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.
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 developes 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 centre 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. 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 - 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. 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 centre of the universe. 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 centre 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. 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. 1610 - The Italian mathematician and physicist Galileo Galilei develops an astronomical telescope powerful enough to indentify moons orbiting Jupiter, sunspots on the Sun and the different phases of Mercury, 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 Ptolemiac 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 Rene Descartes outlines a model of a static, infinite universe made up of tiny “corpuscles” of matter, a viewpoint not dissimilar to ancient Greek atomism. 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. 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. 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. 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 ever larger scales of hierarchy, endlessly being recycled. 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 travel 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).
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 can not be broken down into simpler substances. 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. 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.
1896 - The French physicist Henri Becquerel discovers that certain kinds of matter emit radiation of their own accord (radioactivity).
1900 - The German physicist Max Planck suggests, while describing his law of black body radiation, that light may be emitted in discrete frequencies or “quanta”, and establishes the value of the Planck constant to describe the sizes of these quanta.
1905 - The German physicist Albert Einstein shows how the photoelectric effect is caused by absorption of quanta of light (or photons), an important step in understanding the quantum nature of light and electrons, and a strong influence on the formation of the concept of wave-particle duality in quantum theory.
1905 - Albert Einstein publishes his Special Theory of Relativity, in which he generalizes Galileo's Principle of Relativity (that all uniform motion is relative, and that there is no absolute and well-defined state of rest) from mechanics to all the laws of physics, and incorporates the principle that the speed of light is the same for all inertial observers regardless of the state of motion of the source. 1905 - In a separate paper, Albert Einstein derives the concept of mass-energy equivalence (that any mass has an associated energy) and his famous E = mc2 equation.
1907 - The German mathematician Hermann Minkowski realizes that Einstein’s Special Theory of Relativity can be best understood in a four-dimensional space, which he calls “space-time” and in which time and space are not separate entities but intermingled in a four-dimensional space. 1911 - The New Zealand chemist Ernest Rutherford interprets the 1909 experiments of Hans Geiger and Ernest Marsden, establishing for the first time the “planetary model” of the atom, where a central nucleus is circled by a number of tiny electrons like planets around a sun. 1915 - The German physicist Karl Schwarzschild provides the first exact solution to Einstein’s field equations of general relativity (even before Einstein publishes the theory) for the limited case of a single spherical non-rotating mass, which leads to the “Schwarzschild radius” which defines the size of the event horizon of a non-rotating black hole.
1916 - Albert Einstein publishes his General Theory of Relativity, in which he unifies special relativity and Newton's Law of Universal Gravitation, and describes gravity as a property of the curvature of four-dimensional space-time. 1916 - The Austrian physicist Ludwig Flamm examines Schwarzschild’s solution to Einstein’s field equations and points out that the equations theoretically allow for some kind of invisible connection between two distinct regions of space-time (later to become known as a “wormhole”). 1917 - Albert Einstein publishes a paper introducing the “cosmological constant” into the General Theory of Relativity in an attempt to model the behaviour of the entire universe, an idea he later called his “greatest blunder” but which, in the light of recent discoveries, is beginning to look remarkably prescient.
1919 - Ernest Rutherford is credited with the discovery of the proton when he notices the signatures of hydrogen nuclei when alpha particles are shot into nitrogen gas. 1919 - The English astrophysicist Arthur Eddington uses his measurements of an eclipse to confirm the deflection of starlight by the gravity of the Sun as predicted in Albert Einstein’s General Theory of Relativity.
1919 - The German mathematician Theodor Kaluza proposes the addition of a fifth dimension to the General Theory of Relativity, a precursor to much later superstring theory attempts to combine general relativity and quantum theory. 1922 - The Russian cosmologist and mathematician Alexander Friedmann discovers the expanding universe solution to Einstein’s general relativity field equations. 1925 - The American astronomer Edwin Hubble proves conclusively that nebulae such as the Andromeda Nebula are much too distant to be part of the Milky Way and are in fact entire galaxies outside our own, thus settling the “Great Debate” about the nature of spiral nebulae and the size of the universe.

