Classical Astronomy

In your attmept to understand where modern astronomy came from it is important appreciate how astronomical ideas have been and still are embraced by other cultures. From Alberta's First Nations people to pre-bronnze age cultures of Northern Europe to the ancient Greeks, astronomy has always been an important part of the quest to understand the world.

What is a Cosmology?

One of the roles of astronomy in a culture is to supply a cosmology. Cosmology is defined (ambitiously!) as the science of the entire universe. Often we split cosmology into sub-groupings:
  1. Cosmography: the study, cataloguing and charting of the objects that make up the universe
  2. Theoretical Cosmology: provides explanation or framework in which to explain the objects of the universe
  3. Cosmogony: attempts to explain the origin of the universe
Modern cosmological ideas are strictly rooted in the material universe. Ancient cosmological ideas tended to be bound up with world view which is something far more comprehensive. Their cosmologies served to explain natural phenomena by placing them in a broader context as well as embracing other, mythic ideas. Sky lore and myth were often blended together.

What do we require of a cosmology?

  • it must explain observed phenomena with a few basic principles
  • it must be logically and internally consistent

What Phenomena Have We Listed That Our Cosmology Must Embrace?

Diurnal and sidereal motions: there are two very distinct motions that the stars undergo- why?
Lunar phases: How can we explain the montly progression of lunar phases?
Eclipses: Such dramatic events command our attention and explanation.

Planetary motions: we now encounter new phenomena. Mercury and Venus are always seen close to the Sun and thus visible at either Sunset or Sunrise. They are called inferior planets. On the other hand, Mars, Jupiter and Saturn can be seen at any time and do not appear to be "tied" to the Sun. These are termed superior planets.

The superior planets, especially Mars can dance in a rather peculiar motion. Drifting westward, stopping and then sliding eastward again. This "backtracking motion" is termed retrograde motion
which is illustrated in Figure 4.1.

 Figure 4.1 Mars undergoing retorgrade motion in the Virgo/Leo region of the sky for the time period December 1996 to June 1997.

Stars, star brightness and constellations: Why are there stars of different apparent brightness? Why do they move across the sky, together, in constellations?
Meteors, comets: What are these occasional and un-announced guests?
New stars: History contains accounts of stars that appeared unexpectedly and then faded back to invisibility. How can we fit this into our cosmology?
Table 4.1 Observable phenomena that cosmologies must be able to explain.


Some Key Points to Consider: 

  • the early animistic ideas of the universe (Babylonian/Egyptian/Hebrew). The universe and the "world" of gods and spirits are one. Motion in the heavens and other celestial phenomena were the result of divine intervention and were portents of things to come.
  • all known peoples had some mythology/record of the sky. Ancient petroglyphs occasionally contain recognizable astronomical "motifs".
  • emergence of the Greek mechanistic philosophies in the 6th century BC began to shift cosmology away from its animist center.

Some Examples of Peoples and Their Cosmologies...

The Newgrange Passage Tomb

An impressive Neolithic burial tomb from about 3000 BC that predates the pyramids of Egypt! Newgrange is one of many such tombs that lie along the Boyne river near Drogheda, County Meath, Rep. Ireland. The tomb is an impressive 100 m in diameter and 15 m high with a narrow passge to a central tomb. The structure is aligned precisely to allow the rays of the winter solstice Sun to pass down the passage and fall directly on a large basin at the center of the tomb. Except for about 15 minutes every year this central chamber is dark and silent!

Figure 4.2a Newgrange burial mound (passage tomb) Figure 4.2b Spiral runes carved on wall inside the passage tomb Figure 4.2c Sunlight from winter solstice illuminating the passage tomb
The Newgrange tomb also contains characteristic spiral carvings that some have speculated served as calculating devices for predicting the time of new Moon. The care of contruction and alignment of this massive tomb suggests both a deep familiarity with and reverence for astronomical events.

