Tag Archives: Stonehenge

Ancient astronomy and astrology

by Tim Harding

(An edited version of this essay was published in The Skeptic magazine, December 2013, Vol 33 No 4. The essay is based on a talk presented to the Mordi Skeptics in October 2013).

Today, there are distinct boundaries between the modern science of astronomy and the pseudoscience known as astrology.  But in ancient times, these boundaries were not so clear.  Both fields of study used a common set of astronomical observations – but for different purposes.  The practical purposes of ancient astronomy were celestial navigation and the development of calendars of seasonal dates and events (such as the flooding of rivers) for the planting of crops.  In contrast, the purpose of astrology was to interpret celestial phenomena as signs of divine communications.

Long before the invention of the telescope, ancient observations and predictions could only be of celestial objects visible to the naked eye.  This restricted astronomical and astrological studies to the stars, the Sun, the Moon and five planets – Mercury, Venus, Mars, Jupiter and Saturn.  (The Earth was not counted as a planet until much later).

Prehistoric stone observatories

In some locations, early cultures assembled stone structures that are thought to have astronomical observations as one of their purposes.  The most well-known of these structures is Stonehenge in Wiltshire, England, which commenced construction around 3100BCE and went through several building phases.  As there are no written records to go by, there are several theories as to various religious, mystical and other purposes of Stonehenge.  One of these theories has been proposed by well-known Victorian skeptic Dr. Lynne Kelly. Lynne’s PhD thesis was about the use of structures like Stonehenge as mnemonic aids, to ensure that the oral knowledge of the culture is retained and passed on to succeeding generations. The layout of Stonehenge also includes a celestial observatory function, which would have allowed the prediction of eclipse, solstice, equinox and other celestial events important to a contemporary religion.


Drawing of Stonehenge in midsummer (Source: Wikimedia Commons)

One of the world’s earliest known archeo-astronomical devices is a stone circle at Nabta Playa, in southern Egypt on the Tropic of Capricorn.  The site is between 6,000 and 6,500 years old, or about 1,000 years older than Stonehenge.  Two pairs of upright stones stand directly across the circle from each other, defining a view that would have marked sunrise at the summer solstice, this providing the beginnings of a prehistoric calendar.

Early Egyptian astronomy and astrology

The Ancient Egyptian calendar year was 365 days long, divided into 12 months of 30 days each, plus five extra days at the end of the year.  This was one quarter of a day shorter than solar year, leading to the problem of a ‘wandering year’ requiring frequent astronomical correction.  Observation of stars was important in predicting the annual flooding of the Nile, for the allocation of resources to the planting of irrigated crops. Early Egyptian astronomy was intertwined with astrology.  The Sun was believed to be a major god named Ra, representing light, warmth, and growth.  Ra was thought to travel on two solar boats – one on his journey through the sky during the day and the other in a river flowing underneath the flat Earth from west to east at night.


Ra, the Egyptian sun god (Source: Wikimedia Commons)

Most Egyptologists believe that the Great Pyramid of Giza was built as a tomb for fourth dynasty Egyptian Pharaoh Khufu (Cheops in Greek) over a 10 to 20-year period concluding around 2560 BCE, although other dates have been suggested.  One theory is that this pyramid was carefully aligned towards the northern pole star, which at the time was Thuban, but is now Polaris due precession of the Earth’s rotational axis.

Ancient Mesopotamia

The ancient region known as Mesopotamia comprised the plains of the Tigris and Euphrates rivers, in what is now Iraq, plus parts of Syria, Turkey and Iran.  The lower part between the rivers was known as Sumer, with Babylon, Uruk and Ur as its major cities.  The significance of this region is that it was the cradle of astronomy and astrology as organised fields of study.


The ancient region known as Mesopotamia (in light shading) Source: Wikimedia Commons

Sumer was also the birthplace of writing, in the form of cuneiform clay tablets dating from the mid 4th millennium BCE.  These tablets provide us with the first written evidence of astronomy and astrology in the West, albeit in a fragmentary state. From these tablets we know that the Babylonians developed a sexagesimal (base 60) numerical system, resulting in our current 60 minute hour, 24 hour day and 360 degree circle.  The Babylonians were the first to recognize that astronomical phenomena are periodic and to apply mathematics to their predictions.  They developed the idea of a 7 day week and a 12-month calendar based on cycles of the Moon; together with the seasons of summer and winter.  The Babylonians also measured the variation in day length over a year. At around 1800BCE, the first star catalogues were compiled.

