Classical Astronomy


Astronomy is more than likely the earliest of sciences. Humans, v their innate curiosity and intelligence have looked up and also wondered around phenomena in the sky since prehistoric times. People of different societies scattered across the globe have included their observations of celestial objects and events into their development myths and religions. Worlds such together the Babylonians and also Egyptians make long-term, systematic monitorings of the night sky and some the their documents still survive.

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Five planets, Mercury, Venus, Mars, Jupiter and also Saturn to add the Sun and also Moon were visible to the unaided eye of the old astronomers. The planets might be identified from stars in that through constant observation lock were watched to relocate relative come the stars. The an extremely word planet derives from the timeless Greek term because that wandering star. Unlike stars, planets also varied their brightness end time. A final complication in the observed behavior of planets was the of retrograde motion. This is wherein a earth seemed to earlier track top top its path throughout the sky through the constellations before reverting to its typical direction. The diagram below plainly shows this because that the planet Mars in late 2003.


Figure 1.1 native mid-July come mid-September 2003 the planet Mars appeared to backwards family member to the stars ~ above the celestial sphere. This is an instance of retrograde motion.

This ar does not seek to carry out a detailed background of astronomy increase till Newton. If you wish to delve into this substantial subject in an ext detail us encourage you come follow several of the links to various other sites specialising in the topic. The depth presented below nonetheless most likely goes past that required by the NSW syllabus. In law so the should provide some background to what is a fascinating topic.

Classical Astronomers

The Ionians (6th - fourth Century BC)

The old Greeks, especially the Ionian school of philosophers, are credited with the relocate to a natural, mechanistic view of the Universe. Based upon Miletus in Asia Minor and also founded by Thales, the Ionians space remembered no so lot for the specific models of the universe that lock suggested, however rather the they asked concerns that they can then effort to answer v reason, observation and the applications of geometry. Anaximander polished Thales" ideas and proposed a version which had actually a cylindrical earth at rest in the center of the Universe, surrounding by air climate one or more spherical shells with holes in them. These showed up as stars due to the in salt of fire that lay past the hard sphere.


Figure 1.2 Anaximander had a cylindrical planet surrounded by air and also a solid sphere. Feet in the hard sphere allowed the light from the external rim the fire come shine through, appearing as stars and also the Sun.

Anaximander"s model of the cosmos was revolutionary because that two main reasons. Firstly it introduced a mechanistic view, moving beyond a mythological, supernatural explanation for the Universe. It likewise proposed the ide of spheres neighboring the Earth. This was to profoundly influence astronomy and cosmology because that the following two millennia.Anaximenes polished Anaximander"s design by saying that the stars were addressed on come a solid, transparent crystalline sphere that rotated around the Earth.

Later Ionians contributed much more ideas and discoveries. Anaxogoras (c. 450 BC) realised that the Moon shone by reflect sunlight,had mountains and also was inhabited and that the sunlight was not a god but a large fiery rock much bigger than Greece and a big distance native Earth. Empedocles said that light traveled fast however not at boundless speed. Democritus proposed not just at atomist version of matter but likewise proposed the the Milky way was created of thousands of unresolved stars.

The Pythagoreans


Pythagoras (c. 580 - 500 BC) is attributed with postulating a spherical Earth and also with realising that Phosphoros, the morning star and also Hesperos, the night star were in truth the very same object, the earth Venus. He and also his followers thought in the principle of cosmos, a well-ordered, harmonious Universe. Lock placed an excellent importance top top the power and also aesthetics of geometry and also mathematics rather than experiments. Regular geometrical solids, specifically the sphere, were revered and they sought to uncover harmonies and also ratios in the natural world.

Herakleides, a student of Plato and also Aristotle however heavily affected by Pythagorean concepts refined an previously model by Philolaus to develop one that had actually a spherical planet rotating on it axis. It additionally had Mercury and also Venus revolving about the sun whilst the Sun and other planets revolved approximately the Earth. Stars again were fixed on a revolving crystalline sphere. Models that had the planet at the center of the Universe room termed geocentric or earth-centered.

Interestingly whilst most classic models to be variations top top geocentric models, one of the Pythagoreans, Aristarchus of Samos (c. 310 - 230 BC) propose a version that placed the sun at the centre, the is a heliocentric Universe. His version would be familiar to us today as a reasonable summary of the solar system. All the planets, including the earth, revolved about a resolved Sun in circular orbits. The earth rotated once a day on its axis and also the Moon revolved about the Earth.


Figure 1.3 In Aristarchus" heliocentric design all the planets orbit the sunlight along one paths. The Moon orbits the planet which subsequently spins on its axis.

There are several factors why Aristarchus" design did no gain broad acceptance and also was in impact lost because that 18 centuries till Copernicus redeveloped it. Firstly his original writings were lost in the destruction of the an excellent Library the Alexandria in ad 415. Secondly his ide of a moving planet defies usual sense. We do not feel the earth spinning or moving through space. His idea contradicted the prevailing view of activity as espoused by Aristotle. The final an essential objection come his design was the fail of observers to detect any stellar parallax. Under Aristarchus" model, the closer stars should show a periodic shift in position to and also fro against much more distant stars end the course of a year as the planet orbited the Sun. In reality this was not detected till 1838 following mindful telescopic observations. Aristarchus had actually underestimated the street of the earth from the Sun hence the dimension of possible parallax was overestimated.


