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Hipparchus of Nicea › Who Was
Definition and Origins
Hipparchus of Nicea (190 BCE - 120 BCE) was an ancient Greek mathematician, astronomer and geographer, regarded by many historians as a scientist of the highest quality and possibly the greatest astronomical genius among the ancient Greeks.Basing most of his success on systematically exploiting the Chaldean and Babylonian astronomical knowledge and techniques, many of his achievements in astronomy remained widely accepted for 17 centuries.
HISTORICAL CONTEXT
Around Hipparchus' time, Aristotelian cosmology dominated Greek thought. This model was based on the idea that the earth was the centre of the universe and that circular planetary motions were perfectly uniform. However, it was a rigid model that could not account for certain observations such as the changes in the brightness of the planets, their retrograde motions, and changes in their speeds: these observations clearly contradicted the Aristotelian model. This gap between theory and observation, though, was not significant among Greeks until Alexander the Great conquered the East and Greek geometrical astronomy began to merge with the observational-based Babylonian astronomy.
The Babylonians for centuries kept accurate astronomical observational records and they also had arithmetical tools and a numeral system for writing numbers with sixty as its base, all of which was unknown to the Greeks: Hipparchus incorporated these innovations into Greek thought and, based on the Babylonian numeral system, started to divide circles into 360 degrees.The old mathematical prejudice of uniform circular planetary motion was too strong to be discarded but now there was a greater concern for observational facts.
The flexibility lacked by the Aristotelian model was partially overcome by two geometrical tools created by Apollonius of Perga around 200 BCE. He suggested replacing the conventional circles by eccentric circles. In an eccentric circle the planets moved as usual in a uniform circular motion around the earth, but our planet was not the centre of the circle, rather, offset the centre.This way, the planet's speed changes could be accounted for and also the changes in brightness: planets would appear to move faster, and also brighter, when they were nearer the earth, and slower, and also dimmer, when they were away on the far side of their orbit. Apollonius came up with an additional tool, the epicycle, an orbit within an orbit (the moon revolves around the earth and the earth orbits the sun or, in other words, the moon moves around the sun in an epicycle ). This device could also account for changes in brightness and speed but it could also account for the retrograde motions of the planets which had puzzled most Greek astronomers.
HIPPARCHUS' CONTRIBUTIONS
Only one of his many works has survived, a commentary on the Phainomena of Eudoxus and Aratus of Soli. Most of his ideas in astronomy are known to us through Claudius Ptolemy’s work Almagest, a massive astronomical essay completed during the 2nd century CE which remained as the standard reference for scholars and unchallenged until the Renaissance. The Almagest is mainly based upon Hipparchus calculations and research.
HIPPARCHUS CREATED THE DISCIPLINE OF TRIGONOMETRY. HE CALCULATED THE LENGTH OF THE LUNAR MONTH WITH AN ERROR OF LESS THAN ONE SECOND.
Hipparchus created the discipline of trigonometry. He calculated the length of the lunar month with an error of less than one second and estimated the solar year with an error of six minutes. He also improved the main astronomical instruments of his time (the astrolabes and quadrants). Hipparchus proposed that the difference in longitude between cities could be accurately determined by comparing the local times of an eclipse of the moon, viewed simultaneously from the two locations.
We owe to Hipparchus the general rejection of the sun-centred planetary system proposed by Aristarchus of Samos during the 3rd century BCE. Hipparchus concluded that the geocentric model better explained the observations than did the model of Aristarchus. The only way the sun-centred model could stand mathematical analysis was by supposing an elliptical orbit of the earth, and this supposition was something that Hipparchus was not willing to accept, since it was established consensus among astronomers at that time that the planetary orbits were circular. On top of this, Aristarchus' model expanded the size of the universe far beyond the accepted size, which was also an implication difficult to accept. On the other hand, Hipparchus improved the calculations of Aristarchus of the sizes and distances of the sun and moon: he calculated the distance of the moon from the earth with an error of only five per cent.
It is tempting for us to believe that Hipparchus brought science one step backwards by rejecting the heliocentric model, however, Hipparchus actually tested the heliocentric model and his rejection was supported by mathematical evidence as far his understanding was concerned. After all, it is not what a man believes which defines him as a scientist, it is the why : his conclusions have to be consistent with what the evidence suggests. We could blame him for blindly accepting the idea of circular uniform planetary motion and not being open to considering other possibilities but in fairness the notion of circular uniform planetary motion was, for ancient Greek astronomers, as strong as we today believe our planet is sphere-shaped.Every society is encompassed by a cloud of paradigms which tend to remain unchallenged for many generations. Even the most talented minds do not always manage to overcome this set of comforting convictions.
