There are certain problems to the study of the history of the Science in general and astronomy in particular as understood and practiced by ancient societies. There are problems with sentiments and issues entirely distinct from anything in Astronomy that obfuscate any such studies. The more obvious ones like Nationalistic pride, Colonial or Imperialistic subterfuges or religious & social sensitivities are bad enough, but harder to disentangle are the more subtle effects. The most important of these is the connection of Astronomy and Religion. Before the development of the critical tradition of Science, where each innovation of intuition (theory) faces a critique (observations), it was difficult to separate philosophy and science. Consequently detractors of a culture have been tempted to reduce the level of its thought and protagonists to exaggerate it. Regardless, the regularity of the heavens and its obvious connection to events on Earth, and perhaps most importantly the constant presence of the night sky in people's lives naturally compelled almost all ancient cultures to a study of astronomy. This consisted of the recording of the regularity of the heavens and then speculating on the cause of this regularity, or the ``natural laws'' that governed this order or regularity. The problem is that while the former is easily studied and understood as science, the latter was inextricably connected to theology. This makes it a far more difficult and ambiguous subject of study. Finally and very importantly, there is the matter of our prejudices. There is an overwhelming desire to see a trend of civilization in human history. Of an evolution to a better society. But this turns out to be a sham on closer inspection. We are not evolving towards some ideal existence, but are simply evolving. We may in principle learn not to make mistakes from history, though we seldom do in practise, but this doesn't prevent us from making new ones. This false sense of superiority comes in the way of a full appreciation of Science as understood in ancient civilizations as we are too quick to dismiss their work as primitive. The loss incurred in such mindless indulgence of vanity is entirely ours.
Let us consider, for example, the history of science in ancient India. As with any culture study of Astronomy in Ancient India was intimately connected with the measurement of time. There are calendars, that were roughly accurate, described in the Indian scriptures. There seems to have been very little attempt to actually understand the dynamics of the solar system. Considerable effort on the other hand went into the understanding of theoretical mechanics (the study of motion) and cosmogony (the origin of the Universe). They described motion as being built up of small ``pieces'' of rectilinear motion. Curvilinear motion was built up of these small pieces of motion as well, that changed direction as well as position. Bodies was impelled to move by various agents, which were gravity, volition, fluidity etc. One of these ``actions'' on the body resulted in its motion. The creation of matter involved some entity that filled all space and possessed the potential of becoming matter. In regions where the ``agitation'' of this entity was sufficient, the potential was realized and proto-matter was created. Different types of proto-matter, possessing different dominant attributes, had an affinity for other types of matter which came together to form materials that we observe possessing identifiable attributes.
One should note that these concepts are not dissimilar with many in use today in science. But extreme caution is advised in the understanding of the connection. And it helps to keep in mind the difference between theology and science discussed in the first chapter. As elegant and attractive the above speculation may be it still isn't scientific knowledge as we know it today. The reason being the absence of a notion of bootstrapping that goes on in science today. There isn't the touchstone of observations constantly modifying the speculations of intuition. This makes it distinct from the study of Astrophysics today. We may admire the ingenuity and imaginative flair of any culture when studying their understanding of the Universe. And any appreciation of a civilization is incomplete without knowledge of their scientific thought. But in as much as studying science today is concerned the history of science serves more as showcase of possibilities and sources of intuitive insights rather than as actual contribution to the scientific debate today.
Let us now look at some of the extraordinary people who were involved in the process of developing our modern view of astronomy. Aside from gaining insight into these scientists as humans it will also help us make that extraordinary leap of logic to heliocentric universes from the geocentric views we are so naturally born to.
One of the most important issues in the story of Newtonian dynamics is one of inertia. Aristotle, was born in Macedonia on 384 BC. A student of Plato's, he left Athens after Plato's death and studied various topics. He was also a teacher of Alexander. A racist, he tried to keep Alexander from allowing his soldiers marrying non-Greeks he felt were barbarians. To his credit, Alexander refused to learn this particular lesson from him. Aristotle felt that inertia was an attribute that all bodies possessed that characterized their tendency to resist motion. According to him it depended on the material of the body as well its size. According this view two balls of the same weight but made of different materials would fall at different rates even in the absence of air drag. He also thought circular motion was a natural motion that didn't require a beginning (or causing force to initiate it).
