Hipparchus (or using the more Greek style transliteration, Hipparchos), a Greek astronomer of the 2nd century BC, lived c.190 BC(the Griffiths Observatory site says c.160 BC) to after 126 BC (the GO site says was 125 BC), is often considered to be the greatest astronomer of antiquity. He is better remembered in modern times than, say, Anaximander of the 7th century BC whose achievements were eclipsed by those his successors who built upon his work.
Hipparchus was the first recorded person to construct a theory of the system of sun and moon which was properly based on observational data. He investigated the problem of parallax, and thus came to device the first practical method for determining the sizes and distances of the Sun and Moon. He attempted to measure the distances of the Sun and Moon by geometrical methods. His estimate of the Sun's distance, at 15 million km (9 million miles), was too small although a tremendous achievement for his time, it at least demonstrated that the Sun was far larger than the Earth.
The appearance of a nova in 134 BC prompted him to compile the earliest known comprehensive star catalogue, covering 850 stars. His contribution was to combine his own systematic observations with the earlier Babylonian eclipse records (which went back to the 8th century BC) and on comparing his measurements with the Babylonian records he extracted accurate estimates of the mean motions of sun and moon and the length of the tropical year (which he put at 365¼ - 1/800 days). Accordingly he developed proposals for the improvement of the calendar.
Hipparchus is most famous for his discovery of the precession of the equinoxes, which according to Ptolemy he did by comparing his own observation of the distance of the star Spica from the autumnal equinox with that of Timocharis about 160 years before. Few of his writings survive, but we know about them because other writers used them. He was the most important source used by Ptolemy in writing his Almagest. Ptolemy's solar and lunar theory is only a modification of his predecessor's.
Hipparchus can be deemed the first with a modern astronomer's practical methods: to accuracy of observation (he improved observational techniques) he joined great critical acumen in selecting observational data and in distinguishing what was relevant. His scientific spirit is shown by his refusal to attempt to construct a planetary theory because of the insufficiency of data, and, as related by Ptolemy, was content to record his own observations and compare them with past records in order to point up the contradictions in existing theories. Hipparchus also displayed mathematical flair in applying observational results to determine the numerical parameters of his geometrical schemata for the movement of the heavenly bodies. He is the first person known to have made systematic use of trigonometry, and compiled a Table of Chords in a Circle, the ancient equivalent of a sine table. According to Synesius in Opuscula (as edited by Terzaghi), Hipparchus was probably the inventor of stereographic projection. He also contributed in the field of geography, again applying his rules of observational data, and he wrote works on astrology and weather signs.
A prominent lunar crater, 150 km (94 miles) in diameter is named after Hipparchus, and it is located near the apparent centre of the Moon's hemisphere that faces the Earth. The Hipparcos satellite (High Precision Parallax Collecting Satellite), launched in 1989, was a loose acronym named for him.
Sources: The Oxford Interactive Encyclopedia (OIE) but mostly by me. At university (decades ago) one module was ancient science and I had to study Hipparchus among others, but as I have since forgotten a lot of detail I referred substantially to The Oxford Classical Dictionary 2nd Edition especially for details such as precise dates and figures. The OCD, which is an authority in the Classics world, believes that most of what modern commentators have written about Hipparchus' star catalogue is misrepresented and refers to a particular commentary in preference but while decades ago I read commentaries on Hipparchus' star catalogue (excerpts or summaries), I have not read their preferred commentary. If the OCD is correct, then almost everyone's assessment of the star catalogue, including any made by Griffith Observatory, should perhaps be viewed with an open mind.
Sources for the remaining astronomers: OIE, edited by me.
Nicolaus Copernicus (1473-1543), was a Polish-German astronomer and the founder of modern astronomy. In 1543 he published, in his book De Revolutionibus Orbium Coelestium (The Revolution of the Heavenly Orbs), a heliocentric model of the solar system, now known as the Copernican system, whose centre was near the Sun, not the Earth, as in the older Ptolemaic system.
Copernican system, source: OIE
A preface in the book suggests that the system be treated merely as a simple mathematical device but it seems likely that Copernicus, who did not write the preface, believed it to be true. The heliocentric theory, displacing as it did the Earth from the centre of the heavenly stage, aroused fierce religious opposition.
The Copernican system was a heliocentric model of the solar system introduced by Copernicus in 1543, which treated the Earth as one of the planets circling the Sun. The Moon revolved about the Earth as in the Ptolemaic system. Almost every known motion of the planets was accounted for by the Copernican model although he had to keep some epicycles in the system. According to Copernicus the apparent daily rotation of the heavens was due to the real diurnal rotation of the Earth, and the Earth actually moved along its own orbit about the Sun. The retrograde loops, during which the outer planets move backwards in the sky, became mere optical illusions caused by the relative motion of these planets and the Earth as they orbit the Sun. Venus and Mercury were correctly located as inferior planets, orbiting the central Sun, on smaller orbits than the Earth's and at greater speeds; the Sun was no longer categorised as a planet. This heliocentric theory aroused intense opposition. Some opponents maintained that if the Earth moved, the Moon must be left behind, but a more reasonable criticism was that the brighter and nearer stars should show a yearly shift in parallax against the fainter, more distant stars if the Earth revolved about the Sun. This was subsequently found to be true. The orbits of the planets around the Sun can now be described, to a good approximation, by Kepler's Laws. The Copernican system had profound consequences, leading to a reassessment of the size of the universe. Also, because objects had previously been assumed to fall to the centre of the universe, that is, the Earth, ideas about gravity needed to be revised. This eventually resulted in Newton's Law of Gravitation.
