4. ANCIENT ASTRONOMY

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 are over 4500 years old. The best astronomical records prior to the European Renaissance were developed by the Babylonians, Mayans, Greeks, and Chinese.

A. MOTIONS OF THE PLANETS ON THE SKY

• The general 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 Zodiacal constellations.

• The motion of the planets (especially Mercury and Venus) with respect to the local horizon can be very complicated.

B. ASTRONOMICAL MEASUREMENTS WITHOUT INSTRUMENTS

• Heliacal risings: (helios = related to Sun) stars rising just before Sun. A date-keeping device (e.g. heliacal rising of Sirius forecasts Nile's annual flood).

• Horizon intercepts: alignment of rising/setting object with distinct features on distant horizon as seen from a special location. E.g.: track date using N/S position of setting/rising Sun on horizon. Also track motions of Moon, planets.
  Note: accurate Earth-sky angular measurements of this kind require establishment of a reference direction. For instance, two fixed points making a fixed line toward the horizon is a reference against which to measure angles to stars. 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. Discovered in many ancient buildings (e.g. Egyptian, Mayan): special designs intended for astronomical measurements. Stimulated new research field: "Archaeo-Astronomy" (see ASTR 341).
  Examples of ancient building alignments:
  • To Sun at equinox, solstice

  • To bright stars

  • To planets at extreme N/S rising or setting positions (extremes are determined by inclination of planetary orbit to celestial equator)

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

C. MAYAN ASTRONOMY

• Flourished 250-1000 AD in the area now belonging to Mexico, Guatemala, and Honduras. Elaborate cities, including large pyramidal and other public & ceremonial buildings. Violent, warlike characteristics. Society suddenly disintegrated ca. 1000 AD (disease? drought? political instability?).

• Kept detailed written records, including astronomical texts. But most written documents were destroyed by the Spanish after the conquest (1530); a few "codexes" survive (see above). Much carved material now being translated.

• Fascination with time cycles: persistent, careful observations of Sun, Moon, planets. Elaborate and complex calendar system, with cycles figured up to periods of 3.1 million years.

• Apparently lived in deep fear of eclipses and the planet Venus, leading to wholesale human sacrifices associated with astronomical events.

• 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 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. Mayans assiduously tracked Venus to forecast the god's intent toward themselves. There is no evidence the Mayans understood the physical nature of Venus.
  Below are examples of a Mayan 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.

D. 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

• 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 Hipparchus 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 N/S 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. 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.

E. LUNAR PHASES

• The Moon is the only known natural satellite of Earth. Its sidereal orbital period with respect to the "fixed" stars is 27.3 days. Moves Eastward in its orbit as viewed from Earth, about 13 degrees per day (changing rise/set times by ~50 minutes/day).

• Because Earth moves in its orbit around the Sun, the Moon does not return to the same position with respect to the Sun (e.g. 180 degrees away from Sun) for 29.5 days. This is called its synodic period and defines (roughly) our calendrical "month."

• 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.

• 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 with its angular distance from the Sun.
  The Greeks realized this implies that the Moon is a solid 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 its synodic period of 29.5 days.

• 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.

Seeds textbook: 4.1 (archeoastronomy); 2.2 (precession); 3.1 (lunar phases)
Study Guide 4
Optional: further information on Mayan astronomy: Skywatchers of Ancient Mexico by Anthony F. Aveni (Univ. of Texas Press, 1980/97).

Seeds textbook: 3.3, 3.4, 3.5 (eclipses); 4.1 (archeoastronomy)
Study Guide 5

Lunar Phase Now
Introduction to Archaeoastronomy (UMd)
World Atlas of Archaeoastronomy
Mayan Civilization & Astronomy:
MesoWeb (good general site)
MesoAmerica Photos & Articles (Jacobs)
Photos of Mayan Cities (McKenzie)
Teotihuacan: City of the Gods
Introduction to Mayan Astronomy
Brief notes on the Mayan calendar and on ancient structures in the Americas (Chevalier, UVA Astr 341)
Astronomical Contents of the Dresden Codex (Boehm)
The Maya Calendar (Meyer)
Mayan Culture on the big screen: Mel Gibson's Apocalypto
Trailer
Wikipedia entry (summary, sources, criticism)