1925 - The Austrian theoretical physicist Wolfgang Pauli establishes an important quantum mechanical principle known as the Pauli exclusion principle, which states that no two identical fermions (such as electrons) may occupy the same quantum state simultaneously. 1926 - The German physicist Werner Heisenberg formulates his uncertainty principle, that the values of certain pairs of variables cannot both be known exactly (i.e.
1926 - The Austrian physicist Erwin Schrodinger publishes what is now known as the Schrodinger Equation, a central and revolutionary achievement in quantum physics.
1927 - The Belgian Roman Catholic priest and physicist Georges Lemaitre proposes (even before Hubble’s corroborating evidence) that the universe is expanding, followed in 1931 by the first definitive version of what has become known as the Big Bang theory of the origin of the universe. 1928 - The British physicist Paul Dirac provides a description of the “spin” of elementary particles such as electrons which is consistent with both the principles of quantum mechanics and the Special Theory of Relativity, and predicts the existence of antimatter.
1929 - Edwin Hubble definitively shows that all the galaxies in the universe are moving away from us, according to a formula which has become known as Hubble’s Law, showing that the universe is not in fact static, but expanding. 1934 - The Swiss-American astronomer Fritz Zwicky and the German-American Walter Baade coin the term “supernova” and hypothesize (correctly) that they are the transition of normal stars into neutron stars, as well as the origin of cosmic rays.
1935 - Albert Einstein and the Israeli physicist Nathan Rosen achieve a solution to Einstein’s field equations known as an Einstein-Rosen bridge (also known as a Lorentzian wormhole or a Schwarzschild wormhole).
1935 - The Indian-American astrophysicist Subrahmanyan Chandrasekhar establishes the “Chandrasekhar limit” of about 1.4 solar masses, above which a star must continue to collapse into a neutron star rather than settling down into a white dwarf.
1939 - The discovery of nuclear fission results from the Berlin experiments of Otto Hahn, Lise Meitner, Fritz Strassmann and Otto Frisch.
1948 - The English astronomer Fred Hoyle and the Austrians Thomas Gold and Hermann Bondi propose a non-standard cosmology (i.e. 1953 - The experiments of the American biochemists Stanley Miller and Harold Urey (known as the Miller-Urey experiments) demonstrate the feasibility of producing basic organic monomers such as amino acids from a highly reduced mixture of gases, in an attempt to back up Alexander Oparin’s hypotheses on the origins of life on Earth. 1963 - The New Zealand mathematician Roy Kerr discovers a solution to Einstein’s general relativity field equations which describes a spinning black hole, and argues that these are likely to be common objects throughout the universe. 1965 - The American astronomers Arno Penzias and Robert Wilson discover the existence of cosmic microwave background radiation, considered by most to be the best evidence for the Big Bang model of the universe (and effectively disproving Hoyle et al’s steady state universe theory). 1966 - The Russian physicist Andrei Sakharov outlines the three conditions necessary for the observed matter-antimatter imbalance in the universe to be possible, and hypothesizes about singularities linking parallel universes.
1969 - The Murchison meteorite falls on Australia, revealing significant quantities of organic compounds and amino acids (the basis of early life on Earth) which originated in outer space. 1970 - The English physicist Stephen Hawking provides, along with Roger Penrose, theorems regarding singularities in space-time, indicating that singularities and black holes are actually a fairly generic feature of general relativity.
1980 - The American physicist Alan Guth proposes a model of the universe based on the Big Bang, but incorporating a short, early period of exponential cosmic inflation in order to solve the horizon and flatness problems of the standard Big Bang model. 1980 - The invention of the Scanning Tunnelling Microscope, by the German Gerd Binnig and the Swiss Heinrich Rohrer, shows visually for the first time that matter is composed of spherical atoms stacked row on row.