Copernicus crater
A plaque was added to the lower part of the shaft of the Astronomers Monument, below the Copernicus statue, in 1973 to honour the 500th anniversary of Copernicus' birth. A moon crater was named after Copernicus. The shuttlecraft Copernicus, shuttle #3, formed part of the auxiliary craft complement of the USS Enterprise NCC-1701-A, seen in [Star Trek V: The Final Frontier]. It was named for Copernicus the astronomer, and its name was chosen with relevance to the fact that the Copernicus model miniature used in filming was a re-use of the Galileo shuttlecraft (Galileo being another famous historical astronomer) model for the same film (though two full-size exteriors were also constructed).
the above 2 screenshots from [Future's End] show the location of the Copernicus plaque
Johannes Kepler (1571-1630) was a German astronomer who was the first to describe accurately the elliptical orbits of the Earth and the planets around the Sun. Kepler worked with Tycho Brahe at Tycho's observatory outside Prague and took over the observatory when Tycho died in 1601. Tycho left Kepler his tables of stellar and planetary positions. From these, after a great deal of analysis trying one mathematical model after another, Kepler deduced what are now known as Kepler's Laws, but the physical explanation of these laws had to await Newton's Law of Gravitation. Kepler also made discoveries in optics, general physics and geometry.
Kepler's equation: An equation to find the position of a planet in an orbit described by Kepler's Laws. It gives a relationship between the mean anomaly M and the eccentric anomaly E. The equation is M = E - e sin E, where e is the eccentricity of the ellipse.
click to enlarge artist's impression of the spaceprobe Galileo being launched above Earth
sources for above 2 pictures: OIE
Galileo Galilei (1564-1642), was an Italian astronomer and physicist. He was the founder of dynamics, the science of moving bodies. He was the first person to make extensive use of experiments to investigate natural phenomena.
He studied medicine and then mathematics at the University of Pisa and became well known in 1586 when he published details of his new invention, the hydrostatic balance. He then disproved Aristotle's idea that objects with different weights fall at different speeds (though not, as legend has it, by dropping objects off the Leaning Tower of Pisa). He worked on the problems of motion, and determined that a moving object will continue to move with a constant speed and in a straight line unless acted on by an outside agent (see conservation laws).
In 1592 he became Professor of Mathematics at the University of Padua, and did most of his important work there. In 1604 he proved theoretically that falling bodies accelerate uniformly with time, and formulated the law which describes parabolic motion.
In 1609 he went to Venice, where he learnt of a new invention - the telescope. Improving upon its primitive design, he invented the refracting telescope and became the first person to observe astronomical objects through such an instrument. This telescope, which is now known as a Galilean telescope, had a planoconvex objective lens and a planoconcave eyepiece. By the end of 1609 he had developed it to its maximum practical magnifying power of 30. In the course of his observations, he discovered that the surface of the Moon is not smooth, and that the Milky Way is a collection of stars. He also found a miniature planetary system of four satellites in orbit around Jupiter, which led him to question the firm contemporary belief that the Earth was fixed at the centre of the Universe, and that all the motion of the heavens was around the Earth. His results were published in March 1610 in Sidereus Nuncius (The Starry Messenger).
By late 1610 he had discovered the phases of Venus which provided more evidence for the Copernican system, and he observed that Saturn was oval in shape. This observation was due to the rings of Saturn but these could not be resolved in his telescope.
Further astronomical work led Galileo to propose that the Sun was the fixed body at the centre of motion, and that the Earth orbited the Sun along with all the other planets. He presented his case in a book which was written as an argument between a supporter and an opponent of his views: the Discorsi e Dimostrazioni Matematiche intorno a due Nuove Scienze (1638; Dialogue Concerning the Two Chief World Systems). Galileo's work was considered heretical by the Church of Rome. He was tried by its Inquisition in 1633 and forced to spend the last years of his life under house arrest. His work pioneered the classical mechanics later developed by, among others, the Dutch physicist Christiaan Huygens (1629-95) and the English physicist and mathematician Isaac Newton (1642-1727).
Sir Isaac Newton (1642-1727), was an English mathematician and physicist, the greatest single influence on theoretical physics until Albert Einstein. He was most productive during the period 1666-67 (which he called his "annus mirabilis"), during which he laid the foundations of his future successes in mathematics, optics, dynamics (mechanics), and astronomy. He discovered the binomial theorem, and made contributions to algebra, geometry, and the theory of infinite series, all somewhat overshadowed by his most famous contribution to mathematics - the differential calculus (his 'method of fluxions') for finding rates of change of varying quantities, and his discovery of its relationship with what is now called integration (then 'quadrature'), the problem of finding the area of a figure circumscribed by curved boundaries. A bitter quarrel with the philosopher Gottfried Leibniz ensued, as to which of them had discovered calculus first.