This was essentially how the picture of mechanics remained until Galileo came along. Galileo was born on 1564 at Pisa to a musician father. He studied to be a doctor but quickly got interested in Physics. He discovered that lamps suspended from the ceiling completed each swing in the same time regardless how big the swing was. This led him to the discovery of the pendulum. He drifted towards Mathematics but couldn't complete his degree because he could no longer afford it. In one of his first experiments he showed that there was no difference in the time of fall of balls made of different materials but weighing the same. Although the story of him dropping two balls from the leaning tower of Pisa is probably apocryphal. For his second experiment he studied the motion of particles under the influence of gravity. He then took a plank and rolled balls down on it with the plank at different angles. He saw that more horizontal the plank the more time the ball took to fall down. But regardless, the rate of change of speed was constant at each inclination and the final speed was independent of the inclination of the plank. He also saw that as the plank was made increasingly horizontal the rate of change of speed of the ball grew smaller and he speculated in the absence of friction the speed of the ball would stay constant when the plank was horizontal. This he added to Aristotle's view of inertia to make it what we recognize it today. Inertia is the tendency of a body to continue in its state of rest or uniform rectilinear motion in the absence of any applied force.
At this point Galileo was still just a curiosity in the view of Rome. In time he learnt of the telescope and built his own and pointed it towards the heavens. Immediately he started to discover ``irregularities and imperfections'' in heavenly bodies, like the craters on Moon. He could distinguish stars in the Milky way and found satellites of Jupiter. He was well received by the papacy and they welcomed demonstrations of his telescope. Encouraged by this he launched into a forthright crusade of criticism of the Ptolemaic geocentric system. Threatened by Galileo's pronouncements the Ptolemaic school of academics poisoned his relations with papacy by insinuating that Galileo was preaching against scriptures. Galileo tried to argue that the Church should continue its practise of accepting scriptures to be allegorical when in contradiction with scientific truth, but unfortunately for him the church was locked in struggle with Protestantism and could not afford any such generosity. Galileo agreed to sign a statement saying Copernicanism was ``false and erroneous'' and could only be discussed as a ``mathematical supposition''. Galileo lived a retired life for seven years. Then he wrote a rejoinder to an article targeting him, and he dedicated this to the Pope Urban VIII, a friend. This was so well received that he sought permission to write a book discussing the Ptolemaic and Copernican systems as ``mathematical suppositions''. He was granted license, and he promptly published a devastating attack on the Ptolemaic Universe. It was a smash success, and the church was dismayed. The Pope was furious, but there was little he could do given the license he had granted. Fortunately, a paper surfaced in the files, that indicated that Galileo had infact agreed to never speak of the Copernican system at all, ``mathematical supposition'' or otherwise. So Galileo had obtained the Papal license under false pretenses. Using this document (whose authenticity has never been verified) the church prosecuted him. But Galileo still had friends in the church who argued for him, including the Pope and the Commissary General of the inquisition. He was sentenced to house arrest and seclusion, an incredibly mild punishment. Actually it was during this seclusion that he wrote on his studies of motion and inertia described before. He was dictating his new theory of impact to his students (who included Torricelli, the inventor of the barometer), when he was seized by the slow fever that eventually caused his death in 1642.
Nicolaus Copernicus (Mikolaj Kopernik in Polish), was born on 1473 at Torun, in eastern Poland. He studied contemporary astronomy as well as ancient Greek astronomy including discarded ideas about heliocentric solar systems. He grew dissatisfied with the geocentric system as his observations improved and started to lean towards a heliocentric system, but with the moon orbiting Earth. And the motion was still uniform circular. He hesitated to publish his work fearing retribution for disagreeing with Aristotelian ideas. He initially circulated a small draft amongst friends, which was approved by the Pope Clement VII in 1533. His student, Rhaticus, published his work with the caveat in the preface that the hypothesis of a stationary Sun was only a convenient tool to simplify planetary computations. But it clear that Copernicus believed that the heliocentric solar system was more than just a computational tool. However he died in 1543 before the ripples of the Copernican revolution could be felt.
Tycho Brahe was born in 1546 in Denmark to a privy councillor. But his wealthy and childless uncle abducted him when he was very young and brought him up as his own! He started his education in Copenhagen in law. But a solar eclipse turned him away from law to astronomy. In his first observation he uncovered gross inaccuracies in the Copernican tables. He devoted his life to the accumulation of observations so as to be able to produce accurate ephemerals. Then he discovered a star where none was supposed to be. This altered the confidence people had in the unchanging and perennial nature of stars and heavens. It also made Tycho Brahe's reputation. He eventually left Denmark and settled in Prague. He hired Johannes Kepler at this time. Soon after he died in 1601 leaving the observations to Kepler to study.
