ASTR 1210 (O'Connell) Study Guide


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 one of the most fascinating pre-scientific cosmologies: that of the Mesoamerican cultures that flourished in Mexico and Guatemala between about 500 BC and 1500 AD. 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.

The Moon was far more important in ancient societies than now, so we also discuss the cyclic phenomena (phases and eclipses) associated with the motion of the Moon. Precession, a wobbling motion of the Earth's pole induced by the gravity of the Sun and Moon, complicates our interpretation of ancient observatories and records.

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 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:

Stonehenge by moonlight

C. Astronomical Records

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

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

D. 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.

      Not only did Maya society collapse, but most of their fabulous cities were abandoned and almost completely forgotten---becoming crumbled mounds swamped by jungle vegetation and known only to local people. They were only rediscovered in the 1840's by American explorer John Stephens and popularized by the artwork of Frederick Catherwood (see his watercolor of El Castillo at the top of this page). For other examples of Catherwood's work, click here.

  • The Maya kept detailed written records, mainly of dynastic histories but also including astronomical texts. Regretably, 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. Interestingly, they 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.)


Chichen Itza Today

Dresden Codex

Astronomical Tables in
the Dresden Codex

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.

E. Lunar Phases

Let's now return to the motions of the bright objects in the night sky. Because the Moon is the most conspicuous of the denizens of the night sky, and for several nights each month completely dominates the sky, it always was of major interest to ancient astronomers. Its motion from day-to-day against the star background is also faster (about 13 degrees per day) than those of the Sun or planets.

As seen from Earth, the Moon has almost exactly the same angular diameter as the Sun (although, as we now know, its linear diameter in miles is, of course, much smaller).

The Moon exhibits drastic changes in apparent shape throughout the month, from crescent to round and back. The shapes are called phases of the Moon.

Ancient societies had many colorful supernatural explanations for the lunar phases (e.g. consumption and regurgitation of the Moon by a giant celestial sow). But only one of them arrived at the right answer: the Greeks, who understood the phases as early as 500 BC.

Lunar Phases
Our modern understanding of the Moon is as follows:

F. Polar Precession

Precession is a cyclical, long-period wobble in the orientation of the Earth's polar axis projected on the celestial sphere. It is a complication to interpreting ancient structures containing astronomical alignments because it changes the positions of stars with respect to the celestial poles and equator.

Precession causes changes in the position of stars with respect to the celestial poles and equator and therefore causes misalignments between ancient building sight lines and the current-day positions of stars. We must take it into account in interpreting ancient observatories.

The maximal change in the angle between the pole and a given star is 47o. This means that many stars in the southern hemisphere, which now are always below the horizon from Charlottesville, will become visible at some time in the future.

Precession shifts the location of intersections between the ecliptic and the equator (i.e. the equinoxes) in the stellar reference frame. E.g. the Vernal Equinox moves from one constellation of the Zodiac to the next in about 2000 years.

G. Eclipses

Eclipses are shadow effects in which the shadow of the Earth strikes the Moon or the shadow of the Moon strikes the Earth. There are two types: lunar eclipses and solar eclipses. Both can be beautiful events, for properly situated observers on Earth. They are also dramatic. The sudden extinguishment of the Sun can be terrifying for people who believe the Sun is a living god if they could not predict it in advance.

From that illustration and the diagram above showing the lunar phases, we see that:

For a more detailed description of eclipses, see the optional reading here.

Reading for this lecture: Reading for next lecture:
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Last modified April 2017 by rwo

Text copyright © 1998-2017 Robert W. O'Connell. All rights reserved. Precession and lunar phase diagrams by Nick Strobel. Precession animation by Scott R. Anderson. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.