ASTR 121 (O'Connell) Study Guide

5. ANCIENT ASTRONOMY

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

Evidence from ancient societies that left interpretable artifacts shows that many took astronomy very seriously. In this lecture we discuss some of the ways early societies made and recorded observations of the Sun, Moon, planets, and stars. The Maya are a fascinating 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 associated with the motion of the Moon. Precession, a wobbling motion of the Earth induced by the gravity of the Sun and Moon, complicates the interpretation of ancient observatories and records.
Recording of observations/interpretations is the key to scientific progress.

Although they were able to transmit some scientific information via oral histories and recitation, pre-literate societies rarely progressed far in understanding the world. They had a faulty record of their own histories, let alone nature. Even crude methods of recording data provide enormous advantages. Paradoxically, low-tech stone records survive better than paper records.
The earliest extant astronomical records (Chinese) are over 4500 years old. The best astronomical records prior to the European Renaissance were developed by the Babylonians, Maya, Greeks, and Chinese.

A. MOTIONS OF THE PLANETS ON THE SKY
A conspicuous feature of the planetarium simulations shown in Lecture 4 was the motion of the five bright planets. [Recall that these "motions" are measured with respect to the background of the stars.] 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 (1) the effects of observing from a moving platform and (2) 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 speed of the motions depends on the planet, decreasing from rapid to slow in the order: Mercury, Venus, Mars, Jupiter, Saturn.

The general (average) motion of the planets with respect to the stars is eastward in the sky.

Mercury and Venus never move very far from the Sun and appear to move back and forth in front of/behind it.

At least once per year, each of the planets halts its eastward motion and loops backward for a brief period before starting to move eastward again. This backward loop is called retrograde motion.

The planets are confined to a relatively narrow band on the sky that is roughly centered on the ecliptic (the annual track of the Sun). The planets therefore are always to be found in the 12 Zodiacal constellations.

The extreme north/south positions of the Sun, Moon, and planets differ from one another.

We now know that these differences are determined by the inclinations of their orbital planes to the celestial equator.

The Sun moves along the ecliptic, so its maximal N/S positions are 23.5 degrees from the equator, as described in Guide 04.

The Moon's orbit is inclined 5 degrees to the ecliptic, so its maximal N/S positions are 28.5 degrees from the equator. 5 degrees may sound small, but it is 10 times the angular diameter of the Moon.

The orbit of each planet has a different inclination with respect to the ecliptic. A planet's N/S extremes are affected both by its inclination and its distance from the Earth.

The image below is a time-lapse exposure of a planetarium simulation of several years of planetary motions as seen toward one particular Zodiacal constellation, showing the concentrated "active band" and the retrograde loops of 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:

Heliacal risings: Helios is the Greek word for the Sun. Stars are said to exhibit "heliacal risings" if they rise in the east just before the Sun. This is a (rough) method of tracking the Sun's changing position with respect to the stars (recall that the Sun moves about 1 degree east every day against the stars), and hence it is a date-keeping method. For example, in ancient Egypt a heliacal rising of the brightest star, Sirius, was used to forecast the Nile's annual flood. The method can only be used for stars bright enough to be visible in the twilight sky.

Horizon intercepts: The alignment of a rising/setting object with distinct features on the distant horizon as seen from a special location is called a "horizon intercept." This allows one to track the date using the N/S position of the setting/rising Sun against the horizon (more accurate than using heliacal risings). It also allows tracking of the motions of the Moon and planets. The horizon is a convenient reference plane for tracking celestial objects; it is harder to provide alignment devices that track objects when they are high in the sky.

Note: accurate Earth-sky angular measurements of this kind require establishment of a reference direction. For instance, two fixed points yielding a well-defined fixed line toward the horizon is a reference against which to measure anglular positions of intercepts. The two points could both be natural (e.g. a nearby rock and a tree on the distant horizon) or they could both be artificial, as in ...

Internal building alignments. Special designs, intended to assist in making astronomical measurements or to reflect a recurring important astronomical event (e.g. an equinox or solstice), have been found in many ancient buildings. Discovery and analysis of such features is an important aspect of a new research field: "Archaeo-Astronomy" (see ASTR 341).

Many ancient building alignments were intended to mark the rise or set (i.e. the horizon intercepts) of important astronomical objects as follows; the listed examples will be shown in class:

The Sun at the equinoxes (east-west alignment): e.g. the Egyptian pyramids

The Sun at a solstice (its extreme N/S positions; these are not east-west): e.g. Stonehenge

The Moon at its N/S extremes: e.g. Stonehenge

Bright stars

Planets at their N/S extremes: e.g. El Caracol (Maya)

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

C. 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 250-1000 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 violent, militaristic character. 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 their fabulous cities were abandoned and almost completely forgotten---becoming crumbled mounds swamped by jungle vegetation 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. For examples of Catherwood's work, click here.

The Maya kept detailed written records, including astronomical texts. But most written documents were destroyed by the Spanish after the Conquest (1520 AD), and only a few "codices" survive (a page from one is shown above). Fortunately, large amounts of carved material were undisturbed and are now being slowly translated.

The records show a fascination with time cycles: Maya astronomers made persistent, careful observations of the Sun, Moon, and planets. They built an elaborate and complex calendar system, with cycles figured up to periods of 3.1 million years and even beyond.

Chichen Itza Today

The Maya apparently lived in deep fear of eclipses and the planet Venus, leading to wholesale human sacrifices, including children, associated with astronomical events.

There was a preoccupation with Venus. 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 complex motions of Venus (and Mercury) with respect to the Earth's horizon and other astronomical objects and large changes in the Venusian brightness. (We will show simulations in class.)

