ASTR 1210 (O'Connell) Study Guide
5. ANCIENT ASTRONOMY
Maya pyramid El Castillo at Chichen
(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
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.
[Recall that these
"motions" are measured by observers on Earth with respect to
the background patterns of the stars on the sky.]
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
- 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 against the star background is
eastward in the sky.
- Mercury and Venus never move very far from the Sun (48
degrees in the case of Venus) 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
- 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 (i.e. distance from the
celestial equator) of the Sun, Moon, and planets differ from one
- We now know that these differences are determined by the
inclinations of the orbital planes of the planets and Moon to
the ecliptic plane.
- 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, so it is easy to distinguish on the sky.
- The orbit of each planet has a different inclination with respect
to the ecliptic. A planet's observed N/S extremes are affected both by its
orbital inclination and its distance from the Earth.
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. An illustration is
shown here. 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. Hence this 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
- 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." An illustration
is shown here. 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 built into many ancient buildings. Discovery and
analysis of such features is an important aspect of a new research
"Archaeo-Astronomy" (see ASTR
Many ancient building alignments were intended to mark the rise or
set (i.e. the horizon intercepts) of important astronomical
objects. Some examples:
- The Sun at the equinoxes (east-west alignment): e.g. the
bases of the Egyptian pyramids and the Maya El Castillo pyramid
are aligned almost exactly east-west.
- The Sun at a solstice (its extreme N/S positions). The
rise/set points of the solstices do not lie east-west, because
the Sun is 23.5 degrees from the celestial equator at these times.
E.g. at Stonehenge, the line of sight from the center of the monument
towards "The Avenue" and "Heelstone" points toward the rising Sun on
the Summer Solstice.
- The Moon at its N/S extremes (28.5 degrees from the
celestial equator): e.g. at Stonehenge, the line of sight over two
pairs of special stones point toward Moon rise or set at the extremes.
For details on the complex motions of the Moon,
see Lunar Motions and Their
- Bright stars. The rise/set points of stars are always the
same during a given year, but they do change very slowly over time
because of "precession".
Corrections for precession based on the date of a given ancient
structure must be made before possible sight lines to stars can
- Planets at their N/S extremes: e.g. the Maya El
Caracol observatory building contains alignments of windows and
wall structures with special setting points of Venus on the western
C. Astronomical Records
Recording of observations/interpretations is the key to
Although pre-literate societies were able to transmit some
scientific information via oral histories and recitation, they
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, Greeks, Chinese, and
Maya. At right is a Babylonian planetary almanac written in "cuneiform"
script (ca. 400 BC). The script was incised on a wet clay tablet which
was then fired to make a permanent record. The surviving Maya records
(both carved in stone and written in ink; see below) reveal
sophisticated observational capabilities.
Stonehenge by moonlight
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
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
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
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.
To put Stonehenge in its historical context,
here is a timeline
showing other contemporaneous cultural developments.
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
- Solsticial Alignments: A line from the
monument center to the "Heelstone" points toward the location
of sunrise at the summer solstice (the northernmost sunrise of
the year and the longest day of the year). The reverse points to
sunset at the winter solstice. The Heelstone is a large, isolated
stone lying outside the circular structures on the centerline of the
Avenue. [Click on
the thumbnail at right for a chart of the alignments.]
Note that such "solsticial" orientations are not simply
east-west (which is much more common in ancient buildings).
The heelstone is north-east of the center of
A sketch of the Sun's path as it rises over the heelstone
on the summer solstice as seen from the center is shown below. The
actual "horizon intercept" occurs slightly north of the heelstone
(presumably because one cannot mark that point with a large standing
stone and still see the Sun there).
- Lunar Alignments: The so-called
"Station Stones" are four stones lying just inside the circular bank
(labeled "SS" in the plan
drawing). Lines drawn through Station Stones 92 and 93 or 91 and
align with the N/S
maxima of the Moon's rise or set during the 18.6-year revolution
cycle in the "nodes" of its orbit.
The nodal cycle determines where on the sky the N/S maxima will
occur and also controls the pattern of lunar and solar eclipses.
(See Lunar Motions and Their
Diodorus, a Greek historian during the 1st century BC, refers to
a "19 year" cycle traditionally associated with Stonehenge and
the Moon --- almost certainly the lunar nodal cycle.