The Babylonian astronomers noticed that a few ‘stars’ (later called planets) wandered in relation to other fixed stars and even retrograded in their motions.  These movements were confined to a narrow belt at an angle of about 23 degrees to the equator.  This belt – the Zodiac – was divided into 12 sections, and each section was named after a constellation of fixed stars in the neighbourhood.  The Zodiac also became one of the important features of western astrology. In this early period astronomy consisted of observations, calculations and predictions of events such as solstices and eclipses.  As such, astronomy at this stage was like a branch of applied mathematics plus a database of observations.  There were no cosmological theories to tie all the observations and calculations together and to try and rationally explain them.  This explanation vacuum was instead filled by astrology, which claimed to interpret celestial events as religious or mystical omens.

The Enuma Anu Enlil (c.1600BCE) is a major series of 68 or 70 tablets dealing with Babylonian astrology.  Substantial collection of omens, estimated to number between 6500 and 7000, interpret a wide variety of celestial and atmospheric phenomena in terms relevant to the king and state (known as ‘mundane astrology’).  For example, a typical astrological report to the king reads:

‘If the moon becomes visible on the first day: reliable speech; the land will be happy. If the day reaches its normal length: a reign of long days. If the moon at its appearance wears a crown: the king will reach the highest rank.’

Movements of the Sun, Moon and five planets were regarded as representing the activity of the gods in question.  Evil celestial omens attached to any particular planet were therefore seen as indications of dissatisfaction or disturbance of the god that planet represented. The Venus tablet of Ammisaduqa (Enuma Anu Enlil Tablet 63) refers to the record of astronomical observations of Venus, as preserved in numerous cuneiform tablets dating from the first millennium BCE.

Venus Tablet of Ammisaduqa

Venus tablet

Source: Wikimedia commons

During the 8th and 7th centuries BCE, Babylonian astronomers developed a new theoretical approach to astronomy.  They began to develop an internal logic within their observational data systems to improve their predictive power.  This was an important contribution towards the development of astronomy from a database to a science.  Some scholars have thus referred to this new approach as the first scientific revolution. The new scientific approach to astronomy was adopted and further developed in Greek astronomy.  This process was considerably helped by the conquest of Babylon by Alexander the Great in 331 BCE.  According to the late classical philosopher Simplicius of Cilicia (c.490CE – c.560CE), Alexander ordered the translation of the Babylonian historical astronomical records under supervision of his chronicler Callisthenes of Olynthus, who sent them to his uncle Aristotle in Athens.  Aristotle was also the teacher of Alexander until the age of 16 – what a small world!

Ancient Greece

The name ‘planet’ comes from the Greek term planētēs, meaning ‘wanderer’.  The names of individual planets (within our solar system) are all drawn from Greek mythology, although they have Romanised names outside of Greece. References to identifiable stars and constellations appear in the writings of Homer and Hesiod, in the 7th or 8th centuries BCE.  However, the first Greek attempts to rationally explain the structure and behaviour of the cosmos date from the period 600-450BCE.  The anomalies in the motions of the planets bothered the early Greeks, who were culturally inclined to try to find rational physical explanations for them.

Pythagoras of Samos (c. 570 BCE – c. 495 BCE) was an Ionian Greek philosopher and mathematician who founded a philosophical movement known as the Pythagoreans.  Amongst other things, Pythagoras was the first to think that the Earth was spherical rather than flat; and that the Morning Star and the Evening Star are identical (they are both the planet Venus). Astronomy was listed by the Pythagoreans among the four mathematical arts (along with arithmetic, geometry, and music). One of these Pythagoreans was Anaxagoras (c. 510 – 428 BCE), who discovered that the Moon shines by reflected light from the Sun and gave the correct theory of lunar eclipses (i.e. the Earth is blocking the light from the Sun to the Moon). These eclipses provided the conclusive arguments in favour of the Earth being spherical.  The Pythagoreans also regarded the Earth as one of the planets.

