The Origins of Astronomy

Arguably the first science, was the science of astronomy.  Beginning in the middle east I begin with a look at science in ancient Egypt, before moving onto their more sophisticated neighbours The Babylonians.  Along the way I discuss the astronomical discoveries of these early cultures and ask the question: Why did science start here?

Transcript

Marking any particular point as the place where science starts is an impossible task.  Science as we would understand it, is a relatively modern idea.  The words science and scientist were not widely used until the 19th century, and the idea of science as a career choice didn’t become common until the 20th century.  

Nevertheless, the practice of science, as well as the ideas it produces, have a much longer history.  A reasonable starting point are early civilizations of the middle east, particularly Egypt and Babylon, around 3000 years ago.  These peoples were amongst the first to go from nomadic hunter gatherers into settled societies dependent on agriculture, marking the beginning of what is usually called civilisation.  This societal change was the key event in early science, as the changes in the makeup of society made the practice of science possible.

It would also provide the motivations for these early societies to start pursuing science.  For example, the dependence of these societies on agriculture was likely a particularly important motivator in the development of astronomy.  This is because successful agriculture requires an understanding of a calendar, important for knowing the best time to plant and harvest crops.

In Egypt for instance the seasons of winter, summer etc. aren’t particularly noticeable.  However, every year the Nile would flood which was crucial for creating the rich arable land around the river, upon which Egyptian civilisation was based. The Egyptians noticed that the Brightest star Sirius, was always invisible around this time, being too close to the Sun, …but shortly after it would reappear, and the flooding would begin.  The Egyptians measured this happening every 365 days, creating the basis for a calendar which would prove immensely useful for early astronomy.  This period of 365 days, the tropical year, could be verified using one of the earliest astronomical instruments, The Gnomon.  

Gnomon spelt G-N-O-M-O-N is a simple device, which would prove endlessly useful to early astronomy.  In essence, a gnomon is a vertical stick set in a sunny place, which most importantly shouldn’t be moved over the course of the day. The purpose is to provide some way to record the motion of the Sun as it moves across the sky.  By measuring and recording the shadow cast by the gnomon at some interval of time, a wealth of astronomical information can be worked out.  For example local noon occurs when the shortest shadow is cast.  Likewise, the direction of the shortest shadow gives the position of celestial North, which lies in that direction.  

Similarly, by repeating these measurements over the course of a year, the changing length of the shadows allows  us to track the date of the summer and winter solstice.  The winter solstice for example is the shortest day of the year in the Northern Hemisphere and corresponds to the point when the Sun is lowest in the Sky. Therefore, by using the Gnomon to measure the shadow at noon over a series of days, the winter solstice matches the day where the shadow is at its longest.  The period between two winter solstices then corresponds to a tropical year.   The Gnomon then is an early example of where seemingly abstract measurements, taken over an extended period of time, produce some useful information.

Despite their achievements however, the Egyptians were far outstripped by their neighbours to the east.  The Babylonian civilisation, centred on the Euphrates River  in modern day Iraq, was another of the early agricultural societies which sprang up in the middle east in the 2nd millennia B.C.  Like the Egyptians early Babylonian science was intimately related to the need to keep a calendar which would help guide the annual harvest.  However, their sophistication far outstripped any contemporary astronomy, and the knowledge they passed down was vital in the development of astronomy into a fully fledged science by the Greeks.

Indeed many histories of science begin with the Babylonians, and this has one very good reason, they left records. While it is likely that people have long recognised patterns and cycles in the motion of the stars, the Babylonians are the first people we know of who made methodical recordings of what they saw. Miraculously these records, such as the MUL.APIN tablets, from around 700B.C. survive, and are preserved in the British Museum.  The MUL.APIN tablets then are probably the first extant object in the history, which we might reasonably recognise as scientific data. 

The tablets record a list of the stars and constellations, seen in the night sky.  For example:

The Plow, Enlil, who goes at the front of the stars of Enlil.

The Wolf, the seeder of the Plow.

The Old Man, Enmesarra.

The Babylonians also grouped the stars into constellations and used this to divide the sky into twelve roughly equal proportions, corresponds to what we would now call the 12 signs of the zodiac. Over the course of the year these constellations both rise and fall over the horizon as they make their way across the sky.  So the Mul.Apin also records a list of dates corresponding to these risings.  For example:

On the 1st of Nisannu the Hired Man becomes visible.

On the 20th of Nisannu the Crook becomes visible.

On the 1st of Ajjaru the Stars become visible.

On the 20th of Ajjaru the Jaw of the Bull becomes visible.

On the l0th of Simanu the True Shepard of Anu and the Great Twins

become visible.

This formed the basis of the Babylonian calendar, where an observer watching a particular constellation dip or rise above the horizon in the East, can infer the time of year.  

These dates correspond to dates in the Babylonian calendar, which was based on the lunar cycle, where a new month matches the appearance of the new moon, and therefore a Babylonian month was 29.5 days.  

