This article addresses modern science, by which we mean science as we now understand it; e.g. making use of the scientific method of controlled experimental verification of hypotheses. Before the 1500s, it was typically thought that the natural world could be understood by invoking supernatural deities, or by simplistic (and sometimes, not so simplistic) theories founded on casual observation and 'common sense' - e.g. that the Earth was the center of the Universe, because one could plainly see that the heavenly bodies (sun and planets) rotated about the Earth.

1543

Copernicus (Niklas Kopernik, 1473 - 1543) proposed the heliocentric_theory that the planets revolve around the sun, in De revolutionibus orbium caelestium (Concerning the revolutions of the heavenly spheres)

1572

Tycho Brahe (1546 - 1601) calculated that a newly discovered supernova (in constellation Cassiopeia) must be further away than the Moon. This conflicted with the Aristotelian theory that only the skies between the Earth and the Moon could change.

1574 - 1577

Brahe set up an observatory and showed that a comet was moving among the planets, beyond the moon.

1581

Galileo began studying at the University of Pisa, where his father hoped he would study medicine. While there, he began his study of the pendulum when, according to legend, he watched a suspended lamp swing back and forth in the cathedral of Pisa.

1589

Galileo tested Aristotle's hypothesis that heavier objects fall faster than lighter objects. He performed various experiments in which he dropped objects from a height. He also set balls rolling on gently inclined planes and then determined their positions after equal time intervals. He found that different weights fall with the same acceleration. He wrote down his discoveries about motion in his book, "De Motu" which means On Motion).

1593

Galileo invented the thermometer.

1602

Galileo made his most notable discoveries about the pendulum: it nearly returns to the height at which it was released; and the square of the period varies directly with the pendulum's length (and it does not not depend on the arc of the swing). Eventually, this discovery would lead to Galileo's further study of time intervals and the development of his idea for a pendulum clock).

1608

Hans Lippershey (1570-1619) was a German-born Dutch lens maker who demonstrated the first refracting telescope made from two lenses; he applied for a patent for this in 1608, intending it for use as a military device. A refracting telescope uses two lenses to magnify what is viewed; the large primary lens does most of the magnification. According to legend, Lippershey noticed two children playing with lenses in his shop. The children noticed that when they looked through two lenses, a weather vane on a nearby church appeared to be larger and clearer. Lippershey tried it himself and realized the possibilities. He placed a tube between the lenses to make the first telescope. He applied for a patent for his telescope with the Belgian government. Even though he was paid very well for his invention, a patent was not granted because it was felt it could not be kept a secret.

1609

Galileo heard about Lippershey's invention. He also heard that this man was on his way to Venice to sell his invention to the Venetian state for a very high price. Galileo, who was usually short of money, decided to beat Lippershey to it. In 24 hours, he had a telescope made and had word to a high ranking monk in the Venetian state about his "invention."

1609

Johannes Kepler (1571 - 1630) described the shape of planetary orbits using his 1st and 2nd laws, derived from data collected by the Danish astronomer Brahe. In De Motibus Stella Martis, he published the results of Brahe's calculations of Mars' orbit, which were inconsistent with then current assumption that it was a circle. This publication included the first two of what became known as Kepler's laws. Their gist is that the sun is off-center in the planetary ellipses, that the speed of planetary motion increases as their distance from the sun decreases, and, hence, the areas of the angles subtended by the sun and a given interval of time are the same.

1609

Galileo developed a series of improved telescopes and became the first to view the craters and mountains of the Moon, the moons of Jupiter, the phases of Venus, sunspots, and proved the Milky Way was made up of a multitude of stars. He announced these discoveries in Sidereus nuncius, and seems at this time to have become convinced of the correctness of Copernicus's theory. Also seeking to solve the navigational problem caused by the variability of the time value of a degree of longitude, he calculated tables showing the appearance and disappearance of Jupiter's moons.

1613

Galileo discovered sunspots.