Astronomers Gerald Hawkins, in his best-seller "Stonehenge
Decoded," and Fred Hoyle suggested in the 1960's that the circle
of 56 "Aubrey Holes" (dug at the inner periphery of the circular
mound) could have been used as an analog computer to track the motion
of the Moon, Sun, and the nodes of the lunar orbit in order to predict
eclipses. 56 years, or 3 saros cycles, is required to bring solar
eclipses back to approximately the same locations on Earth's surface.
Though technically correct, this idea has found little support among
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
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,
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
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
- 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.
description of the design of their elaborate "El Caracol"
observatory in Chichen Itza. Interestingly, the Maya never invented the
- 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
Astronomical Tables in
the Dresden Codex
- Venus was believed to be a malevolent god, whose demands for
blood sacrifice at critical times led to ritual murder
by the Maya of both captives and their own citizens, including
children. The Maya assiduously tracked Venus to forecast the god's
intent toward themselves.
- But there is no evidence the Maya understood the origin of the
celestial motions, which they attributed entirely to supernatural
- From the standpoint of astronomy, there is an unpleasantly
sinister aspect to this. The astronomers, or "daykeepers," were so
good at making observations that it's inconceivable they hadn't
realized that the celestial cycles of the Moon and Venus
were strictly repeatable. And the more cycles they recorded,
the more confident they could be about it. In other words, they knew
that human intervention made no difference to the motions in the sky.
The astronomers must have colluded with the political and religious
leaders in pretending that the sacrificial rituals were effective.
There is a hint of this in the movie Apocalypto. At the start
of the ostensibly terrifying solar eclipse, two priests exchange a
knowing glance. They knew it was coming all along.
Uxmal, Maya city ca. 850 AD, with the Pyramid of the
Magician at the left
The Long Count and the End of the World
The Maya believed in a recurring cosmic cycle of birth and
destruction during which the gods struggled to nurture a fruitful human
species. Three cycles, each ending in catastrophe for the world, were
thought to have preceded the then-current, fourth, cycle. The cycles
as interpreted by the (later) Aztecs are described
Time within a cycle was tracked by the Long Count, in which
each day was assigned a unique number. Counts were expressed in a
modified base-20 system, the longest unit of which was
the baktun. A baktun is 20x18x20x20 = 144,000 days
or 394 solar years long. Some Maya documents suggest that a
cosmic creation cycle would end in worldwide disaster after
exactly 13 baktuns, or 5125 years.
By cross-correlating Long Count dates with unique astronomical events
and historical dates after the Spanish conquest, archaeologists have
been able to convert Long Count dates to those in the Julian (Western)
calendar. The starting date of the fourth cycle (and the end
of the third) was determined to be 11 August 3114 BC. But that
implies that the end of the fourth cycle occurred on 21 December
You can find much speculation on the Internet before December 2012
about the meaning of the cycle turnover, including irresponsible
predictions of a looming
Doomsday. A handy "countdown to the apocalypse" calendar
is shown at the right. The predictions were nonsense, and there was,
obviously, no catastrophe at the predicted time. The doomsday
hucksters have since retreated into silence to count their money.
Remember that for all their skill in tracking the planets, the Maya
world view was riddled with superstition, and they showed no insight
regarding the true physical nature of the universe or even the size
and shape of the Earth. They had counting systems but had not
developed mathematical geometry, which would have helped them
understand the nature of sky cycles. Their writings were vague and
contradictory concerning the cosmic cycles, and some inscriptions
anticipate eras as much as 2020 years from now in an
inconceivably distant future. Finally, as is obvious from the
historical record, there was no worldwide cataclysm in 3114 BC, at the
end of the previous 13-baktun cycle. Same with 2012, and we have now
started the first baktun of a new cycle.
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.
Reading for this lecture:
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.
You have two objects, A and B, both of which are the same shape.
B weighs twice as much as A. You drop both simultaneously
from a height of 3 feet. What happens?
You should attempt your own experiments to determine the
answer...don't just take the word of the readings!
- A (the lighter object) hits the ground first.
- B (the heavier object) hits the ground first.
- They hit at the same time.
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