Archeao-astronomy

Astronomy, without any doubt, can be considered to be one of the oldest sciences. As long as humans have existed, there has existed a fascination for the skies above. And hence, there is an unavoidable marriage between the history of civilization and astronomy, making the imprint of astronomy ubiquitous in ancient cultures. Even the oldest civilizations maintained written or unwritten knowledge about the movements of objects in the skies and the corresponding changes those movements were associated with on the earthly plane they lived upon.

Archaeoastronomy is a discipline rooted in the fascination of human cultures with patterns and movements in the night sky. It is a 'study of the incorporation of celestial orientation, alignments or symbolism in human monuments and architecture' as well as in written records. Perhaps better described as 'cultural astronomy,' it deals with how a civilization could have incorporated astronomical symbolism into their culture.

The most prominent parts of ancient cultures that survive to the present day, however, are the structures they built, making archaeology indispensable to this discipline. And hence, the term 'archaeo-astronomy.' The nomenclature, however, should not drive away the emphasis from the fact that archaeoastronomy is an interdisciplinary field that derives aspects from areas such as anthropology, sociology, and pedagogy as well.

Periodicities in the sky

Before we go on to indulge with the subject, it is important to appreciate how the objects in the sky derive their relative relevance from their behaviour in the sky.

The relevance of astronomy itself boils down to the fact that the sky is ever-changing and that the changes themselves are periodic. The periodicity could vary from a few hours to a few thousand years. These periodicities are, in turn, derived from the motions of objects in the skies. Based on the periodicities they produce, these motions can be classed mainly into two categories:

• those that produce periodicities that can be observed over the lifetime of a typical human being (Class A), versus 
• those with periodicities that are larger than the typical lifetime of civilization itself (Class B)

Motions of Class A are the ones that are observed and are relevant to the people of a specific civilization. In contrast, Class B's motions make it possible for us in the present day to derive information about ancient people by contrasting their observations with ours.

A few examples of Class A motion would be:

• Earth's rotation around its axis: causing day and night
• Earth's revolution around the Sun: causing seasons
• Moon's rotation around the Earth: causing lunar phases and eclipses
• Revolution of planets around the Sun: causing the movement of planets across the fixed background of stars

A few examples of Class B motion would be:

• The relative movement of stars within our Galaxy
• Precession of Earth's rotation axis (periodicity ~ 25,700 years)
• Changes in the ellipticity of Earth's orbit (periodicity ~ 100,000 years)
• Changes in the obliquity of Earth's rotation axis (periodicity ~ 41,000 years)

To help understand how these motions help us get information about the people that came before us, let us consider the precession of Earth's axis in the next section.

The Ecliptic and precession of the Equinoxes

The path that our parent star takes against the fixed backdrop of stars is called the Ecliptic. It is important to note that the Ecliptic is tilted with respect to the celestial equator at an angle whose current value is about 23.5 degrees. This angle of inclination is the reason why we experience seasons. 

The two points where the celestial equator and the Ecliptic intersect each other on the celestial sphere are called the equinoxes. When the Sun is in one of these locations, day and night on Earth are of the same duration. In fact, the word equinox is itself derived from the Latin words' aequi', meaning equal, and 'nox', meaning night.

At all other times, the Sun is present in either the Northern or Southern celestial hemispheres, marking summer in the respective hemispheres on the Earth. The points on the celestial sphere where the Sun stops moving away from the celestial equator and back towards it, or in other words, the points where the Sun is at the highest or the lowest point away from the celestial equator, are called the Solstices. The word is again derived from the Latin' sol' meaning Sun, and 'stit' meaning stationary.

Where the equinoxes mark the beginning or end of Summers or Winters, the solstices mark the peak of these seasons.

The relevance of equinoxes and solstices becomes clear once we understand that these presented clear markers for the onset and offset of different seasons to early humans as they transitioned from a hunter-gatherer lifestyle and settled to perform agriculture. Agriculture made it necessary to know what time of the year it was and, correspondingly, which crops should be sown at that time and which should be harvested.

But as much as this calendrical system would have helped our ancestors excel at agriculture, how does it help us gain information about them?

This is where class B motions become relevant. The Earth's rotation axis, which essentially defines the celestial equator, is not fixed but processes with a periodicity of about 26000 years. This means that the location of equinoxes and the solstices change against the fixed background of stars, varying with the same periodicity of 26000 years.

This means that, at the onset of Summer, the constellation that the Sun was in at around 3000 BCE is not the same constellation that the Sun rises in at the onset of Summer in the present times. 

Suppose somehow, through architectural remains or written records, we find evidence that the people of a particular ancient culture observed the onset of a given season with the Sun being in a particular constellation. In that case, we can calculate the approximate time that civilization would have existed. 

Nodal Precession

A phenomenon similar to the precession of equinoxes occurs when the points of intersection of the path of the moon and the Ecliptic (called lunar nodes) move along the Ecliptic. Note that eclipses can only occur when the Sun and the Moon are present simultaneously at one of the nodes. This means that ancient records of eclipses can help us in dating these cultures.

Folklore

It is interesting to note that folklore regarding constellations in the sky forms an essential part of a culture. Numerous cultural references and practices are projected onto the sky when people begin to find star patterns. 

This can help us determine the approximate latitudes at that the culture would have originated in because the appearance of the same collection of stars can look very different depending upon their elevation in the sky.

For example, where people in Europe would have seen the big dipper, the Pardhi tribe of Central India saw a bed being stolen by three thieves. All this is owing to the relatively flat appearance of the asterism from lower latitudes.

It is curious to note how the ancient Indians connected the fact that the lunar nodes are not fixed with mythology. There is good reason to believe that the mythological creatures 'Rahu' and 'Ketu' represent the two lunar nodes. In the system of ancient Indian Astronomy, they are considered as two dark planets, and their movements are calculated just like the rest of the planets, and when they coincide with the Sun and the Moon, you have the predictions of eclipses.

There are many more ways that astronomy could have interacted with the cultures of ancient people. The field of Archeao-astronomy is a rather young science, and we have only begun to understand how our ancestors could have incorporated their astronomical knowledge into their worlds. Hopefully, this article presented a decent overview to understand some of the fundamental aspects of the field.

    - Sanatan Jaminy (MS19016)