Category Great Scientific Discoveries

          It is believed that light travels in straight lines. However, light bends very slightly at the edges of objects. Thus, the shadows formed will be a little smaller than if it were a simple straight line. This effect is called diffraction.

          The effects of diffraction of light were first carefully observed and characterised by Francesco Maria Grimaldi. Grimaldi also coined the word ‘diffraction’ from the Latin word diffringere, meaning, ‘to break into pieces’, referring to light breaking up in different directions. Grimaldi’s observations were posthumously published in 1665. A more conclusive study was done by Augustin-Jean Fresnel who made his calculations on diffraction public in 1815 and 1818.

          The discovery of diffraction supported the wave theory proposed by Christiaan Huygens. According to the wave theory, light is a stream of waves, with each wave made up of smaller wavelets. When light hits a glass at an angle, the wavelets that reached first would slow down. This causes light to bend. As the wave theory conflicted with Newton’s ideas, it was not accepted until the 19th century.

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          In the early 17th century, the concept of planetary motion was based on the ideas of Copernicus. People believed that planets orbited the Sun at a constant speed in perfect circles. This belief was challenged by the astronomer Johannes Kepler. He used the data meticulously gathered by his former employer Tyco Brahe, who had worked, without even the advantage of a telescope. Kepler calculated that rather than circles, the planets’ paths were ovals or ellipses and they did not have a constant speed. His discoveries certainly made people aware that the universe was not as simple as initially thought.

          Kepler’s first two laws on planetary motion were published in 1609 and the third one in 1619. The application of Kepler’s laws extends to the motions of natural and artificial satellites and unpowered space crafts in orbits in stellar systems or near planets. It is to be noted that these laws, as formulated by Kepler, do not take gravitational interactions between planets into consideration.

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          Jupiter has 79 known moons, almost making it a solar system by itself. In January 1610, Italian astronomer Galileo Galilei discovered four of Jupiter’s moons which are now called lo, Europa, Ganymede and Callisto. Originally, Galileo referred to them numerically as I, II, III, and IV. The first moon he discovered was lo and it is closest to Jupiter among the four. He published his observations in a book called The Starry Messenger. However, Simon Marius, who discovered the moons independently around the same time as Galileo, gave these moons their present names.

          Galileo’s discovery proved the importance of the telescope as a tool for astronomers. It made possible for humans to perceive celestial objects that remained unseen by the naked eye.

          Galileo’s discovery was a turning point in astronomical history as it put an end to the geo-centric model of the universe.

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          For almost 2000 years, Western thinking was influenced by the concept of universe being centred around Earth, advocated by Aristotle and Ptolemy. In the 16th century, the Polish astronomer Nicolaus Copernicus proposed a new concept.

          In his book On the Revolutions of the Heavenly Bodies, Copernicus proposed that it was the Sun at the centre of the solar system and not Earth. This model was called the heliocentric system in 1543.

          The Copernican model displaced Ptolemy’s geocentric model. Copernican heliocentrism became the launching point for modern astronomy. It described Earth as just another planet; placed third outward from the Sun.

          Copernicus also explained that stars are distant objects that do not revolve around the Sun. Instead, Earth rotates once in 24 hours. This causes the stars to appear as if they revolve around Earth in the opposite direction.

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          Earth’s circumference was first accurately measured more than 2,000 years ago by the Greek astronomer Eratosthenes.

         Eratosthenes heard that midday sunlight shines straight down to the bottom of deep wells, on the same day each year in the nearby town of Syene. This indicated that the Sun was directly overhead in Syene on that day. However, on the same day, sunlight fell only on the sides of the wells in Alexandria.

          Eratosthenes reasoned that the difference in the angle of incoming sunlight was due to the curved surface of Earth. By measuring this angle, he related the distance between Alexandria and Syene to the total dimension of the globe.

          On the day the Sun shone at the bottom of the wells in Syene, Eratosthenes measured the sun’s position in the sky over Alexandria.

          It was seven degrees away from the zenith, which meant that Syene was seven degrees away from Alexandria. He then made several calculations considering this angle and the distance between Alexandria and Syene, which is about 800 kilometres.

          There is only a difference of five per cent between the answer he got (42,000 km) back then and the value accepted today (40,075 km).

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          Why do some things float in water while the others sink? The answer to this question was found by the Greek scientist Archimedes. The principle he formulated is called the Archimedes principle.

          The Archimedes principle states that, when anything is immersed in a fluid even partly, it feels an upward push equal to the weight of the fluid it displaces. For instance, a ship launched into the ocean will sink until it displaces water equal to its own weight.

          There is an anecdote related to this discovery. King Hiero II of Syracuse wished to give a gold crown to a temple. The king him-self supplied the gold. However, he had a suspicion that the gold-smith mixed some silver to it. The king asked Archimedes to find the truth without damaging the crown. While taking a bath, Archimedes noticed that the level of the water in the tub rose as he got in. He realised that the submerged crown would displace an amount of water equal to its volume. Thus, the Archimedes principle was formulated.

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