissimilar to ancient Greek atomism. Descartes’ universe shares many elements of Sir Isaac Newton’s later model, although Descartes’ vacuum of space is not empty but composed of huge swirling whirlpools of ethereal or fine matter, producing what would later be called gravitational effects.
  • 1638 - Galileo Galilei demonstrates that unequal weights would fall with the same finite speed in a vacuum, and that their time of descent is independent of their mass. Thus, freely falling bodies, heavy or light, have the same constant acceleration, due to the force of gravity.
  • 1675 - The English physicist Sir Isaac Newton argues that light is composed of particles, which are refracted by acceleration toward a denser medium, and posits the existence of “aether” to transmit forces between the particles.
  • Newton's Law of Universal Gravitation - click for larger version
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    Newton's Law of Universal Gravitation
    (Source: Astronomy Notes:
  • 1687 - Sir Isaac Newton publishes his “Principia”, which describes an infinite, steady state, static, universe, in which matter on the large scale is uniformly distributed. In the work, he establishes the three Laws of Motion (“a body persists its state of rest or of uniform motion unless acted upon by an external unbalanced force”; “force equals mass times acceleration”; and “to every action there is an equal and opposite reaction”) and the Law of Universal Gravitation (that every particle in the universe attracts every other particle according to an inverse-square formula) that were not to be improved upon for more than two hundred years. He is credited with introducing the idea that the motion of objects in the heavens (such as planets, the Sun and the Moon) can be described by the same set of physical laws as the motion of objects on the ground (like cannonballs and falling apples).
  • 1734 - The Swedish scientist and philosopher Emanuel Swedenborg proposes a hierarchical universe, still generally based on a Newtonian static universe, but with matter clustered on even larger scales of hierarchy, endlessly being recycled. This idea of a hierarchical universe and the “nebular hypothesis” were developed further (independently) by Thomas Wright (1750) and Immanuel Kant (1775).
  • 1761 - The Swiss physicist Johann Heinrich Lambert supports Wright and Kant’s hierarchical universe and nebular hypothesis, and also hypothesizes that the stars near the Sun are part of a group which travels together through the Milky Way, and that there are many such groupings or star systems throughout the galaxy.
  • 1783 - The amateur British astronomer John Michell proposes the theoretical idea of an object massive enough that its gravity would prevent even light from escaping (which has since become known as a black hole). He realizes that such an object would not be directly visible, but could be identified by the motions of a companion star if it was part of a binary system. A similar idea was independently proposed by the Frenchman Pierre-Simon Laplace in 1795.
  • 1789 - The French chemist Antoine-Laurent de Lavoisier definitively states the Law of Conservation of Mass (although others had previously expressed similar ideas, including the ancient Greek Epicurus, the medieval Persian Nasir al-Din al-Tusi and the 18th Century scientists Mikhail Lomonosov, Joseph Black, Henry Cavendish and Jean Rey), and identifies (albeit slightly incorrectly) 23 elements which he claims cannot be broken down into simpler substances.
  • Wave interference in Thomas Young's double-slit experiment - click for larger version
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    Wave interference in Thomas Young's double-slit experiment
    (Source: Wikipedia:
  • 1803 - The English scientist Thomas Young demonstrates, in his famous double-slit experiment, the interference of light and concludes that light is a wave, not a particle as Sir Isaac Newton had ruled.
  • 1805 - The English chemist John Dalton develops his atomic theory, proposing that each chemical element is composed of atoms of a single unique type, and that, though they are both immutable and indestructible, they can combine to form more complex structures.
  • 1839 - The English scientist Michael Faraday concludes from his work on electromagnetism that, contrary to scientific opinion of the time, the divisions between the various kinds of electricity are illusory. He also establishes that magnetism can affect rays of light, and that there is an underlying relationship between the two phenomena.
  • 1861 - The French scientist Louis Pasteur’s experiments show that organisms such as bacteria and fungi do not appear of their own accord in sterile nutrient-rich media, suggesting that the long-held acceptance of the spontaneous generation of life from non-living matter may be incorrect.
  • 1864 - The Scottish physicist James Clerk Maxwell demonstrates that electric and magnetic fields travel through space in the form of waves at the constant speed of light and that electricity, magnetism and even light are all manifestations of electromagnetism. He collected together laws originally derived by Carl Friedrich Gauss, Michael Faraday and André-Marie Ampère into a unified and consistent theory (often known as Maxwell’s Equations).
  • 1896 - The French physicist Henri Becquerel discovers that certain kinds of matter emit radiation of their own accord (radioactivity).
  • 1897 - The British physicist J. J. Thomson discovers the electron, the first known sub-atomic particle.

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