1983 - The Russian-American physicist Andrei Linde develops Guth’s cosmic inflation idea further with his chaotic inflation (or eternal inflation) theory, which sees our universe as just one of many “bubble universes” that have developed as part of a multiverse. 1984-6 - A series of important discoveries in string theory leads to the “first superstring revolution”, and it is first realized that string theory might be capable of describing all elementary particles as well as the interactions between them.
1995 - The American theoretical physicist Edward Witten and others develop M-theory, and spark a flurry of new research in string theory, sometimes called the “second superstring revolution”. 1998 - Observations of distant Type 1a supernovas, both by the American astrophysicist Saul Perlmutter and by the Australians Nick Suntzeff and Brian Schmidt, indicate that they are actually further away from the Earth than expected, suggesting an accelerating expansion of the universe.
2002 - The American physicist Paul Steinhardt and South African-British physicist Neil Turok propose another variation of the inflating universe known as the cyclic model, developed using state-of-the-art M-theory, superstring theory and brane cosmology, which involves an inflating universe expanding and contracting in cycles. Gravity is a non-contact force (a force that can be exerted on a body without touching it), one of four fundamental non-contact forces we know of.
The latter two are active only at distances as small as those present between protons and neutrons in the nuclei of atoms.
Isaac Newton and others determined, mostly from astronomical data — observations of the motions of planets in the sky — that the gravitational attraction is a property of mass, that it is proportional to the product of the two masses involved, and that it is inversely proportional to the square of the distance between their centers of mass. The graph on the right shows how the force of gravity (Fg) varies as one of the masses is increased from zero and the other is held constant.
We generally think of gravity in terms of one variable mass and one fixed mass, like a planet.
The universal law of gravitation is called an inverse-square law because the force is inversely proportional to the square of the distance between two masses. The important thing about this feature of gravity is that if we double the distance between objects, the force of gravity is reduced by a factor of four (22), not two.
Orbiting is a perfect balance between inertia, the tendency of a body to move in a straight line (not a curve) until acted upon by a force, and the attractive force of gravity. As the Moon orbits Earth, it has inertia, which always moves it in a direction tangential to the orbital path (nearly a circle).
Only a centripetal (center-seeking) force, like gravity, is required to keep bodies in orbit around one another. Here's another way to think about it: Imagine you've got quite an arm (QUITE an arm), and can throw a baseball very fast. Look at these two scenarios (left) First, you throw the ball at a velocity that produces a forward distance in a given time (let's say a second) that does not keep up with the downward distance caused by the pull of gravity. Now imagine that you throw the ball with sufficient speed that the curvature of Earth gives it enough extra height in any given period of time that the amount of distance it falls due to gravity is just enough to keep it a fixed distance away from the ground. That's really quite a speed, and it partly explains why it's so difficult to get objects to orbit Earth. A 3 Kg newborn baby at the surface of Earth is gravitationally attracted to Earth with a force of about 30N. Calculate the weight, in Newtons and in pounds, of the Mars curiosity rover, which has a mass of 899 Kg, on both Earth and Mars. How far apart (in meters) must two 1000 Kg spherical masses be in order to exert a 0.01 N force on each other? We can use the universal law of gravitation to find the acceleration of gravity near the surface of Earth (below). This is a good derivation to practice your skills with scientific notation and cancelling units.
Notice that 9.8 mA·s-2 is the acceleration of gravity near the surface of Earth, where r = the radius of Earth. When checked, Shutterstock's safe search screens restricted content and excludes it from your search results.

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. His Copernican Principle (that the Earth is not in a central, specially favoured 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. This involves a static Earth at the centre of the universe, around which revolve the Sun and the Moon, with the other five planets revolving around the Sun. Newton later used them to deduce his own Laws of Motion and his Law of Universal Gravitation. 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.