1616

Galileo called to Rome and ordered to stop supporting the Copernican theory.

1619

Kepler, in Harmonica mundi, published his third law: The square of the length of a planet's year varies with the cube of the mean radius of its orbit. His three laws "are the only three exact and general mathematical laws of planetary motion, applying not only to this but to all similar planetary systems. And he contributed a further revolutionary idea: that the planets move in their orbits...because the Sun exerts a force that causes them to move as they do" (Park 1990:157).

1620

Francis Bacon published Novum Organum (scientific method and inductive reasoning).

1621

Galileo discerned that the acceleration of a falling body is proportional to the time and independent of weight and density.

1632

Galileo published a work in Italian for the non-specialist, the Dialogo (Dialogues), comparing the Ptolemaic system unfavorably to the Copernican. For this, he was tried by the Inquisition in 1633 and forced to abjure belief that the Sun was central and that the Earth moved. Due massimi sistemi contains Galileo's construction of the concept of 'inertia,' perpetual motion being the limiting case: In an ideal world without friction, given the acceleration and retardation of a body by gradually sloping planes tending toward horizontal, momentum persists indefinitely. "Force could therefore be defined as that which produced, not velocity, but a change of velocity from a state of rest or of uniform velocity" (Crombie 1952:301). When a body is acted on by two forces, each is independent of the other.

1633

The Inquisition denounced Galileo.

1636

Galileo finished his final book, Discorsi e dimostrazioni matematiche interno a due nuove scienze (Dialogues Concerning Two New Sciences), which contained most of his physics and some strenghtened arguments. The two sciences are statics and dynamics. The Discorsi, together with the Dialogo, both works of popular science, "helped create a new age of scientific thought with their emphasis on observation, common sense, clear language, and persuasion by reasonable arguments" (Park 1990:206).

1637

Descartes published "Geometry".

1638

Galileo publishes Dialogues Concerning Two New Sciences, summarizing the principles of mechanics.

1641

Descartes publishes Principles of Philosophy arguing that the universe is governed by simple laws and that natural processes could have shaped the earth.

1643

The mercury barometer was invented by the Italian physicist Evangelista Torricelli (1608 - 1647), a pupil of Galileo. Torricelli inverted a long glass tube filled with mercury into a dish of mercury; the mercury in the tube falls till its height above the mercury in the dish balances atmospheric pressure on the dish mercury.

1665

English mathematician and physicist Isaac Newton deduces the inverse-square gravitational force law from the ``falling'' of the Moon.

1668

Newton built the first reflecting telescope.

1684

Newton proves that planets moving under an inverse-square force law will obey Kepler's laws.

1686

Newton uses a fixed length pendulum with weights of varying composition to test the weak equivalence principle to 1 part in 1000.

1687

Newton published Principia Mathematica, describing the laws of motion. This established the modern science of dynamics. He formulated the law of universal gravitation: all objects are affected by a force, gravity, and the strength of this force varies in accordance to the mass and distance between the objects.

1704

Newton published "Opticks".

1735

Carl Linnaeus publishes Systema Naturae, laying the groundwork for the system of binomial nomenclature that will continue for over two centuries.

1777

Antoine Lavoisier proposed idea of chemical compounds made of elements.

1781

William Herschel discovered Uranus.

1789

The French chemist Antoine Laurent Lavoisier published a Treatise on Chemical Elements with which the revolution in quantitative chemistry opened.

1799

Alessandro Volta developed the 'voltaic pile', a forerunner of the electric battery, which produced a steady stream of electricity. In honor of his work in the field of electricity, Napoleon made him a count in 1801. The electrical unit known as the volt was named in his honor.

1800

Andr%E9-Marie_Amp%E8re discovered properties of magnetic field produced by electric current.

1801

Thomas Young demonstrates the wave nature of light and the principle of interference, by the famous two slit experiment.