It was Hipparchus who used and perfected the geometrical tools proposed by Apollonius of Perga in order to eliminate most of the contradictions of the geocentric model. Based on these devices he performed a number of refinements in the model which allowed sufficient observational accuracy for it to be accepted for the next centuries. These refinements encouraged the progress of the geocentric model, but never achieved total success. Astronomy would have to wait for Kepler (17th century CE) to come up with a fully successful planetary model able to describe the motions of the sky.
Hipparchus perfected the method of Eratosthenes for charting the Earth's surface. He decided to mark off lines completely around the sphere parallel to the equator and at equal intervals. Then he marked off other lines at right angles to these spaced equally at the equator. The result was a regular grid covering the entire globe. He also numbered all of these lines and, thus, it was possible to determine terrestrial positions by following a simple set of coordinates. He tried to organize the astronomers of the Mediterranean to record all information that would help to determine the location of all important cities. However, during that time the level of political order and cooperation required for such a task could not be realised. Hipparchus had, however, set the the basic pattern for mankind's cartographic mastery of the planet.
MEASURING THE EQUINOCTIAL PRECESSION
If we stand at any point of our planet on the 21st of March (spring equinox in the northern hemisphere), shortly before dawn, and look due east, we will see a constellation sitting on the horizon in the place where the sun will shortly rise. That constellation today is Pisces, and it has been so roughly for the last two thousand years. Within the next couple of hundred years, this constellation will be Aquarius. The reason for this change is an almost imperceptible wobble on the axis of the earth which causes the sun to move backwards as a pointer against the constellations, gradually tracking backwards one degree every 72 years or so. This gradual retrograde shifting of the stars is known as the Precession of the Equinoxes.
One evening, Hipparchus noticed the appearance of a star where he was certain there had been none before. It was critical for him to determine whether this appearance was real, since heavenly bodies at that time were believed to be unchanged and not subject to either creation or destruction. Determined to certify possible later changes, he made a catalogue of the sky providing the positions of 1080 stars by stating their precise celestial latitude and longitude. Timocharis, 166 years before Hipparchus, had also made a chart. Comparing both charts, Hipparchus calculated that the stars had shifted their apparent position by around two degrees. This is how he discovered and measured the precession of the equinoxes. He calculated the precession to be thirty-six seconds per year, an estimation a little too short according to modern calculations, which is fifty. This astronomical discovery is one of the finest of all his discoveries. Countless pages have been written on whether Hipparchus was the very first to know about the equinoctial precession. Some scholars believe that the Babylonian astronomer Kidinnu, during the 4th century BCE, already knew about it but certainly Hipparchus' was the first mind in Greek tradition to discover it.
Roman Glass › Antique Origins
Ancient Civilizations
Roman glassware includes some of the finest pieces of art ever produced in antiquity and the very best were valued higher than wares made with precious metals. However, plain glass vessels such as cups, bowls, plates, and bottles were also used as everyday containers, in particular, for storing and serving food and drinks. Glass was also used by the Romans for its decorative qualities and could be incorporated in mosaics and decorative panels in both walls and furniture. The material was also used for windows, to create jewellery, mirrors, game pieces, magnifying glasses, sculpture and, in the form of powder, even as a medicine and toothpaste.
Roman Glass Kantharos
MATERIALS
The use of the man-made material called glass - a mix of soda, silica, and lime - pre-dates the Romans by over 1,500 years, but even they seem not to have fully understood the complexity of the component mix and the necessity for lime to make the glass impervious to water and much more durable to the ravages of time and use. Lime can be found naturally, for example, as part of the silica component in the form of sand which contains a significant percentage of crushed sea shells. Indeed, two areas became noted for the high quality of their glass - along the Belarus River in Phoenicia and the Volturnus River in Campania - not coincidentally, areas where the sand was particularly rich in lime. However, some Roman glassmakers, perhaps without knowing exactly why, did understand that the addition of small pebbles and shells could affect the final quality of the glass produced.
Roman Glass Vase
TECHNIQUES
The use of glass before Roman times was mainly restricted to small opaque bottles or large bowls, very often made in imitation of metal-wares. Early glass was usually opaque due to the high number of tiny air bubbles within the glass as a result of the firing process and usually had a pale green or yellow hue due to the presence of impurities. The colour tint of the glass could, though, be manipulated by increasing or decreasing the level of oxygen in the furnace. Colours could also be achieved by adding small amounts of metals to the mix; adding copper produced blue, green, and red, manganese produced pink and red, cobalt a deep blue, calcium white, and lead brought a yellow tint.