Figure 1.4 In a heliocentric model, closer stars should present an apparent change in place relative come background stars as result of parallax. This diagram considerably exaggerates the effect and is no to scale.

An Athenian and also a pupil that Socrates, Plato had a profound affect on philosophy and he wrote widely on many different fields. Fairly than gift remembered for a details model that the universe it to be his views on the nature, placed forward in his conversation Timaeus, the were come so strongly influence subsequent generations. Come Plato the universe was perfect and also unchanging. Stars were eternal and also divine, installed in an outer sphere. Every heavenly motions were one or spherical together the sphere was the perfect shape. Such to be his influence that the concept of circular routes was not challenged until Kepler, after plenty of years that painstaking calculations, uncovered the elliptical orbits the planets nearly 2,000 years later.

Plato thought that the visible world was only a dim representation of the genuine world. He was therefore not involved with direct observations or exactly how they associated with his ideas however realised geometrical, arithmetical models can be devised to fit observations and save appearances.

Aristotle (384 - 322 BC)

Aristotle"s work had a profound affect on west thought, at some point being absorbed and molded into supporting Christian theology and dogma. He would probably have actually been disturbed by this. A pupil the Plato, he subsequently tutored Alexander the Great. Whilst thought of as a theoretical philosopher he additionally conducted experiments in number of fields. His works on astronomy and also the physics of movement were written in On the heavens and Physics.

Like Empedocles prior to him Aristotle witnessed all matter on planet as being written of combinations of only 4 elements; earth, air, fire and water through the nature of cool, moist, hot and also dry. The stars to be made of a separate fifth element, quintessence and also were incorruptible and also eternal. Motion in the heavens to be natural, unforced and circular so that the planets and also Sun orbited a fixed, unmoving spherical earth in circular orbits. On Earth, however, matter was corruptible and also subject come decay. Activity was linear with objects request a pressure acting on lock to remain in motion. That is to be not until Newton in the second fifty percent of the seventeenth century the this ide of forced motion was overthrown.

Aristotle"s own version of the cosmos was a development of that of Eudoxus who had likewise studied under Plato. It had a series of 53 concentric, crystalline, transparent spheres rotating on various axes. Each round was centered on a stationary planet so the design was both geocentric and also homocentric. Stars were resolved on the external sphere. The Moon marked the boundary in between the unchanging, continuous heavens and the corruptible Earth. According to Aristotelian cosmology the was just within the sub-lunary sphere, that is between the Earth and also Moon, that changeable phenomena such together comets might exist.

Ptolemy (AD 120 - 180)

The critical of the good classical astronomers, Claudius Ptolemy stayed in Alexandria. He added to mathematics, optics, geography and also music but is chiefly remembered for his large work ~ above astronomy, known as the Almagest. In the he in-depth a model of the universe that profoundly influenced Western and Arabic believed for the following 1,500 years.

Ptolemy relied heavily on devices invented and observations make by previously astronomers. Apollonius (262 - 190 BC) had occurred the ideas of the eccentric and the epicycle to explain planetary activities (see figure 1.5 below). Hipparchus (161 - 126 BC) had organised previously Babylonian records in addition to his own monitorings to develop a catalogue of 850 stars. That plotted them on a celestial sphere and also introduced the principle of comparing brightnesses top top a magnitude range that develops the communication of that still supplied today. Ptolemy synthesised every this work and incorporated his own mindful observations to develop a version that to be to end up being accepted together the standard model until the 1600s.

Figure 1.5 The Epicycle, Deferent and also Eccentric. A earth orbits point x in a circular path dubbed the epicycle. The deferent is the circular route that suggest x takes about the center of motion, C. This is no the same suggest as the location of the Earth. The offset is referred to as the eccentric. Various planets would have different eccentrics, deferents and also epicycles. The resultant path traced the end by a planet could account because that retrograde motion and also variations in brightness.

The Ptolemaic model had actually a spherical, unmoving earth in the main region that the Universe, that natural place. Note contrary to usual misconception, it was not strictly geocentric together the version used eccentrics; rather it was geostatic. 10,000 earth diameters indigenous the the centre, stars were fixed on a celestial round that rotated when every 24 hours. The Sun and the 5 planets had their motions explained by combinations of epicycles, deferents and eccentrics. In full some seventy circles and also spheres were required.

Ptolemy"s desire, inspired by Plato, the his version should right observations and also save appearances led the to present a ethereal device, the equant, into his model. Each heavenly body had actually its own equant, the allude around which movements of 4 epicycles appeared uniform. The equant did no coincide through the centre of a planet"s deferent. Whilst the principle of the equant broke with the precept the the movement of spheres about their centres be uniform that was effective in bookkeeping for the variations listed in the retrograde motion of part planets.

To us Ptolemy"s model seems overly complex and clearly wrong yet it survived as the standard model used through scholars because that 1,500 years. Why to be this? there are number of reasons:

It worked, that is it can predict the position of a planet to in ~ 2°.It accounted for the observed planetary motions, retrograde motion and also variations in brightness.Unlike Aristarchus" version it did no predict the unobserved stellar parallax.It inserted the earth in its herbal place in ~ the center of things, solve Aristotelian philosophy.It matched with common sense. We perform not feeling the planet move and also Ptolemy"s model had actually a static Earth.

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the was no seriously challenged until the mid-1500"s through the job-related of Copernicus.