In the second century B.C. Greek astronomy came of age. While it had previously been closely connected with philosophy and mathematics, the increased use of observation pushed astronomy into the realm of science. And the man most responsible for this was Hipparchus of Nicaea.
This episode delves into some of Hipparchus's achievments, as well as arguing that more than any other persons Hipparchus was responsible for turning astronomy into a fully fledges science.
Contact: thecompletehistoryofscience@gmail.com
Twitter: @complete_sci
Music Credit: Folk Round Kevin MacLeod (incompetech.com) Licensed under Creative Commons: By Attribution 3.0 License
In the second century B.C. Greek astronomy came of age. While it had previously been closely connected with philosophy and mathematics, the increased use of observation pushed astronomy into the realm of science. And the man most responsible for this was Hipparchus of Nicaea.
This episode delves into some of Hipparchus's achievments, as well as arguing that more than any other persons Hipparchus was responsible for turning astronomy into a fully fledges science.
Contact: thecompletehistoryofscience@gmail.com
Twitter: @complete_sci
Music Credit: Folk Round Kevin MacLeod (incompetech.com) Licensed under Creative Commons: By Attribution 3.0 License
Hello and Welcome to the Complete History of Science Podcast
How is History made?
Well in the 19th century the historian and philosopher Thomas Carlyle formulated a theory, which become known as The Great Man theory of history.
Carlyle believed that historical events were largely shaped by the actions of Great Men.
Men who, through their intellect, or perhaps their political leadership, or military skill, shape the course of history.
And its true that history seemingly provides many examples of such men, whether it be Napoleon, who shaped the political map of 19th century Europe, or Julius Caesar, whose actions led to the downfall of the Roman Republic.
In more recent times we might also reformulate The Great Man theory to include female figures such as Margret Thatcher, who dismantled the post war welfare state in the UK and whose influence has shaped the political status quo across the western world.
However, The Great Man theory of history is largely out of favour with most modern Historians.
History is frequently too complex to be explained by the actions of a single individual, however strong willed that person may be.
Instead, history can be viewed as a consequence of the collective actions of millions of little people, as well as the institutions, culture, and societies, which those individuals produce.
In this view, society at large creates the material conditions necessary for so called ‘great’ individuals to act.
The Great Man theory of history, however, is still interesting to consider in the context of the History of Science.
In science there are undoubtedly men of genius, Archimedes, Newton, Einstein, Von Neuman, each possessing a creative and analytical intellect which is difficult for most of us to comprehend.
But they were also born at the right time, into societies in which they could fully use their gifts.
Scientific advancement then contingent on both of these conditions.
Breakthroughs happen when an idea is ripe, and the prerequisite discoveries have already been made, but science also requires great thinkers to make these intellectual leaps.
And in the Greek world of the 2nd century B.C., astronomy was ripe for a great leap forward.
Eudoxus’s mathematical models for the solar system had stimulated many other geometers to try and improve on his models.
Likewise, as we discussed in the last episode, Aristarchus had demonstrated to the Greek thinkers who followed that geometric models might go hand in hand with observation.
An stepping into this world was Hipparchus of Nicaea.
Hipparchus was born around the beginning of the 2nd century B.C., in the Greek city state of Nicaea, in modern day Turkey.
Hipparchus was probably the greatest astronomer of antiquity and the discoverer of several astronomical phenomena.
What set Hipparchus apart from his contemporaries was his reverence for measurement and observation.
To give an example Hipparchus made improvements on Aristarchus’s measurements of the size and distances of the Sun and moon.
Like Aristarchus he constructed geometrical models of the Earth, Moon and Sun; but unlike Aristarchus it’s clear that Hipparchus certainly derived his values from observation.
Consequently, Hipparchus’s estimates are much more accurate.
For example, his value for the distance to the moon was around 67 times the radius of the Earth, only around 10% greater than the correct value of 60 times.
Hipparchus obsession with measurement and records also led to perhaps his most surprising achievement, the discovery of the precession of the equinoxes.
As discussed, Hipparchus was the first Greek astronomer to fully accept observation as an essential component of astronomy.
His predecessors had left some records, for example, Timocharis, who lived in the 3rd century B.C. had left some scattered observations of the positions of several stars.
Hipparchus however, greatly expanded on these observations and in doing so noticed that during the two centuries which had elapsed between these measurements the constellations had consistently shifted around 2 degrees eastward on the celestial sphere.
He then noticed the same anomaly in the position of other stars of the zodiac.
In modern terms this happens for the following reason.
The Earth is tilted on its axis with respect to the Sun, at an angle of 23.5 degrees.
This axis however rotates, which is known as precession.
This means that over time the Earth will point in a different direction compared to the stars, changing their relative position in the night sky, for example, the current pole star, Polaris, will over time shift away from the north pole.