Thus, freely falling bodies, heavy or light, have the same constant acceleration, due to the force of gravity. This idea of a hierarchical universe and the “nebular hypothesis” were developed further (independently) by Thomas Wright (1750) and Immanuel Kant (1775). 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. He also establishes that magnetism can affect rays of light, and that there is an underlying relationship between the two phenomena.
He collected together laws originally derived by Carl Friedrich Gauss, Michael Faraday and Andre-Marie Ampere into a unified and consistent theory (often known as Maxwell’s Equations). Objects (including planets, like the Earth, for instance) fly freely under their own inertia through warped space-time, following curved paths because this is the shortest possible path in warped space.
In these experiments, he also became the first person to transmute one element into another (nitrogen into oxygen) through a deliberate man-made nuclear reaction. The solution for a universe with positive curvature (spherical space) results in the universe expanding for a time and then contracting due to the pull of its gravity, in a perpetual cycle of Big Bang followed by Big Crunch now known as the oscillating universe theory. Later, in 1935, he proposes the famous “Schrodinger's Cat” thought experiment or paradox concerning quantum superposition, decoherence and entanglement.
Zwicky also uses the virial theorem to deduce the existence of unseen matter (what is now called dark matter) in the universe, as well as the effect of gravitational lensing. He also predicts that black holes should in theory emit radiation (known today as Hawking radiation) until they eventually exhaust their energy and evaporate. If we bring a negative and a positive charge together, the electrostatic force is attractive (brings them together). You can think of such constants as fixed numbers that are there for us to get the units correct. The force of gravity is greater on a more massive person than a less massive person, therefore the more massive person weighs more — weight, unlike mass, is a measure of force.
The graph shows that when r, the distance between masses, is small, the gravitational force is high, but as r is increased, the force drops non-linearly. This idea underscores the notion that for a long time observers were stuck on the idea that there must be some force pushing in the direction of the motion. But for every bit of tangential motion, there is some inward motion that keeps the path bent exactly to the orbital path. How fast would you need to throw it so that as it travels, the bending away of the spherical Earth below it is just balanced by the downward distance it is pulled by gravity — it orbits just above the surface? The forward inertia of the ball keeps it moving forward and gravity bends that path into a curve (an ellipse, actually).
Generally when we fire off a satellite, the rocket that carries it accelerates straight upward until it clears most of the friction-producing atmosphere, then turns over to accelerate to a tangential velocity that is fast enough to achieve a stable orbit. Remember that ever body in the universe exerts a mass-proportional amount of gravity on every other body.
The location of the center of mass of the Earth-Moon system (the point where the two, if joined by a stick, would balance) is somewhere inside of Earth because Earth has about 6 time the mass of the moon. The dashed circle stays where it was at the beginning to help you see the circular motion of the larger planet. If we travel a substantial distance from the surface of Earth (it has to be big because r is so large), we would expect the acceleration of gravity to be smaller.
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For a slightly different perspective, also see the section on Cosmological Theories Through History. Although he portrays the universe as finite in size, he stresses that it exists unchanged and static throughout eternity. 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.
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. He also details the complex motions of the stars and planetary paths using equants, allowing astronomers to predict the positions of the planets.
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 cannon balls and falling apples). This theory describes a universe that has no beginning and no end, and that expands continuously, but in which new matter is constantly created and inserted as it expands in order to maintain a constant density, so that its overall look does not change over time. But if we bring two positive charges or two negative charges together, they repel one another (opposites attract, likes repel). That makes sense, of course: If there's a baby carriage moving down the street, we pretty much expect the force to be applied from behind, not from the side. Thus while Earth exerts a force on the Moon that keeps it in orbit, the Moon also exerts a force on Earth. Think of a seesaw: Where does the light person need to be in order to balance the weight of a heavy person? There is no such repulsive component to the gravitational force; you never see someone walking down the street suddenly be ejected from the planet by gravity. All text and images on this website not specifically attributed to another source were created by me and I reserve all rights as to their use.

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