ASTR 1210 (O'Connell) Study Guide


5. ANCIENT ASTRONOMY


Maya pyramid El Castillo at Chichen Itza
(Catherwood, ca. 1844)


Evidence from ancient societies that left interpretable artifacts shows that many took astronomy very seriously, to the extent of including precise astronomical alignments in their buildings and ceremonial structures. In this lecture we discuss some of the ways early societies made and recorded observations of the Sun, Moon, planets, and stars.

Then, we explore two of the most dramatic examples of ancient astronomy. Stonehenge, the striking arrangement of massive stone monoliths in southern England from before 1500 BC, encodes astronomical knowledge, but we have no idea how its builders acquired that or how they perceived the universe around them.

By contrast, the Mesoamerican cultures that flourished in Mexico and Guatemala between about 500 BC and 1500 AD not only documented extensive and painstaking observations of the sky, but they also left records of a fascinating, pre-scientific cosmology built on those. Their vibrant, if violent, view of the cosmos is beautifully captured in the so-called "Aztec Calendar Stone". The Mesoamerican Maya culture is an amazing example of great accomplishments in astronomy conjoined with ferocious societal behavior.


A. Motions of the Planets on the Sky

A conspicuous feature of the naked-eye sky in the planetarium simulations shown in Lecture 4 was the motion of the five bright planets. Although not as fast as the diurnal, solar, and lunar motions, the planetary motions are considerably more complex and placed greater demands on the abilities of ancient astronomers.

As discussed in Lecture 4, these motions are a combination of (a) the effects of observing from a moving platform and (b) intrinsic movement of the planets themselves in their orbits around the Sun. We will not try to separate these now but instead will simply review a few key facts about the motions revealed by our Starry Night simulator:

The image below is a time-lapse exposure of a planetarium simulation of several years of planetary motions over about 40o of the sky, showing the concentrated "active band" and the retrograde loops of several planets. North is up and east is to the left in the image. The planets cross the image moving to the left, except during their retrograde loops. The ecliptic runs along the center of the bright band. Large N/S departures from the ecliptic are apparent for several planets.


B. Astronomical Measurements Without Instruments

The most elaborate astronomical instruments prior to the advent of telescopes were made out of metal and wood. However, even societies that lacked metalworking skills could make reasonably careful astronomical observations using other kinds of technologies, some of which we explain next:

C. Astronomical Records

Recording of observations/interpretations is the key to scientific progress.



Stonehenge by moonlight

D. Stonehenge

Stonehenge, on the Salisbury plain in south-central England, is the best known of thousands of "megalithic" ("giant stone") monuments surviving from prehistoric times (roughly 3500-500 BC) in northern Europe. ( Click on the thumbnail at right for information on megalithic sites in Great Britain and Ireland.) These consist mainly of standing stones, dirt mounds and ditches, and evidence of former wooden structures, now long decayed. Four examples are shown here.

Very little is known about the peoples who built these. Unlike the Maya or the Middle Eastern cultures they did not incise their hard stone surfaces with symbols or writing, and they left no other records of any kind. Consequently, scholarly debate has raged over the purpose of such structures. There is, however, good evidence that their builders incorporated astronomical knowledge of the Sun, Moon, and bright stars in some of them. That includes Stonehenge, which is probably the best-studied ancient structure in terms of its astronomical alignments and significance.

Construction at Stonehenge took place ca. 3100-1500 BC (over 1500 years!) in several major phases. This was a massive effort, involving, for instance, transport of specially-selected 5 ton stones up to 240 miles. The image above shows Stonehenge as it might have appeared in the period 2000-1550 BC.

Here are some more views of the modern Stonehenge.

The current-day structure consists of a series of concentric circular ditches, banks, and post-holes with a number of large standing stones clustered in the center and a few at the periphery. Originally, the large standing stones were capped with lintels, but only a few of those remain in place today. A long straight "Avenue," marked by two parallel banks, runs north-east from the main structure and ultimately connects with an ancient settlement complex several km away.

Astronomical alignments: there are both solar and lunar alignments built into Stonehenge.