Johannes Kepler was born in Germany in 1571 to an innkeepers daughter and a drunken mercenary. As a gifted student he won a scholarship which allowed him to pursue education further than his social standing would have otherwise allowed. He also had the good fortune of studying under a professor who was convinced Copernicus was right. On qualifying he became a teacher of mathematics. He sent his papers to various people including Tycho Brahe, who promptly offered him a job. Delighted Kepler joined him in Prague. Tycho died almost immediately and Kepler took on his role. His first task there was to write an article debunking astrology. Apparently this did not diminish his demand as an astrologer! He also proceeded to explain optics and why pieces of curved glasses appeared to help correct faulty vision. In his Astronomia Nova ("New Astronomy") of 1609, Kepler had demonstrated that the orbit of the planet Mars is an ellipse. He tried unsuccessfully to fit the orbit of Mars to Brahe's observations in every possible combination of circles his ingenuity could devise. Because none of them worked, he tried noncircular paths until he found the true solution: Mars moved in an elliptical orbit with the Sun occupying one of its two foci. This was a devastating blow to the sacrosanct nature of the uniform circular motion, Aristotle's ``natural motion''. But Kepler did find a uniformity in planetary motion. The area swept by the line connecting the planet and the sun (which sits in one of the foci of the ellipse) is uniform with time. Ten years later he published his third principle which was that the cube of the mean distance of a planet from the sun was in a constant ratio to the square of the time required for the planet to complete its orbit. Kepler's patron was the Holy Roman Emperor who was forced to abdicate by his brother. Kepler moved to Austria and married again, his first wife having died in Prague. In 1620 he had to defend his mother against charges of being a witch. He also had to leave Linz when he lost his patronage because of a peasant revolt. He was promised support by a duke in Silesia where he went, but he found out that the duke's word wasn't one he could rely on. He left his family and traveled to Austria to retrieve his money he had left deposited there. Unfortunately en-route he fell ill and died in 1630.
A year after Galileo's death in Florence Isaac Newton was born on 1643 in Lincolnshire, England. His father had died five months before his birth and his mother married again. His stepfather and mother left him in the care of his grandmother. And he would remain separated from his mother for the next 9 years until his stepfather died. This traumatic experience left an indelible mark on him. He later spoke of ``Threatening my father and mother Smith to burne them and the house over them''. When she was widowed the second time his mother tried to employ Newton to look after her estate with disastrous results. He was sent back to school, and then onto Trinity College, Cambridge. There he was influenced by Descartes view of ``physical reality as composed entirely of particles of matter in motion'' and who held that ``all the phenomena of nature result from their mechanical interaction''. He proceeded to work on optics which he never published until he was required to lecture for the fellowship that he was granted. Immediately it resulted in a harsh criticism by Hooke who felt himself to be the expert of optics then. Newton's response was extremely sharp, almost hysterical. The controversy with Hooke worsened with his studies of planetary motion. Edmund Halley encouraged Newton to publish his work and in response Newton sent him a short tract. This however did not contain the three Newton's laws or the Universal gravitation law. The short tract evolved over two and a half years to the Principia that did include these. Immediately Hooke accused Newton of plagiarism. While this was a unreasonable accusation, Hooke was an ailing man on the decline. Instead of graciously acknowledging his contribution, in a fury Newton removed all references to Hooke in his manuscript. And refused to publish his book on optics or accept the Presidency of the Royal Society until Hooke died! He was very close to Fatio Duiller, a Swiss born mathematician living in London. He moved to London as the Warden of the mint. Unfortunately Fatio fell seriously ill and his family tried to take him back to Switzerland. Newton tried to get him to move to Cambridge where he would support him. His letters grew in intensity and then suddenly stopped. He sent wild accusatory letters to Samuel Pepys (whose diaries provide a look into London of the past) and John Locke (the philosopher who was one of the inspirations of the U.S. constitution), who feared of his sanity. He however recovered and returned to work. But it appeared he took great interest in counterfeiters and sending them to the gallows. His regime as the President of Royal Academy was rife with trouble. He tried to force Flamsteed, the Astronomer Royal to publish his data so Newton's friend (and Flamsteed's rival) Halley could use them before Flamsteed could. Eventually Flamsteed had to get a court order to prevent Newton from publishing his data! Characteristically Newton sought his revenge by removing all references to Flamsteed from Principia. Finally came the particularly ignoble quarrel with the German philosopher and mathematician, Leibniz who independently discovered calculus and published it. Of course Newton had already discovered calculus long before Leibniz started working on mathematics, but somehow charges of plagiarism started to be exchanged between the two. Newton proceeded to write letters in the name of his acquiescent young followers. He appointed a ``impartial'' board whose report he himself wrote in secret. Every paper on that subject had paragraphs interjected into it by him critical of Leibniz. This continued after Leibniz's death and until Newton himself died. He remained the President of Royal Society until his death in 1727, often dozing during the proceedings.