First Nations People of North America - Medicine Wheels

The prairie provinces contain many examples of First Nations Medicine Wheels. Figure 4.3 shows a very large medicine wheel located near Majorville, Alberta that is more than 4500 years old! In the centre of the wheel you see a large cairn 9 metres in diameter, surrounded by a stone circle 27 metre across. Radiating outward are about 28 spokes that link the central cairn to the surrounding circle. What was the "function" of the spokes?

Some have speculated that these radial spokes point in the directions of significant astronomical events (perhaps rising or setting locations for stars or the Sun). This is far from settled and many archaeologists and astronomers have challenged the idea.

Figure 4.3 The Majorville Medicine wheel (image used with permission R. Nelson,

We do know that these are sacred sites and it wopuld be fair to conjecture that the First Nations people had an intimate awareness of the night sky. Whether they were also "observatories" similar to Newgrange or Stonehenge is still a question in need of research.

Example 4.1 Suppose a spoke in a medicine wheel such as the Majorville site was used to mark the first visible rising of Sirius in late summer. Use Stellarium to estimate the direction in which this spoke should be oriented. Does this "prove" the idea that medicine wheels had an astronomical purpose?

Solution: Look up the location of Majorville - latitude = 50o 38' , longitude = 112o 42' . Set the location in Stellarium to this and begin to explore the eastern sky in the very early morning just as the Sun rises. Beginning about August 24 you will see the bright star Sirius rising through the morning twilight. Figure 4.4 shows this.

Figure 4.4 The early morning sky on August 24 from a simulation of the Majorville, Alberta Medicine Wheel site.

At this point the azimuth for Sirius is 120o or south-east. If a spoke were oriented in this direction you might reasonably conjecture it was used to mark such an event as the rising of a prominent star. Does this "prove" the idea that the spokes represented astronomical markers? NO! One would need to find many more examples - perhaps other medicine wheels with very similar alignments before the plausability of this idea could be accepted. However, the term "proof" is not appropriate here.

Early Greek Myhtologies and the Constellations

The constellations that we are most familiar with have their origins in Greek mythology and were used a "signifiers" or markers to help tell the mythical stories of the ancient Greeks. The constellation Cassiopeia is not intended to look like a queen on a throne but to act as marker in the sky against which the story of the vain Queen Cassiopeia, her weak husband King Cepheus and their beautiful daughter Andromeda can be told. This is a common motif that one sees in early, narrative cultures.

As in many early cultures, the Greeks attributed the various phenomena described in Table 4.1 as the result of direct interventions by "gods" and spirits.

The Ionian Greeks...

With the Greeks and especially the Ionians we encounter a radically different approach to the heavens. The Ionians are the first to explain the workings of the heavens in terms of natural processes. In many ways the Ionians are the most direct forerunners of the modern astronomy that would begin at the time of Galieo and Newton.

Thales of Miletos (624 BC– 546 BC)

           One of the first of the Ionian philosphers on record is Thales who, it is widely calimed, was the first person to be able toi predict the occurence of an eclipse. Did Thales actually predict an eclipse? Perhaps not but what was significant is his recognition that one can do so and that the heavens have a mathematical predictability. The universe is rational and can be comprehended by the human mind. This becomes an increasingly dominant idea that slowly displaced the role of animist influence.         

Pythagorus (580 BC - 500 BC)
Pythagorus may be best known for the theorem that bears his name but he made many important contributions to Greek philosophy including:

  • The birth of the idea of the mathematically governed universe. The emergence of mathematical metaphors - the orbit is a circle, the planet is a sphere and the universe is mathematical.
  • The discovery of irrationals numbers.

With Pythagorus a split of traditions occurs. Once branch followed Pythagorus and the Ionian tradition and developed mechanical conceptions of the universe culminating in the heliocentric model of Aristarchus. The other branch led to Plato and Aristotle and culminated in Ptolomey with the development of the geocentric cosmology.