Herakleides of Pontus was a Pythagorean who lived in the 4th century BCE and studied under Plato.  Herakleides discovered that Venus and Mercury revolve around the Sun. He also held that the Earth rotated on its own axis every 24 hours, which accounted for the apparent procession of the stars across the night sky, but did not explain the retrograde motion of the planets.  By now, these anomalous planetary motions had become the central problem of astronomy and cosmology. Plato encouraged Eudoxus of Cnidus (c. 410 BCE – c. 347 BCE) to develop a two-sphere model with the Earth at the centre, and the planets occupying a separate sphere to the stars, similar to that shown by the following diagram. two sphere model

Source: Wikimedia Commons

Aristarchus of Samos (310 BCE – ca. 230 BCE) has been called ‘the Greek Copernicus’ because he proposed a heliocentric model of the cosmos, with the Sun at the centre instead of the Earth, about 1800 years before Copernicus did.  Aristarchus also calculated the sizes of the Sun and Moon, as well as their distances from the Earth in Earth radii.   Aristarchus’s working drawings of the relative sizes of the Sun, Earth and the Moon are shown below. Aristarchus drawing

Source: Wikipedia Commons

The radius and circumference of the Earth were first calculated (but slightly underestimated) by Eratosthenes of Cyrene – c.276-c.194 BCE.  He was a mathematician, poet, music theorist and inventor of the  discipline of geography, including the terminology used today.  Unfortunately, Aristarchus was unable to persuade his contemporary colleagues of the merits of his theory, which was largely forgotten until rediscovered by Copernicus in the 16th century CE.  Seleucus of Seleucia (b.190BCE) was the only Greek Babylonian philosopher to support heliocentric model of planetary motion. He also correctly theorized that tides were caused by the Moon, a theory that was overlooked by Galileo 1700 years later.

Hipparchos of Nicaea (c. 190 BCE – c. 120 BCE) was a Greek astronomer, geographer, and mathematician of the Hellenistic period.  He is considered the founder of trigonometry but is most famous for his incidental discovery of precession of the equinoxes.  He compiled a star catalogue recording the position and brightness of the stars, which was used by astronomers for centuries afterwards. As a result of the non-acceptance of Aristarchus’s heliocentric model, subsequent Greek astronomers persisted with trying to reconcile the anomalous movements of the planets with a geocentric model of the cosmos.

Apollonius of Perga (c. 262 BCE–c. 190 BCE) introduced two new mechanisms: the eccentric deferent and the epicycle, which are illustrated in the diagram below. Claudius Ptolemy of Alexandria (c. 90CE – c. 168CE) was a Greco-Roman mathematician, also known as an astronomer, geographer and astrologer.  Ptolemy explained how to predict the behavior of the planets by introducing the equant. Below is a simple illustration showing the basic elements of Ptolemaic cosmology.  It shows a planet rotating on an epicycle which is itself rotating around a deferent inside a crystalline sphere. The center of the system is marked with an X, and the earth is slightly off of the center.  Opposite the earth is the equant point, which is what the planetary deferent would actually rotate around.

Ptolemy model

Illustration showing the basic elements of Ptolemaic cosmology (Source: Wikimedia Commons)

Ultimately, these attempts at retrofitting cosmological theory to seemingly endless observational anomalies became too much.  Dislike of the equant, on top of the deferent and the epicycle, was a major motivation for Copernicus to construct his heliocentric system after the scientific renaissance some 1500 years later. Although astrology was not as popular in ancient Greece as it was in Egypt and Mesopotamia, belief in astrology continued through the Roman period and the Middle Ages.  Through most of its history, astrology was considered a scholarly tradition. It was accepted in political and academic contexts, and was connected with other studies, such as astronomy, alchemy, meteorology, and medicine.  At the end of the 17th century, new scientific concepts in astronomy and physics (such as heliocentrism and Newtonian mechanics) called astrology into question.[1] Astrology thus lost its academic and theoretical standing, and common belief in astrology has since largely declined.


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Endnotes [1] Rational arguments that the claims of astrology are false include firstly, because they are incompatible with science; secondly, because there is no credible causal mechanism by which they could possibly be true; thirdly, because there is no empirical evidence that they are true despite objective testing; and fourthly, because the star signs used by astrologers are all out of kilter with the times of the year and have been so for the last two or three thousand years.

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