This change takes place over a very long period of time, roughly 26,000 years to complete one rotation.
While he can’t have fully grasped these implications, Hipparchus’s respect for observation meant that he didn’t dismiss this as some observational error but insisted that it must be a real phenomenon.
Because the observations by Timocharis were dated, Hipparchus was able to calculate that the longitudes of the stars were changing at a rate of around 1/100 of a degree per year.
However, Hipparchus’s importance in the History of Science goes beyond any individual discovery. Hipparchus’s greatest achievement was as a key figure in the development of astronomy as a science.
To understand this contribution, it’s best to look at an example.
As we have already discussed several times, the seasons, or more specifically the times between the solstice and the equinox varies.
Hipparchus improved upon previous approximations and took his own definitive measurements of the lengths of the seasons. These were:
Spring 94 1/2 days (Hipparchus, ca. 130 B.C.)
Summer 92 1/2
Autumn 88 1/8
Winter 90 1/8
Which sum to 365 ¼ days.
Hipparchus then used these measurements as a basis to improve upon previous models of the Sun’s motion.
When Eudoxus had created his solar models, some 300 years earlier, he had made several assumptions not based on observation.
Eudoxus like many early Greeks, considered that the heavens must be perfect, and hence decided that the Sun should orbit in a circle at a constant speed.
This was a purely speculative and in some ways quasi-relgious assumption, reflecting Greek astronomy’s origins, in philosophy.
It was also impossible to reconcile with the data, which showed that the seasons lasts for different periods.
Hipparchus instead demonstrated that the model can be made to work if we simply assume that that the Earth is not at the centre of the orbit.
The Sun still travels at a constant speed, in a circular path, but because it is now centred at a point other than Earth, it appears to spend longer in some quadrants than in others.
Hipparchus, a great mathematician as well as scientist, worked out a detailed a geometric model for the Sun’s motion, based these his observations.
Its worth noting we can’t credit Hipparchus with the idea of an off-centre orbit, which was first suggested by Apollonius of Perga, in the 3rd century B.C..
However, the key difference is like his contemporaries, Apollonius was unconcerned with fitting his models to data and was instead only interested in the broad strokes of the theory.
Hipparchus real contribution in the history of science was to assert that a mathematical models should be work in detail and be consistent with observations.
Unlike earlier astronomers, Hipparchus’s had no time for philosophical ideas about the perfection of the heavens and was prepared to drop assumptions from his models when they were shown not to fit with the data.
After Hipparchus astronomy didn’t change into a science overnight, but he did start the long process of insisting that scientific models should be based upon and measured against empirical data.
This was a new way of working in the second century bc and one that persists to the present.
Science can advance mathematical and physical models for how the world works, but unless these models conform to reality, they aren’t accepted.
Models also need to be able to make predictions and Hipparchus’s solar model was so successful in that regard, that it could predict the position of the Sun at any time to within one arc minute, which was a much smaller interval than could be directly measured.
Likewise he was able to improve upon the estimate for the length of the tropical year, and attained a value which was only out by 6 minutes from the modern accepted value.
Part of his success depended on his use of improved scientific equipment compared to his predecessors.
Hipparchus mainly worked on the island of Rhodes, on which he set up an observatory, using the latest modern instruments available at the time.
For example, the simple Gnomon was superseded around this time by the superior quadrant, sometimes called a meridian quadrant.
A quadrant is a device, usually a quarter circle, which can measure an angle up to 90 degrees.
This was set in the plane of the meridian, that is along a line of longitude, so that it can measure the angle of the sun’s rays.
It’s possible, though unlikely, that Hipparchus was the inventor of the quadrant, but what is certain is that he used the quadrant to make his more accurate estimates of the seasons.
The quadrant itself would continue to be an incredibly important device in the history of astronomy until fairly modern times and was also a key device for navigation in the following centuries, as it allows a measurement of latitude, even at sea.
In addition to making his own observations, Hipparchus’s was also the first Greek astronomer to recognise the immense value of the astronomical records which had come down from the Babylonians.
The Greeks had long recognised that the Babylonians, whom they called the Chaldeans, were great astronomers.
However, Hipparchus was the first to begin to use their huge body of data to improve his own celestial models.
An example is Hipparchus’s theory of lunar motion.
Hipparchus used the Babylonians observations of the lunar cycles and updated them with his own.
Using this he was able to create a reasonably accurate model of the moon’s motion.
I say ‘reasonably’, because in reality, the moons motion is more complicated than many people realise.
For example, most people take the lunar month to be 28 days; but if we were to measure the time it takes for the moon to go through its phases, that is from one new moon to the next, it is actually 29.5 days, called the synodic month.