His optical experiments, begun in 1666, led to his discovery that white light is made up of a mixture of coloured rays. He applied his knowledge of optics to the production of the first reflecting telescope in 1668.
He became Lucasian Professor of Mathematics at Cambridge in 1669.
In his major treatise Philosophiae Naturalis Principia Mathematica (1686-7), widely acknowledged as being one of the greatest science books ever written, he gave a mathematical description of the laws of mechanics (see Newton's Laws of Motion below) and gravitation, and applied this theory to explain planetary and lunar motion.
Newton's Law of Gravitation (see also below) is central to his work on astronomy. It states that the force between any two bodies in the universe is proportional to the product of the masses of the two bodies divided by the square of the distance separating them. He also proved that the gravitational effect of a three-dimensional body such as a planet is equivalent to that of its total mass concentrated at a point at its centre. He used his theory to account for the polar flattening of the Earth, the precession of the equinoxes, the revolution of the lunar line of nodes, and also to measure the mass of the Sun and those planets that have moons. He proved that any body moving in space subject to a single central force moves on a conic, such as an ellipse, and he went on to devise a method of calculating cometary orbits.
For most purposes Newtonian mechanics has survived even the 20th century introduction of relativity theory and quantum mechanics (to both of which theories it stands as a first, but very good, approximation) as a mathematical description of terrestrial and cosmological phenomena.
In 1699 Newton was appointed Master of the Mint, and was responsible for an urgently needed reform of the coinage, and in 1703 was elected President of the Royal Society, whose reputation he greatly increased over the following twenty-four years.
Newton interested himself also in alchemy, astrology, and theology, and attempted a biblical chronology. He was involved in several bitter controversies with fellow scientists.
Newton's Law of Gravitation, one of the most far-reaching and important physical laws ever formulated, stating that every particle of matter attracts every other particle of matter with a force proportional to the product of the particles' masses and inversely proportional to the square of the distance between them. Together with Newton's Laws of Motion, the Law of Gravitation, published in 1687, laid the foundations for subsequent major advances in physics and astronomy.
Newton's Laws of Motion, three laws of motion, which are fundamental to the understanding of classical mechanics.
The first law states that every body continues in a state of rest or uniform motion in a straight line unless it is acted upon by an external force. This law is also known as the principle of inertia and provides a description of the absence of force, since any deviation from rest or straight-line motion must mean that a force is acting on the body.
The second law states that the rate of change of momentum of a body is proportional to the applied force and acts in the same direction. If the mass of the body remains constant this law equates force F with the product of mass m and acceleration a, according to the equation F = ma. It thus provides a definition of force.
The third law states that for every applied force, or action, there is an equal force, or reaction, which acts in the opposite direction. Concisely expressed action and reaction are equal and opposite.
The crucial consideration when applying Newton's laws is that they only hold relative to inertial frames of reference, i.e. ones which are at rest or moving with constant velocity. Since the Earth itself is rotating, it does not strictly provide an inertial frame, although in local problems the effect of this is negligible. When considering the flight of a space rocket, however, the Earth's rotation must be taken into account. Newton's laws, however, do not explain some of the phenomena observed in planetary motion. A more sophisticated theory is needed to explain motion at speeds close to that of light, and the behaviour of objects close in size to atoms. Relativity theory and quantum theory have been developed to deal with these situations.
Sir Isaac Newton is seen in [TNG: Descent] and [#34 Death Wish].
Sir William Frederick Herschel (1738-1822) was a German musician who moved to Bath in England, and turned to astronomy. His desire to study the nature and distribution of distant stars led him into the field of telescope design and production. Herschel concentrated on reflecting telescopes with mirrors made of speculum and he built for himself the most powerful telescope of the day. On 13th March 1781 Herschel thought he had discovered a comet but its orbit quickly indicated that it was a new distant planet. He named it Georgium Sidus after his patron King George III but it was later renamed Uranus. Herschel completed his 48-inch (1.2 m) mirror in 1789 and discovered two new satellites of Saturn (Mimas and Enceladus) soon afterwards. It was not until the age of 43, after his discovery of Uranus, that he became a professional astronomer.
Herschel's main achievement was in the field of stellar distribution. He discovered that most nebulae were star systems and developed a theory of stellar evolution. He also concluded that the Milky Way was a flat and finite system of stars. Herschel discovered that binary stars were not just chance alignments but actually two stars in orbit around their common centre of mass. He also found that the Sun was moving through space in the direction of the constellation Hercules.
His sister, Caroline Lucretia Herschel (1750-1848), independently discovered eight comets and several nebulae and star clusters, and published a star catalogue in 1798.
Herschel's only son was Sir John Frederick William Herschel (1792-1871), who shared his father's interest in astronomy, and went on to be awarded a Fellowship of the world-renowned Royal Society by the age of 21, and was a founding member of the Royal Astronomical Society.
