The Planets Move (Third millennium BC – fifth century BC)

Ancient astronomers have observed that some stars move in regular patterns over time in relation to other stars which remain relatively fixed in position; they would name these moving stars as planets (Greek “planetes,” meaning “wanderer”). The five planets known since ancient times are Mercury, Venus, Mars, Jupiter and Saturn, as they can readily be seen with the naked eye without much difficulty. Their observation revealed that the Earth is part of the solar system of planets distinct from fixed stars.

Heliocentrism (1540s)

Heliocentrism is the scientific theory proposing that the Sun is at the center of the Solar System, in contrast with geocentrism, which places the Earth at the center. Polish mathematician, astronomer and physician Nicolaus Copernicus (1473 – 1543) advocated heliocentrism through his groundbreaking work, “De revolutionibus orbium coelestium” (On the Revolutions of the Celestial Spheres), published in 1543, challenging the established orthodoxy of geocentrism that had prevailed since the time of Ptolemy in the 2nd century AD. However, it would be another hundred years before Galileo Galilei and Johannes Kepler provided the theoretical and observational evidence for the theory and established a historic milestone of modern science known as the Copernican revolution.

Elliptical Planetary Orbits (1600s)

Based on his study of the astronomical observations compiled by Tycho Brahe (1546 – 1601), German astrologer Johannes Kepler (1571 – 1630) was able to mathematically develop his eponymous laws of planetary motions that can effectively explain and accurately predict the motions of the planets in elliptical orbits around the sun. His work provided significant support for Copernicus’ heliocentric theory and gave Isaac Newton (1643 – 1727) the necessary foundation for his theory of universal gravitation.

Moons of Jupiter (1600s – 1610s)

Having made significant improvements to the telescope, Italian physicist Galileo Galilei (1564 – 1642) discovered that Jupiter has moons orbiting it like the earth. This discovery attested to the correctness of the heliocentric theory advanced by Copernicus, who had considered that the Earth as not unique in itself but as resembling the other planets that revolve around the sun.

Halley’s Comet (1700s – 1750s)

English metereologist and physicist Edmund Halley (1656 – 1742) provided evidence that comets, like the planets, orbit the sun; and was able to correctly predict the return of Halley’s Comet. Due to the near similarity of the observed characteristics of the comet he observed in 1682, he concluded that it is the very same object that appeared 1531 and 1607 following a 76-year orbit. True enough, the comet did return in 1758, sixteen years after his death.

The Milky Way: a Gigantic Disk of Stars (1780s – 1830s)

British telescope-maker and musician William Herschel (1738 – 1822), who is best known as the discoverer of the planet Uranus, mapped the heavens with his sister Caroline (1750 – 1848) and determined that our solar system dwells in a massive disk of stars with a bulging center called the Milky Way, by counting the number of stars in the field of view of his telescope in more than 2,000 sample areas in the sky. Subsequent studies would confirm that our galaxy is disk-shaped, but would reveal that the sun is no where near its center and that the system is significantly more enormous than in Herschel’s opinion.

General Relativity (1910s)

German-born American theoretical physicist Albert Einstein (1879 – 1955) proposed his general relativity theory that states that gravitational attraction between masses causes warping of bordering space and time by those masses, hence, large masses can bend light. This theory was proven to be true by astronomers in an experiment involving a solar eclipse in 1919, observing how starlight was deflected as it travels near the sun that positions of the stars seem to be shifted by less than 2 arc seconds or 1/1800 of a degree. Employment of modern and higher precision techniques of radio astronomy have confirmed the result.

The Universe Is Expanding (1920s)

American astronomer Edwin Hubble (1889 – 1953) identified many astronomical objects (galaxies, stars, and nebulas) and calculated their distances, learning that they are much farther than originally thought, thereby disproving the prevailing notion of the day that the universe consisted only of the Milky Way. He also observed that the more distant galaxies appear to be moving away at a faster rate than nearer ones; and his calculations would prove that the universe is expanding, thus offering the first observational proof to the Big Bang model of the birth of the universe advanced by Alexander Friedmann (1888 – 1925).

Radio Waves from the Center of the Milky Way (1930s)

Karl Jansky (1905 – 1950), American physicist and radio engineer, was assigned by his employer Bell Telephone Laboratories to investigate static sources that cause interference on radio voice transmissions. His studies revealed three types of static: distant thunderstorm, local thunderstorms and a faint hiss-type static of unknown source. After over a year of following the signal, he determined that the static was strongest in the direction of the center of the galaxy. His discovery led to the development of the new field of study called radio astronomy.

Cosmic Microwave Background Radiation (1960s)

German-born American physicist Arno Penzias (1933 – ) and American astronomer Robert Wilson (1936 – ) identified a form of electromagnetic radiation filling the whole universe as cosmic microwave background radiation, which they supposed to be the remnant of the Big Bang. Their measurements of its various properties such as temperature and range frequencies, coupled with Edwin Hubble’s earlier discovery of the expansion of the universe, served as the major confirmation of the Big Bang theory and allowed astronomers to correct many of the assumptions concerning it.

Gamma-Ray Bursts (1960s – 1990s)

Most often associated with nuclear blasts, gamma-ray bursts are luminous flashes of gamma rays that seem to come from random directions in deep space at random times. This three-decade old mysterious space phenomenon was finally resolved by highly sophisticated ground-based telescopes and orbiting satellites. These electromagnetic radiations have now been linked to distant extra-galactic events, such as the collapsing of enormously massive stars into a black hole.

Extrasolar Planets (1990s – 2000s)

As a result of enhanced high-resolution telescope technology and by measuring their gravitational effects on their parent stars, astronomers were able to detect and confirm more than 300 extrasolar planets or exoplanets as of September 2008, proving the existence of other solar system, although none as yet bear any similarity to our own.

The Universe Is Accelerating (1990s – 2000s)

Astronomers have observed that the rate of expansion of the universe at great distances appears to be accelerating, instead of decelerating due to gravitational pull. If these observations are right, the increasing divergent expansion will overcome the gravitation of the local area, and the universe, the Milky Way, galaxies, stars and everything else as we know it, will gradually be torn apart to the last atom. The new theory of this ultimate fate of the universe is known as the “big rip.” With such a significant discovery, we can certainly look forward to the gradual development of the theory in the years to come, in the same way the Big Bang theory has continually evolved.

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