Roman Glass Bowl
With the invention of glassblowing (blowing the glass whilst still hot through a hollow iron rod 1 to 1.5 m long) in the 1st century BCE, a better quality of glass was produced, and the production process became faster and cheaper with the consequence that vessels made from glass became much more common, everyday objects. This trend was increased further by the invention of the glassblowing furnace in the 1st century CE. As Strabo noted in his Geography, a glass vessel could be bought in the 1st century CE for only a copper coin. The exact location and time of the invention of this new production method is not known, but the earliest examples of blown glass date from the 1st century BCE in the areas of Syria and Palestine. This is also the time when the Latin word for glass - vitrum - is first recorded.
Roman Glass Perfume Containers
The technique of blowing glass not only allowed vessels to be made with thinner walls and so the glass became more translucent, but it also allowed for a whole new range of shapes to be created. The older method of casting glass using moulds and creating decorative effects by joining together prefabricated component parts of white, brown, blue, red, and green did continue into the 1st century CE but was restricted mainly for the production of larger vessels such as bowls, plates, and jugs.With the revolutionary new method of glass-blowing the possibilities for design became limited only by the imagination of the craftsperson. As Seneca marvelled in his Epistulae Morales, the glassblower could, 'by his breath alone, fashion glass into numerous shapes which could scarcely be accomplished by the most skilful hand'.
Roman Glass Perfume Bottle
DESIGNS
Roman glass-makers ( vitriarii ) and glass-cutters ( diatretarii ) could employ a wide range of techniques to transform simple glass vessels into highly decorative pieces. Colourful glassware, as we have seen, was first created by putting together different pieces of pre-made glass. However, it was in the late 1st century CE that colourless glass first appeared in the glassmaker's repertoire and it became highly sought after by Roman households. One of the most popular shapes using clear glass was a large drinking cup with carved horizontal handles and sometimes with carved decorative incisions representing vine scrolls and laurel wreaths. Another decorative technique was to decorate the rims of plates with the 'egg and dart' motif so popular in architectural decorative sculpture.
Roman Glass Two-handled Cup
The production method of blowing glass into carved moulds permitted mass production and another range of designs which were often rendered using multi-coloured rods fused together in the mould and with patterns in high relief, decorative bosses being a particularly popular design in the 1st century CE. Decoration could also be added by placing small pieces or trails of hot glass onto the vessel and the shape of the vessel itself could be altered whilst still hot, for example by pinching the glass at the base of bottle necks. Handles were usually added to vessels separately and the lack of cutting shears amongst the glassworker's tools is evident in the folding over of handle ends so that the glass could be tapered and snapped off.
Glass could also receive pre-printed designs but surviving examples are rare. Abrasion was another technique to create decoration by creating areas of bright and dull contrast on the vessel. Gold leaf was also used; sandwiched between two layers of glass, the gold was used to create designs and even portraits and figure scenes and was used especially in the bases of cups and bowls. The technique was also commonly used in glass funerary medallions.
Glass could also receive pre-printed designs but surviving examples are rare. Abrasion was another technique to create decoration by creating areas of bright and dull contrast on the vessel. Gold leaf was also used; sandwiched between two layers of glass, the gold was used to create designs and even portraits and figure scenes and was used especially in the bases of cups and bowls. The technique was also commonly used in glass funerary medallions.
Portland Vase
MASTERPIECES
As glass-makers mastered their art, glassware became ever more intricate and ambitious in design and glass could now be transformed into dazzling works of art. Gem-cutting techniques were used to create effects such as those seen in cameos.Perhaps the most famous example of this technique is the 'Portland Vase' which was made sometime during the reign of Augustus (27 BCE - 14 CE) and which depicts the marriage of Peleus and Thetis from Greek mythology.
Lycurgus Cup
Later vessels were cut even more dramatically and by the 4th century CE, the art form reached its zenith with the highly carved diatreta or cage-cups and perhaps the most famous Roman glass vessel of all, the Lycurgus Cup. These cups were created by carving away thick layers of the glass leaving a design or figure attached to the main body of the vessel only by a small hidden bridge of glass, creating a lattice work of decoration surrounding the entire vessel. The Lycurgus Cup, now currently in the British Museum in London, was carved in the 4th century CE from green and red glass and depicts the myth of Lycurgus and his fatal entrapment in a vine. The decorative figures in green are additionally carved out from behind to make them as thin as possible so that they become even more translucent.
Although the Roman glass industry never reached the scale of other mass production industries such as pottery and coinage, glassware, nevertheless, became relatively common and remarkably uniform across the Roman Empire, and the sheer quantity of glassware produced would not be matched until the boom in Venetian glass in the 15th century CE.
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