The desire to see the Earth as stationary is a very powerful one. After all we know when we are moving. It seems almost blindingly obvious that there is a difference between the states of rest and motion. And nothing in our experience about the Earth tells us that it is moving. Infact when it does, we can immediately tell that it is, as when during an Earthquake. Our entire experience tells us that the Earth must be stationary. In that case the motions of the Sun, Moon and the planets must be their own. Further, their motions clearly are about Earth. The sun rises in the east, moves across the sky and sets in the west. The moon, and the planets move across the sky, though not as regularly. Even the stars gradually move across the sky at night. All this, under the assumption of Earth being stationary, immediately leads to the Geocentric Universe. In this model the Earth is stationary and at the center of the Universe. Everything else is moving around the Earth. But almost immediately on accepting this picture you run into complications. The stars, were presumed to all lie at the same distance, because there appeared to be no reason to imagine otherwise. This put them on a sphere with Earth at its center. This sphere was called the Celestial sphere. Clearly the stars had a 24 hour period movement which was as if the Celestial sphere was rotating with that period about the axis that went straight through the Earth piercing it at its North and South pole and touching the Celestial sphere near the Pole star in the North. But there was an seasonal movement of the stars as well. In fact that helped fix the year. The plane that contains the Sun's orbit around the Earth is called the Ecliptic plane. It is so called because when the Moon's orbit crosses the Ecliptic plane that we observe Eclipses. The Sun of course obscures all the stars that are behind it at noon. But the stars obscured by the Sun at different seasons is different. This cycle is repeated on a twelve month period. This was immediately seen to be a powerful predicting tool of seasonal variations. The band of stars on the celestial sphere that falls behind the Sun were divided up into twelve sections. Each group is called a constellation. They were all given names and images for ease of recognition. Almost all ancient societies had names for the constellations and therefore must have used them for predicting changes in climate and fixing the calendar. Another important observations were the phases of the moon. The moon is seen to wax and wane, going from the new moon, through crescent, quarter and gibbous to full moon and then back to new moon again. With a period that is roughly 30 days. It made sense to take this as the smaller unit after year, which was the period of the Sun's motion across the Celestial sphere. All of this resulted in the Greek geocentric model. Eudoxus, a Turkish astronomer from the school of Plato who lived around 400-350 b.c. first proposed a geometric model of the geocentric Universe. He placed in order around the stationary Earth the seven heavenly bodies, Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. The stars were on the outer most sphere which rotated daily. And the planets and Sun were on spheres that were tilted and rotating about the Earth. Unfortunately in order to account for the retrograde movement of the planets (i.e., the planets appear to stop on the sky move backwards, then stop again and continue moving forward), the motion of the Sun against the constellations, and the seasons led him to use multiple sphere for each planet or the Sun and the technique soon got quite complicated. This picture was substantially improved by Ptolemy, who used the concept of epicycles. This has each heavenly body rotating along a small circle called an epicycle whose center is rotating along a larger circle called a deferent. This got rid of the messy extra spheres for each planet. But even this proved to be inadequate for the data available then. He then moved the center of the deferent off the Earth and introduced an equant which was a circle with its center at a distance from the center of the deferent that is equal to that of Earth from the center of deferent and on its opposite side. And then placed the deferent (and its epicycle) to orbit around the equant. This Ptolemaic picture of the Solar system essentially stayed intact and increasingly at odds with observations, until Copernicus came along. He decided that this system had lost it aesthetic appeal. And looked for other models. He finally settled on the heliocentric model. Actually he had the Sun static but off center. The major hurdle was the absurdity of a moving Earth. Once people became comfortable with that idea the heliocentric model had a lot of attractive features. It was a simpler, more aesthetic model. Unfortunately the simplest model Copernicus used wasn't better at fitting the observations than the Ptolemaic geocentric model. He had to make the model more complicated to do better. This is because he had the planets move in circular orbits around the Sun. An immediate objection to the Earth's motion was that the stars were not observed to change perspective as they would if we were moving (above the 24 hour variation). Copernicus' solution to this problem was to put the stars at substantially larger distances than had been thought before. This was a move as revolutionary as the one that set the Earth moving.