This is also different from the time which it takes to make one complete orbit about the Earth.
The Moon’s orbit, like most celestial bodies is elliptical, but this ellipse is itself precessing.
In simple terms this means that the elliptical orbit doesn’t stay still but rotates, and does so with a period of around 9 year.
Overall this means it takes approximately 27.5 days to make a complete orbit of Earth, which we called as the anomalistic month.
The complexity of Lunar motion meant that it wasn’t possibly for Hipparchus to model with a simple off-centre rotation.
So, Hipparchus’s introduced what is known as an epicycle.
Essentially an epicycle is an additional smaller orbit which a celestial body makes, while also following a larger circle known as the deferent.
Introducing epicycles would from this point become a standard trick used for centuries by astronomers to describe the motion of celestial bodies as circular, when in reality their orbits are elliptical.
While the epicycle was again the invention of the mathematician Apollonius, Hipparchus’s was the first to apply the theory in detail to the orbit of a celestial body.
By using Babylonian observations, as well as his own, Hipparchus could calculate the necessary parameters, such as rate of rotation of the epicycle and deferent, which allowed him to make reasonable prediction of the longitudinal position.
Hipparchus’s model is actually even more complex than the epicycle-deferent one I’ve described, as the moon is also orbiting in a slightly different plane from the Sun, so is offset at an angle of around 5 degrees.
The reason this is important is because taking it into account allows eclipses to be described and predicted.
Based on the parameters which Hipparchus calculated for his model, could make reasonable predictions of the lunar eclipses, though solar eclipses with their much smaller shadows were more difficult.
Hipparchus’s successors would attempt to improve upon his lunar theory to make better estimates of the solar eclipses, but it wasn’t until the time of Tycho Brahe, and eventually Edmund Halley that solar eclipses would finally be predicted.
Despite having derived very accurate solar and lunar theories, Hipparchus didn’t create a similar theory of the planets motion.
The reason for this is unclear, as he did make extensive observations of the planets.
Hipparchus did however, use his observations to demonstrate that the models of his predecessors were highly inaccurate, and the planets, with all of their retrograde motions, are immensely difficult to predict.
It would take another astronomer Ptolemy to finally create an accurate planetary theory, some two hundred years later. We will discuss this in detail in the next episode. However, it is worth noting now the debt to which Ptolemy owes Hipparchus.
Much of Ptolemy’s work makes direct use of Hipparchus’s theories, in particular his lunar and solar models.
And even his planetary theory makes use of both Hipparchus’s observations as well as adapting his epicycle-deferent models.
Historically everything we know of Hipparchus’s work, actually comes through Ptolemy, whose great book, the Almagest, became the most influential scientific work of the ancient world.
The Almagest was so great and complete, that it meant that older works, such as those of Hipparchus were never copied or handed down.
However, if it hadn’t been for Hipparchus its arguable that the Almagest may never have existed.
Hipparchus more than any other individual in the ancient world, put Astronomy on firmly scientific footing, and in the process created arguably the first true science.
Planets…..
Nevertheless the records which Hipparchus himself left enabled those which followed him to answer this question conclusively.
Ptolemy….
Finish – no theory of planets
- Ptolemy to the rescue…
Add in lunar/solar eclipses?
Cut
I think in the History of Science its necessary for us to consider both of these theories when we discuss how the great discoveries of the past were made.
He created an extremely accurate model of the Suns motion across the sky, as well as a theory of eclipses, which in the case of lunar eclipses, were very successful. He also discovered what is now known as the precession of the equinoxes, which is the slow motion of the Earth axis. However, Hipparchus real contribution to astronomy and science goes beyond any individual discovery.
Hipparchus was working around 100 years after Aristarchus, who was the last great figure of Greek astronomy which we looked at. Like Aristarchus, Hipparchus constructed geometric models to explain astronomical phenomena. However, unlike his predecessor Hipparchus’s real achievement was in understanding the importance of combining these mathematical models with detailed observations.
Today he is remembered as a great mathematician and the founder of trigonometry. However, what is interesting about Hipparchus is that this discovery was in many ways incidental, as Hipparchus discovered Trigonometry in the pursuit of his primary interest, Astronomy.
And it was Hipparchus’s attachment to making and recording observations allowed him to give definitive answers to questions which had troubled earlier astronomers, and in the process, forced them to give up some of the assumptions which were laid down by his predecessors.
In particular Apollonius of Perga, a mathematician from the 3rd century B.C., had introduced many new ideas to attempt to resolve some of the deficiencies in Eudoxus’s models.
Hipparchus started the long insisting that scientific models should be based upon and measured against empirical data.
Before Hipparchus astronomy was still closely linked with the fields of philosophy and mathematics.