Stonehenge is situated at a unique latitude: where the lunar and solar sight lines just described cross at right angles. It is possible that the Stonehenge people chose this site for the monument because of this fact and that this is the reason they invested so much effort (estimated at 1.5 million person-days) in building it.

Before solar and lunar orientations could be built into Stonehenge, its planners must have observed the sky for many cycles---in the case of the Moon, many times 19 years. And they needed a method to pass the information on from one generation to the next (the lifespan then was only ~30 yrs). No stone, paper, or other forms of records have been found.

The most obvious stone structures (the 5 pairs of massive trilithons arranged in a horseshoe shape, see above right) were constructed last but have no clear astronomical significance.

Stonehenge is the most elaborate structure in northern Europe remaining from the period before 1500 BC. It clearly reflects the two most important sky cycles (solar and lunar). But its central function is still obscure. It may have served as an astronomical calendar tracker, a memorial, a site for religious rituals --- or all of these.


Madrid Codex
Part of the Maya Madrid Codex with an astronomer-like figure
"eyeing" the cosmos. Click for more images of the Codex.

E. Maya Astronomy

The Maya were the most advanced ancient astronomers in the Western hemisphere. They represented the pinnacle of a 2000-year "Mesoamerican" cultural tradition, preceded by the Olmecs and succeeded by the Toltecs and Aztecs.

The Maya flourished 250-900 AD in the area now belonging to Mexico, Guatemala, and Honduras. They built many elaborate cities, including large pyramidal and other public & ceremonial buildings. Maya societies had a harsh, militaristic character, and city-states frequently waged war on one another. The civilization suddenly disintegrated ca. 900 AD (disease? drought? political instability? invasion?), some 600 years before the Spanish Conquest.

Maya Observations, Sky Cycles and Calendars

  • The Maya kept detailed written records, mainly of dynastic histories but also including astronomical texts. Regrettably, most written documents were destroyed by the Spanish after the Conquest (1520 AD), and only a few "codices" survive (example pages are shown above and to the right). Fortunately, large amounts of carved material were undisturbed and are now being slowly translated.

  • The records show a fascination--even an obsession--with astronomical time cycles. Maya astronomers made persistent, careful observations of the Sun, Moon, Venus, and other planets. They built an elaborate and complex calendar system, in which civic and religious ceremonies were tied to celestial cycles. The two major ritualistic cycles had lengths of 260 days and 52 years. In contrast to most calendars, the concept of a lunar month did not play a major role in this system.

  • Astronomer "daykeepers" were needed to maintain the alignment of the sacred calendars with the real sky and to divine the meaning of changes in the sky. They consequently had high status in Maya society.

  • Despite their remarkable architectual accomplishments, the Maya had only limited metalworking skills (primarily jewelry) and therefore lacked metal observational instruments. They presumably made most of their astronomical observations using wooden sighting devices and building or horizon alignments. See this description of the design of their elaborate "El Caracol" observatory in Chichen Itza. Interestingly, the Maya never invented the wheel.

  • The Maya apparently lived in deep fear of eclipses and the planet Venus. A preoccupation with Venus would be natural for an observationally-skilled culture because it is, by far, the brightest starlike object in the sky and exhibits very complex motions by virtue of its proximity to Earth. Viewed from Earth, Venus has a 584 day (19 month) cycle of "configurations" with respect to Sun; the Sun and Venus have a 2922 day (8 year) cycle with respect to the bright stars. The cycle features complicated motions of Venus with respect to the Earth's horizon and other astronomical objects and large changes in the Venusian brightness. (We will show simulations in class.)

 
Chicen-Itza

Chichen Itza Today

 
 
Dresden Codex

Astronomical Tables in
the Dresden Codex

Uxmal
Uxmal, Maya city ca. 850 AD, with the Pyramid of the Magician at the left

The Long Count and the End of the World

Below are examples of a Maya observatory ("El Caracol" at Chichen Itza, left) and the remarkable Aztec "Sunstone" calendar, carved in 1479 (right). Click on thumbnails for more images and an explanation of the Sunstone.


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Last modified August 2018 by rwo

Text copyright © 1998-2018 Robert W. O'Connell. All rights reserved. Megalithic and Maya images from various public sources. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.