Theories and discoveries: a history
Where did it all begin?
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Aristotle was a scientist who lived in Ancient Greece. He lived in the era that today is referred to as the 'Before Christ' era. 'Before Christ' is usually abbreviated to 'BC'. In Aristotle's time science was still in its very early stages of development. It included the study of nature and philosophy. Aristotle's writings on the general subject of logic still influence all Western philosophical theories. According to Aristotle, logic and reasoning were the main instruments of scientific investigation. Based on observations of some natural events, mathematical calculations and logic, Aristotle proposed that the universe is a sphere with the Earth at the centre. He divided all natural substances into four main groups: earth, air, fire and water. Aristotle also proposed that the heavens moved naturally in a complex circular motion and were made of a fifth element called ether.
Ptolemy lived in Alexandria, which is in the territory of modern-day Egypt. He lived in the era that is now is referred to as Anno Domini or 'AD' which is Latin for 'In the year of our Lord'. The 'Year of our Lord' refers to the year of Christ's birth. Ptolemy was an astronomer, mathematician and geographer. He studied and further developed Aristotle's theory of the structure of the universe. Ptolemy proposed that the universe revolved around the Earth, which was stationary. According to Ptolemy, the universe was divided into a number of spheres: the moon sphere, the planets' spheres and the stars' spheres. Ptolemy also put together a catalogue of 48 constellations, the names of which are still used today. The model of the universe proposed by Ptolemy was held for about 1500 years and today is referred to as the Ptolemaic system.
A sixteenth century Polish scientist, Nicolaus Copernicus was the first scientist to propose the heliocentric (centred around the Sun) model of the universe. He claimed that all the planets, including Earth, moved in orbits around the sun. He showed how this new system could be used to accurately calculate the positions of the planets. Copernicus is considered to be the founder of modern astronomy. See image 2.
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A seventeenth century Italian scientist, Galileo Galilei was the first to use a telescope. He made many important astronomical discoveries, like the natural satellites (sometimes referred to as moons) of the planet Jupiter and the phases of the planet Venus, which are similar to those of Earth's moon. As a result of his new, daring scientific statements, Galileo was accused by the Roman church of being a heretic (a Christian who disagrees with traditional church statements). He was sentenced to life imprisonment because he refused to publicly withdraw his support of the heliocentric model of the solar system proposed by Copernicus.
Mathematician and physicist Isaac Newton lived in England. His unified system of scientific laws called 'The Principia: Mathematical Principles of Natural Philosophy' was written in the seventeenth century. Newton defined the laws of motion and universal gravitation which he used to predict precisely the motions of stars and the planets around the sun. Newton also calculated the relative masses of celestial bodies according to their gravitational forces. Using his discoveries in optics, Newton constructed the first reflecting telescope.
Edmond Halley was an eighteenth century English astronomer who calculated the motion of stars and comets. He catalogued the positions of about 350 stars in the southern hemisphere and observed a transit of the planet Mercury. After working out a theory of cometary orbits, he concluded that the comet of 1682 (which was named Halley's Comet after him) was periodic. He also predicted that it would return in 76 years.
Thomas Wright was an eighteenth century English astronomer, mathematician, instrument-maker and architect. Wright is best known for his publication 'An original theory or new hypothesis of the universe' in which he explained the appearance of the Milky Way as an optical effect. The Milky Way is a galaxy made up of millions of different stars. Our solar system is part of this galaxy. The Milky Way appears as a broad band of light that looks like a trail of spilled milk in the night sky. According to Wright, the brightest stars are those that are the closest to Earth. The most distant stars appear as very faint spots of light. See image 3.
Sir William Frederick Herschel was an eighteenth century English scientist. He discovered the planet Uranus and explained the nebulae phenomenon. A nebula is a mass of clouds of gas or dust which reflects the light of nearby stars. Herschel also developed a theory of the evolution of stars. Herschel suggested that the whole universe is changing and evolving over time.
Henrietta Leavitt was born in the nineteenth century in the United States. Leavitt discovered more than 2400 variable stars, about half of the known total in her day. These stars change from bright to dim and back again fairly regularly. By observation of variable stars, also called Cepheids, Leavitt was able to measure the period of each star by measuring the timing of its variations in brightness. What Leavitt determined was that the brighter the Cepheid, the longer its period. Leavitt also calculated the distance to Cepheids.
American scientist Edwin Hubble was the first to classify galaxies according to their content, distance, shape and brightness patterns. He discovered that galaxies move away from each other at a rate constant to the distance between them (Hubble's Law). The further away they were, the faster they moved. This led to the calculation of the point where the initial expansion began, or the Big Bang theory. Hubble calculated 'the beginning' to be about two billion years ago, but according to more recent research data, it probably happened about 20 billion years ago.
German-born scientist Albert Einstein formulated the special and general theories of relativity. The general theory of relativity states that the force of gravity is the result of the warping of space-time. Einstein was also the first to discover the relationship between the mass of an object and the energy that it can generate. He also made important contributions to quantum theory and statistical mechanics. Einstein is often called the greatest scientist of the twentieth century.
In 1965 American scientists Arno Penzias and Robert Wilson detected a persistent signal from space that they could not explain. Finally, they recognised it as what is now called the Cosmic Microwave Background (CMB), radiation left over from the initial 'explosion of the universe', which is commonly referred to as the Big Bang.