1803

John Dalton introduces atomic ideas into chemistry and states that matter is composed of atoms of different weights - the law of definite proportions.

c.1837

Charles Darwin formulates the theory of natural selection to explain evolution. Fearful of the controversy his theory will cause, he delays publishing.

1842

Christian Doppler examines the Doppler shift of sound.

1847

Hermann Helmholtz formally states the law of conservation of energy.

1851

Jean-Bernard Foucault shows the Earth's rotation with a huge pendulum.

1858

Although he uses different terminology, Alfred Russel Wallace independently reaches the same conclusion as Darwin: natural selection is the driving force behind evolution. Wallace's and Darwin's papers are both read at the same Linnean Society meeting.

1859

Darwin proposed the theory of evolution, in his On the Origin of Species by Means of Natural Selection. It was the most comprehensive biological theory of the time, and stirred as much controversy in society at large as the work of Copernicus.

1864

James Clerk Maxwell publishes his papers on a dynamical theory of the electromagnetic field.

1866

Austrian monk Gregor Mendel proposes his thesis on the basic laws of heredity. His work will be largely ignored until 1900.

1871

Charles Darwin publishes The Descent of Man.

1873

Maxwell states that light is an electromagnetic phenomenon.

1888

Heinrich Hertz discovers radio waves.

1895

Wilhelm Roentgen discovers X-rays.

1897

Joseph Thomson discovers the electron. Marie Curie begins research of "uranium rays" that will lead to the discovery of radioactivity.

1900

Sigmund Freud published The Interpretation of Dreams. Laid the basis of the new science of psychoanalsis. Claims that certain nervous disorders may be cured by patients talking about their dreams. Freud maintains that dreams present symbols that can reveal much about a person's suppressed memories and desires. He believes that all dreams are the fulfillment of a person's wishes.

German physicist Max Planck published his quantum theory, claiming that energy is made up of particles; each one is called a quantum.

1905

Albert Einstein's second 1905 paper proposed what is now called the special theory of relativity. He based his new theory on a reinterpretation of the classical principle of relativity, namely that the laws of physics had to have the same form in any frame of reference. As a second fundamental hypothesis, Einstein assumed that the speed of light remained constant in all frames of reference, as required by Maxwell's theory.

1914

Ernest Rutherford%2C_1st_Baron_Rutherford_of_Nelson suggests that the positively charged atomic nucleus contains protons.

1915

Einstein's general theory of relativity was published in late 1915. In this theory the interactions of bodies, which heretofore had been ascribed to gravitational forces, are explained as the influence of bodies on the geometry of space-time (four-dimensional space, a mathematical abstraction, having the three dimensions from Euclidean space and time as the fourth dimension).

1919

Arthur Eddington leads a solar eclipse expedition which claims to detect gravitational deflection of light by the Sun. The general theory of relativity accounted for the previously unexplained deviations in the orbital motion of the planets as calculated by Newtonian mechanics, and predicted the bending of starlight in the vicinity of a massive body such as the sun. The confirmation of this latter phenomenon during an eclipse of the sun became a media event, and Einstein's fame spread worldwide.

Early 1920's

Edwin Hubble proved that there were galaxies beyond the Milky Way. Using the Doppler effect and light's tendency to shift toward red as it moves away, Hubble also determined that these other galaxies were moving away from us and each other in all directions. His observations about the red-shift of light gave us a means of determining our distance from these galaxies and objects within our own. Hubble's work in combination with Einstein's theory of gravitation led to the inescapable conclusion that all the galaxies, and the whole Universe, had originated in a great 'explosion'. This theory, commonly referred to as the Big Bang theory, espouses that the universe was created approximately 15 billion years ago.

1925

Tennessee schoolteacher John Thomas Scopes is tried for teaching evolution. Two-time presidential candidate William Jennings Bryan leads the prosecution. Labor lawyer Clarence Darrow leads the defense and goads Bryan into declaring that humans are not mammals.