Aristarchus of Samos (310 BC - 230 BC)

Aristartchus is often called the "Greek Copernicus" - his cosmology is purely heliocentric and essentially the model that emerged 1900 years later with Copernicus.

Among his other achievements he added 1/1623 (or 53 seconds!) to the measure of the length of the year - without the benefit of as much as a pendulum clock!!

One of the strange twists in the history of astronomy is the fate of Aristarchus' cosmology. With Plato it fell into relative obscurity and during the middle ages was known as the "Greek Heresy" because it proposed a model contrary to the accepted Earth-Centred of geocentric view.

  Figure 4.5 Aristarchus' model of the solar system.


Plato (428 BC– 348 BC)

It is hard to think about western philosophy without thinking about Plato. So influential is Plato's thinking that Bertrand Russell has written that philosophy after Plato is merely footnotes on Plato. Given the influence of Plato, consider the following quote from Book VII of The Republic:

Let us then study astronomy by means of problems as we do geometry and let the things of the sky go if the study of astronomy is to make the naturally intelligent part of the soul useful rather than useless.
Plato distrusted the senses. His allegory of the cave is a subtle and profound investigation of the limitations of human knowing and is an attack against any naive, empirical science. In the hands of the Neo-platonists and scholastics of the middle ages it became a dogma that stifled the growth of science. For Plato perfection was in the forms - all else was illusion or deceptive. We best learn astronomy by turning it into geometry - forget the stars. The amazing regularity of the universe fostered this ideal. But the universe is not regular in the way Plato thought.

Aristotle (384 BC – 322 BC)

One of the greatest and most versatile philospohers of all time was Aristotle. In the middle ages he was referred to as "The Philosopher". He wrote extensively on matters of science, law and aesthetics.

Aristotle established a neat separation of the perfect and the imperfect by returning to the geocentric model. All that was base and subject to decay was found within the sublunary sphere and consisted of the 4 base elements:


Beyond the sublunary sphere were the stars and planets - immutable and perfect and part of a 5th element - the quintessence.. We will return to this when we discuss Galileo.

While it is tempting to regard this as naive it did provide an unified explanatory framework for the world and conforms to simple observation of how matter seemed to behave.

  Figure 4.6 Schematic representation of Aristotle's 4 elements.


Dominant Metaphors and the Eventual Paralysis of Astronomy

Dominant metaphors of this time were the circle and the sphere. All heavenly motion - being perfect - was circular. This was a pervasive and eventually paralyzing idea in astronomy.

 Aristotle's cosmology (theory of the universe) was concerned solely with describing the motions of the Moon, Sun and planets. In all, it required the concerted motions of 54 spheres to account for the motion of the 7 known planets. Even with this it failed completely to account for:

  • the changing brightness of planets
  • the changing apparent size of the Moon.

Eratosthenes (276 BC - 194 BC)

 Contrary to popular belief the ancients were well acquainted with the idea of a spherical Earth. Aristotle taught that the shadow cast during a lunar eclipse was a direct consequence of the spherical shape of the Earth. Eratosthenes extended the understanding of the spherical Earth by actually measuring the size of the Earth. His technique is both ingenious and simple

  • It was known that on the Summer Solstice the Sun shone directly into a well in the city of Syrene (south of Alexandria). On this same date the Sun was 7 degrees south of zenith as viewed from Alexandria - 5000 stadia away. Eratosthenes performed the following simple calculation:
  • 7 degrees = 7/360 or about 1/50th of a complete circle.
  • if 1/50th of the way around the Earth = 5000 stadia then the complete trip = 50 X 5000 or 25 0000 stadia
  • Since the circumference of a circle C = 2p R we can divide 250 000 stadia by 2p to find that the radius of the Earth is about 40 000 stadia, the diameter is 80 000 stadia.

So ... What's a "stadia"?