Galileo was convinced of the reality of the Copernican model when he observed the phases of Venus. The relation between the position of the Sun and Venus and its phases were impossible to account with the Ptolemaic model. He also found that Jupiter had satellites revolving around it, so there wasn't anything special about Earth that would require everything to rotate around it. Tycho Brahe observed that the paths of comets meant that they would have to have pierced several of the crystalline spheres the planets were presumed to have been rotating on. And finally Kepler's discovery of the three laws named after him based on his meticulous study of Tycho Brahe's voluminous data led the way to the creation of Newtonian dynamics and the birth of modern astrophysics.
The English who took to Age of Reason and Locke particularly adeptly
were amongst the first to welcome the new and expanded
Universe. Robert Hooke searched in vain to observe parallactic motion
in a star. More than fifty years after Hooke's abortive experiment,
James Bradley made a discovery. He rigidly fixed a telescope to the
chimney of his collaborator Molyneux' house. This lead him to discover
not the parallax of any star, that was still too small to be measured
then, but evidence of motion of the Earth and the finite speed of
light. He discovered aberration. As the Earth moves through space at
high speeds, and since light has a finite speed of travel, their
combined motions cause the star to appear at a different place in the
sky than where it would be were the Earth stationary. It is said that
Bradley came to this conclusion looking at the wind vane on a ship. Of
course the precession of the equinoxes was known even to Hipparchos in
120 BC. This is the gradual precession of the celestial sphere, with a
period of about 26,000 years is because not only is the Earth's
rotation axis at an angle of about 23
to the direction of
its axis of revolution about the Sun, but the axis of rotation moves
in a circle about the axis of revolution. This is caused by two
facts. The Earth's rotation makes it bulge a little in Equatorial
region. This deviation from sphericity gives the Moon (and the Sun to
a lesser degree) a handle to try and pull the Earth's rotation axis
back in the direction of the axis of revolution about the Sun. This
however doesn't result in an alignment between the two axes, the
Earth's rotation forces it into a perpetual precession with the
rotation axes trying to catch up with the axis of revolution but
perpetually careening off sideways and missing it. Bradley went one
step further and found that because the Moon's orbit about the Earth
is not quite in the plane of Earth's orbit around the Sun. This meant
that the Moon's tug trying to straighten out Earth's axis isn't
constant but wobbles. This produces a wobble over the precession in
the Earth's axis. This wobble is called nutation.
Another strong evidence of the Earth's motion are the seasons that we
observe on the Earth. The Sun of course is the source of nearly all
the warmth and light in the Solar System. Because of the curved nature
of Earth's surface (it being shaped like a sphere with a slight bulge
along the equator) the equatorial regions are warmer than the polar
regions. This is because the Sun's rays are more oblique near the
poles than the equator. Consequently larger areas share the same
amount of sunlight near the pole than at the equator. Now remember
that the Earth's axis is inclined at an angle of 23.5 from the
vertical to the ecliptic plane. So as the Earth goes round the Sun at
different times of the year, the Sun is more directly over the
Northern and Southern hemispheres. On June 21
the Earth is
oriented such that its north axis is tilted the most it ever does
towards the Sun. This is the Summer Solstice (at least for those
living in the Northern hemisphere) when the Sun is overhead at
23.5 latitude. This is time of the longest day for those in
the North, with Sun never setting north of the latitude 66.5!
Both these two latitudes (i.e., 23.5 and 66.5), have
special names. Tropic of Cancer and Arctic circle respectively in the
Northern hemisphere and Tropic of Capricorn and Arctic circle in the
Southern hemisphere. Similarly on December 22
we have the
winter Solstice. These were times of celebration for nearly all the
ancient cultures and tended to get absorbed into (and in the process
legitimized) the more organized religions.
Infact the connection of religion with seasons and calendars is extremely strong. The last change of calendar was instituted by Papal edict in 1582, by Pope Gregory (hence Gregorian calendar), but this wasn't picked up by England until 1752 and until 1918 by Russia (USSR actually). Even Soviet Union felt forced to accept the Gregorian calendar. The only European calendar of secular origin was the French republican calendar, which unfortunately had even more days in a week than we do, ten to be precise. Born in 1792 counting the day of French Revolution as day one it quickly fell into dis-use because of conflict with the Gregorian calendar. The other calendar in substantial use in the world is the Muslim calendar which starts from Hijira which celebrates Muhammad's miraculous flight from Mecca to Medina. St. Bede's wonderful book on the history of English church is largely a story of dispute between the Papacy represented by Pope Gregory through St. Augustine and the Irish priesthood on the exact dating of Easter. It appears unlikely that any new changes to the calendar will be acceptable very quickly.