1927

The German physicist Werner Heisenberg formulated the uncertainty principle, which held that limits existed on the extent to which, on the subatomic scale, coordinates of an individual event can be determined. In other words, the principle stated the impossibility of predicting, with precision, that a particle such as an electron would be in a certain place at a certain time, moving at a certain velocity. Quantum mechanics instead dealt with statistical inferences relating to large numbers of individual events.

1932

James Chadwick discovers the neutron. Heisenberg presents the proton-neutron model of the nucleus and uses it to explain isotopes. Carl Anderson discovers the positron.

1938

Hahn and Strassmann discover nuclear fission.

1939

Walter Maurice Elsasser suggests that the liquid iron core of Earth has eddy currents that set up Earth's magnetic field.

1942

Enrico Fermi makes the first controlled nuclear chain reaction at the University of Chicago.

1943

The world's first operational nuclear reactor is activated at Oak Ridge, Tennessee.

1945

Hiroshima and Nagasaki bombed using the first (and only existing!) nuclear fission bombs. Radar contact is established with the Moon. Arthur C Clarke proposes communications satellites in geosynchronous orbit above the Earth - by 1965 his visionary ideas become reality.

1948

Ralph Alpher, and Robert Herman predict that a Big Bang universe will have a blackbody cosmic microwave background with temperature about 5 K.

1952

Development and explosion of the world's first thermonuclear device - the "H bomb".

1953

Stanley Miller and Harold Urey combine gases generally believed to be in the earth's early atmosphere (methane, ammonia and water vapor) and zap them with electricity. These experiments produce several amino acids.

1953

James Watson and Francis Crick publish their paper on the molecular structure of DNA in Nature Magazine.

1953

Fiesel Houtermans and Clair Patterson publish independent estimates inferring the age of the earth through radiometric dating of meteorites. Both estimates are over 4.5 billion years.

1957

The Soviet Union launches Sputnik.

1965

Arno Penzias, Robert Wilson, Bernie Burke, Robert Dicke, and James Peebles discover the cosmic microwave background radiation.

1967

Bell and Hewish discover the first pulsar.

1969

Neil Armstrong becomes the first human to set foot on the Moon on July 20th, closely followed by Buzz Aldrin.

1972

Stephen Jay Gould and Niles Eldredge publish their theory of punctuated equilibrium, stating that evolution often occurs in short bursts, followed by long periods of stability.

1972

Stephen Hawking proves that the area of a classical black hole's event horizon cannot decrease. James Bardeen, Brandon Carter, and Stephen Hawking propose four laws of black hole mechanics in analogy with the laws of thermodynamics.

1974

Stephen Hawking applies quantum field theory to black hole spacetimes and shows that black holes will radiate particles with a blackbody spectrum which can cause black hole evaporation.

1980

Alan Guth proposes the inflationary Big Bang universe as a possible solution to the horizon and flatness problems.

1980

Louis W. Alvarez, Walter Alvarez, Frank Asaro and Helen V. Michel publish their asteroid impact theory of dinosaur extinction. They discover a thin layer of clay at the Cretaceous-Tertiary boundary enriched with heavy metal iridium, leading the team to speculate that a giant body from space collided with Earth, causing the layer (later found to be worldwide). The theory will not gain widespread acceptance among scientists for several years.

1990

Hubble Space Telescope (HST) launched.

1990

The COBE satellite shows that the microwave background has a nearly perfect blackbody spectrum and thereby strongly constrains the density of the intergalactic medium.

1992

The COBE satellite discovers anisotropy in the cosmic microwave background, i.e. ripples from the Big Bang.

1993

Fermilab in Chicago discovers the "top quark", first predicted in 1984.

1994

Hubble Space Telescope (HST) finds evidence for a black hole at the centre of the M87 galaxy.

1998

Supernova observations suggest that the universe is expanding at an increased rate.

2001

Evidence for a black hole at the centre of our galaxy is found.