However, this meant that 12 cycles of the lunar calendar, was only  354 days in a year, 11 days short of the real value.  This meant the lunar months would get out of step with the rise and fall of the constellations.  The Babylonians weren’t unaware of this and so eventually developed a complex system where a thirteenth month was inserted every several years to account for this.  

The Babylonians real triumph, however, was in developing advanced mathematical models which could describe these observations.  For example, as we discussed earlier, ancient astronomers could measure the motion of the Sun with a Gnomon, allowing the solstices and equinoxes to be measured accurately.  

Naively we might expect that there is an equal period in each of these four seasons, that is that the same number of days elapse between the equinox and the solstice.  However, this is not the case. For example, the time between spring equinox and summer solstice is around 93 days, but the time between the winter solstice and the vernal equinox is only 89 days.  The Babylonians accounted for this by assuming that the Sun moves at different rates depending on the time of year.  To achieve this the Babylonians developed numerical techniques, specifically arithmetic progressions, which allowed them to account for the suns increasing and decreasing speed.

Even more impressively they were also able to apply this model to the motion of the planets.  Most of the celestial objects we’ve discussed so far, like the Sun, Moon and Stars move in predictable ways.  However, the planets motions are more complex. From night to night a planet may seem to also move across the sky in one direction. But then, occasionally, the planet will suddenly change direction and start to move in the opposite direction, before eventually changing direction again.  This is known as retrograde motion, and gives the planets their name, where the word ‘planet’ derives from the Greek for wanderer. The Babylonians were able to apply their numerical sophistication to predict when these retrograde motions would occur.

The natural question which arises here is why the Babylonians should have been interested in the motion of the planets.  Unlike the motion of the Sun, which give rise to the seasons, the motion of the planets have far less obvious utility.  The answer lies in understanding that for the Babylonians observation of the night sky was primarily about predicting what might happen on Earth.  

Astronomy has for most of its history been intimately related to the practice of astrology.  Even great astronomers, such as Johannes Kepler, were known to practice astrology as late as the 17th century.  In the case of the Babylonians, the careful observations and recording of astronomical data were made in order to provide the best possible information to the Babylonian kings. The questions which they hoped to answer were not the personal ones which we may now associate with astrology, but questions of national importance.  Would there be a good harvest this year?  Would now be a good time to go to war with a neighbouring city?

For this the Babylonians created records of astronomical observations, unparalleled in the ancient world.   And they achieved this because theirs was a complex, centralised and stratified society.  

For example, one of the primary differences between an agricultural society such as Babylon, and more primitive hunter gatherer societies, is that they produce a surplus of food.  This allows for a class of people whose time can be spent on an abstract endeavours like astronomy rather than more pressing matters like food collection. Babylonian rulers invested in a state bureaucracy, likely some form of priesthood, which was able to spend time making and recording the astronomical observations.  The durability of the state also meant that these observations were gathered and improved over many years, providing information which would have been impossible to collect within the lifetime of any individual.  

As compared to contemporary Greeks, the Babylonian astronomers were far more quantitative and precise measurement because that was their livelihood, and the state funded this endeavour over many generations.  The Greek astronomers who followed would rely on this Babylonian data for centuries afterwards.

However, the role of the state was also a limit on how much Babylonian astronomy could achieve.  While the Babylonians had immense success in predicting the motion of celestial objects, what they seem to have lacked is any theory underpinning the motion.  In contrast, the Greeks astronomers who worked more freely and often with less data, preferred to philosophise on the nature of the objects in the night sky, and suggest models, without necessarily checking them against observation.  

In our modern conception of science both of these elements are necessary.  Models are theorised which help us picture phenomena, and offer some explanatory power, but these models then need to be checked against rigorous scientific data.  

Astronomy in the ancient world fell far short of this ideal model of science. However, many of the important elements which set the stage for the scientific revolution were falling into place. While we may think of astrology as fundamentally opposed to science, the relationship is far more complex and subtle than they may at first seem.      

I think that we can understand this if we consider that many of the same impulses which motivate us to study astronomy, are the same ones which also lead us to practice astrology.  The Babylonians saw the planets as either representations of Gods, or perhaps the Gods themselves, but their astrology supposes that the will of these Gods is comprehensible.  While this is an alien idea to us, it differs from more primitive mystic thought which supposes that the universe might be chaotic and unknowable.  Astrology and astronomy are both founded on the belief that the universe around us is both understandable and predictable.  

As we shall see time and again, early science is frequently intertwined with religion, superstition and magic.  To an early practitioner there wouldn’t have been any meaningful distinction to be drawn between these fields and science. The history of science then is in many ways, the history of how we came to discard these layers of pseudoscientific thought until we reached the point where science could be regarded as a field in its own right.  

In the following few episodes, we’ll move westward to the Greek world, where Greek astronomy would emerge and slowly surpass the Babylonians, culminating in the work of Ptolemy, who would write the definitive astronomical work of the next 1,500 years.

 Along the way I discuss the astronomical discoveries of these early cultures and ask the question: Why did science start here?