Venus was assumed to be a malevolent god, demanding sacrifices at critical times. The Maya assiduously tracked Venus to forecast the god's intent toward themselves. But there is no evidence they understood the physical nature of Venus.

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

E. LUNAR PHASES
Let's now return to the motions of 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.

Here is a montage of photographs of the lunar phases.

Other than the apparent daily and annual motions of the Sun, the lunar phases are the most dramatic of the cycles visible in the sky. They were especially important before the invention of artificial lighting, because they determined people's ability to move around at night.

The phases were understood as early as 500 BC by the Greeks

The key clue is that the phase of the Moon correlates directly with its angular distance from the Sun. See this drawing, which shows the phases and location of the Moon at sunset during the first two weeks of a lunar cycle. The Sun is always at the western horizon at the times shown.

The Greeks, familiar with solid geometry, realized that this behavior implies that the Moon is a sphere, half of which is always illuminated by the Sun. It is not self luminous and shines only by reflected light.

The fraction of the sunlit hemisphere which we can see from Earth determines the lunar phase. We see a "full", "crescent", or dark ("new") Moon depending on the angle between the Sun and Moon as viewed from Earth. See the illustration above. (The Moon moves counterclockwise in its orbit in the diagram.) The Moon repeats its phases after a period of 29.5 days.

Our modern understanding of the Moon is as follows:

As in the case of the planets (see A above), the motions on the sky of the Moon are a composite of its intrinsic motions in its orbit and the motion of our observing platform, the Earth.

The Moon is the only known natural satellite of Earth, moving in a slightly elliptical orbit with an average radius of 238,000 miles (384,000 km). Its sidereal orbital period with respect to the "fixed" stars is 27.3 days. It moves eastward in its orbit as viewed from Earth, about 13 degrees per day (changing rise/set times by ~50 minutes/day). The lunar orbit is tilted slightly (5 degrees) out of the ecliptic plane.

Because Earth moves in its own orbit around the Sun, the Moon does not return to the same position with respect to the Sun as seen from Earth (e.g. 180 degrees away from Sun) for 29.5 days, 2 days longer than its sidereal period. This is called its synodic period, and the phases therefore repeat after one synodic period. The situation is shown here. The 29.5 day cycle of phases roughly defines our calendrical "month."

This time lapse movie (472K) composed of still photographs of the Moon during a 29.5 day cycle vividly illustrates the relationship between shadowing and phases. The changes in the apparent size of the Moon and the slight "rocking" motion (known as libration) are caused by the fact that the lunar orbit is significantly non-circular in shape.

E. POLAR PRECESSION

Precession is a cyclical, long-period wobble in the orientation of the Earth's polar axis. It is a complication to interpreting ancient monuments because it changes the positions of stars with respect to the celestial poles.

The gravity of the Moon & Sun act on the "bulge" at Earth's equator. This produces a cyclical change in the direction toward which the Earth's pole points. It is slow: 0.5 degree per century or a 26,000 yr cycle. Though subtle, precession was first detected in 150 BC by Greek astronomer Hipparchos through comparison of measured star positions over several centuries.

Though Polaris is near the N. pole now, it will not be in a few 1000 years. Click here for an animation of the pole's position on the sky at different dates. [Note: the point marked "zenith" in this animation is not the zenith but the north celestial pole.]

Precession causes changes in the position of stars with respect to the poles/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 47^o. This means that many stars in the southern hemisphere, now 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.

F. 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.

A lunar eclipse occurs when the shadow of the Earth strikes the Moon

A solar eclipse occurs when the shadow of the Moon strikes the Earth. In the right circumstances, the Moon just covers the Sun's face as seen from Earth, leading to a (short-lived) total eclipse.

The geometry of a lunar eclipse is illustrated in the following diagram:

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

A lunar eclipse can only occur near Full Moon, and

A solar eclipse can only occur near New Moon

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

Reading for this lecture:
Bennett textbook: Ch. 2.3 (lunar phases, eclipses), 3.1 (ancient astronomy)
Study Guide 5
The Aztec Calendar Stone
Optional reading: Eclipses and Stonehenge
Optional reference on Maya astronomy: Skywatchers of Ancient Mexico by Anthony F. Aveni (Univ. of Texas Press, 1980/97).

Reading for next lecture:
Bennett textbook: Ch. 3.2
Study Guide 6
Optional references: Bertrand Russell, A History of Western Philosophy; Arthur Koestler, The Sleepwalkers; Timothy Ferris, Coming of Age in the Milky Way; J. L. E. Dreyer, A History of Astronomy from Thales to Kepler.

Web links:
Introduction to Archaeoastronomy (UMd)
World Atlas of Archaeoastronomy
Astronomy of the Egyptian Great Pyramid
Eclipses and Stonehenge
Lunar Phase Now
Maya Civilization & Astronomy:

MesoWeb (good general site)
MesoAmerica Photos & Articles (Jacobs)
Photos of Maya Cities (McKenzie)
Teotihuacan: City of the Gods
Introduction to Maya Astronomy
Brief notes on the Maya calendar and on ancient structures in the Americas (Chevalier, UVA Astr 341)
Astronomical Contents of the Dresden Codex (Boehm)
The Maya Calendar (Meyer)
El Caracol, A Maya Observatory
The Aztec Calendar Stone
Maya Culture on the big screen: Mel Gibson's Apocalypto

Trailer
Wikipedia entry (summary, sources, criticism)

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Last modified May 2009 by rwo
Text copyright © 1998-2009 Robert W. O'Connell. All rights reserved. Precession and lunar phase diagrams by Nick Strobel. Precession animations by Scott R. Anderson. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.