Figure 4.7 Eratosthenes method to measure radius of the Earth.

Some scholars put the stadia used by Eratosthenes as 1/6 kilometer. If this is so then Eratosthenes was only off by 4% in his measurement of the Earth's radius!  Other scholars use the length of the Olympic Stadium as the unit of measure. This puts Eratosthenes' estimate off by 14% (over). Either way - its a very simple and compelling demonstration of the shape of the Earth.

Hipparchus ( 190 BC - 120 BC) and the Distance to the Moon

Anyone acquainted with ancient astronomy can have only admiration for the ingenuity and intelligence of early astronomers.  With the simplest of tools they none the less were able to produce good estimates of the Earth-Moon distance and at least order of magnitude estimates for the Earth-Sun separation.
Hipparchus devised several particularly neat ways to find the Earth-Moon distance using solar and lunar eclipses. 

Solar Eclipses:

Hipparchus noted that during the total eclipse of 129 BC, totality occurred at Syrene but Alexandria only saw a partial eclipse (1/5). Since he knew the distance between Syrene and Alexandria he now had a baseline from which to peform his calculation.

Lunar Eclipses:

His method rests on noting that the shadow cast by the Earth on the Moon gives us a sense of the relative sizes and distances of these objetcs.  Figure 4.8 shows the difference in curvature between the shadow on the Moon's surface and the curvature of the Moon itself.

Figure 4.8 Two images from the January 2000 lunar eclipse show that the shadow on the lunar surface has a different curvature than the moon itself. The Moon's curvarture is 2.5 x greater than the shadow's curvature.


The key to Hipparchus' method was his understanding that the arc cast by Earth's shadow told him something about the Earth - Moon  separation in relation to the size of the Earth.  By using some basic geometry Hipparchus determined that the Moon is about 60 Earth radii away from us.  If we use Eratosthene's measurement of the size of the Earth then it follows that we know the Earth-Moon distance.

Example 4.2:  If we use the "generous" interpretation of the unit stadia we take Eratosthene's measured Earth radius as 6600 km. How good is Hipparchus' measurement of the Earth-Moon distance?

Solution: The accepted value for the Earth-Moon distance is 384 000 km. If we use Hipparchus' value of 60 earth radii we conculde that Hipparchus knew that the Moon was about (6600 km) X 60 = 396 000 km! That is - his value was only off by about 3 percent!!


Ptolemy (83 AD – 168 AD)

During the middle ages the greatest astronomer from antiquity was Claudius Ptolemy. Ptolemy worked from the "circular" dogma and created an elaborate geocentric model in which the planets moved in complicated circles on circles. His model was very successful in predicting the positions of the planets and his major work The Almagest was THE work in astronomy until the 17th century.

Figure 4.10 presents an a simulation of the Ptolemaic model. With an ingenious construction of circles, epicyles and other "contrivance" Ptolemy was able to produce a model of the heavens that enabled him to predict astronomical phenomena with a precision comparable to what could be observed in his time. His primary concern was methematical - it is not clear if he really belived that the planets moved in the complicated patterns implicit in his cosmology.


Figure 4.9 Claudius Ptolemy from a medieval print. (image in public domain)

By about the 13th century accumulating errors in the Ptolemaic predictions of planetary positions prompted an "overhaul" of the Ptolemaic model. Under the patronage of Alfonso X (sometimes known as Alfonso the Wise) a series of small "tweaks" were applied to the Ptolemaic model and the Alfonsine Tables were produced as elaborate numerical corrections to Ptolemy's original predictions.

In this task, Alfonso is reputed to have commented:

"If the Lord Almighty had consulted me before embarking on creation thus, I should have recommended something simpler."

Figure 4.10 Ptolemaic Simulator - click on image to open applet in separate window. (Applet produced by and used with permission from Nebraska Astronomy Applet Project).



To understand the role that early astronomy has played in the emergence of our modern astronomy

Chp 2