Beginnings of Astronomy

Key points: Why ancient humans pursued astronomy; apparent motions of stars and planets on the sky; examples of major astronomical monuments

Knowing the time of year relative to the seasons is critical to determine when to plant crops. It was obvious that the month was about 30 days and the year was about 365 days or 12 months, but none of these relations is exact so any counting scheme had to be "reset" so it wouldn’t drift.

The first observatories we know addressed this problembuttonbook.jpg (10323 bytes)

Stonehenge is a famous example. Among other things, it was used to identify the first day of summer, the "summer solstice." Ancient astronomers could use it like a giant gunsight to determine when the sun was rising in a certain direction. They sighted through the monument to a large stone placed at a distance, the heelstone, to identify the day of the summer solstice. (photo by George Rieke)

 

View toward Heelstone Sun rising over Heelstone
Diagram of Stonehenge at summer solstice (from http://www.soulsofdistortion.nl/Summer%20Solstice%20Galactic%20Alignment.html)

View toward the "heelstone"  

Sunrise at the summer solstice lined up over this prominent stone (from C. Witcombe, http://witcombe.sbc.edu/earthmysteries/EMStonehenge.html)

An even earlier example can be found at Newgrange, just north of Dublinbuttonbook.jpg (10323 bytes)

The great temples of ancient Egypt also served to update calendarsbuttonbook.jpg (10323 bytes)

Through such observations, the ancient astronomers developed calendars that eventually evolved into the one we use todaybuttonbook.jpg (10323 bytes)

Early man also knew that what stars could be seen and how they appeared to move during the night depended on where he was on the earth:

The further north one travels, the higher the North star appears in the sky and stars appear to move closer to parallel to the horizon.

Paths of moon, stars, and sun over Tucson Here is how the motions look from Tucson (assuming we are looking south and have super-wide-angle eyes that can see all the way from east to west) (animation by G. Rieke); you may have to reload to start it.
Paths of moon, stars, and sun at North Pole and here is how they would look from the North Pole (assuming our super-wide-angle eyes take in the pole star at the top of the animation) (by G. Rieke).buttonbook.jpg (10323 bytes)

It is this behavior that suggested that the stars and planets were placed on great crystal spheres that were centered on the earth and rotated around it.

The dependence of apparent positions on the sky on north-south position (see animations just above) resulted in using stars for navigation. Reconstructed Greek Trireme, Olympios, (from Perseus Encyclopedia, http://www.perseus.tufts.edu/cgi-bin/image?lookup=1989.02.0009)
"Glorious Odysseus, happy with the wind, spread sails and taking his seat artfully with the steering oar he held her on her course ['her' is a raft he built to sail away from the nymph Kalypso], nor did sleep ever descend on his eyelids as he kept his eye on the Pleiades [a cluster of stars] and late-setting Bootes [a constellation, an identifiable pattern of stars in the sky] and the Bear [the constellation now known as the Big Dipper], to whom men give also the name of the Wagon, who turns about in a fixed place and looks at Orion [another easily identified constellation], and she alone is never plunged in the wash of the Ocean. For so Kalypso, bright among goddesses, had told him to make his way over the sea, keeping the Bear on his left hand."

-- Homer, the Odyssey, from the translation by Richmond Lattimore, Perrenial Classics edition, 1999, HarperCollins, publishers.

Reconstructed Greek trireme
Phoenician shipPhoenician navigation route There was always a problem with east-west positions, though, because they required accurate time keeping ribbon.jpg (3557 bytes)

Phoenician ship (far left) (http://en.wikipedia.org/wiki/File:Phoenician_ship.jpg);  Phoenician navigation route (left): shoreline was only reliable east-west guide (from Geocities, http://www.geocities.com/SoHo/Cafe/9535/navigate.htm)

Some objects appeared to be points of light that move with respect to the stars. The Greeks gave this type of object the name "planet" for wanderer. Mercury, Venus, Mars, Jupiter, and Saturn  have been known since pre-historic times.

The sun and the moon always appear to move eastward against the pattern of stars, and the planets also generally move eastward, but once in awhile, a planet will appear to turn around and move westward for awhile before returning to a generally eastward motion. This "retrograde motion" posed an intellectual puzzle that occupied astronomers for thousands of years! In fact, the motions on the sky were so dramatic and precise that we can truly claim that trying to explain them was responsible for the beginnings of precise scientific theory for all fields.

Retrograde motions of planets trace onto the sky as shown to the left. As an ancient astronomer observed Mars over a period of about seven months, it would have traced a large loop against the background of stars.

For a demonstration, see http://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.html, part of which is reproduced below.

Animation of Mars motion Motion of Mars, converted to a movie of apparent motion. Compare the run of dates in the lower right of the animation. The animation shows the total motion of Mars against the stars over a period of about 9 months.

These complications led to sophisticated systems of astronomy/astrology. To predict dramatic celestial events, the astronomers/priests/astrologers developed accurate long term calendars and texts.

The Maya of meso-America provide an example of great accomplishments in astronomy, which they embodied into religious/ceremonial aspects of their culture.

For example, the El Castillo pyramid, Chichen Itza, Yucatan, Mexico, played the role, among other things, of marking the spring equinox in a public and dramatic fashion. To the left is a picture of El Castillo at the time of the spring equinox. The shadows trace out an immense snake slithering down the side of the pyramid (this happens only on this special day). Such dramatic demonstrations had more to do with impressing the public with the power of the priests than low-budget astronomy, but they did require accurate observations and major efforts to understand them. (http://en.wikipedia.org/wiki/File:ChichenItzaEquinox.jpg)
El Caracol, Chichen Itza Building such large monuments therefore required substantial investments in astronomy as well as in architecturebuttonbook.jpg (10323 bytes) An example is El Caracol, a Mayan observatory at Chichen Itza, not far from El Castillo buttonex.jpg (1228 bytes) (G. Rieke)
Temple VI, Tikal Temples at Tikal, Guatemala, are elevated above the treetops to allow unobstructed viewing of the stars. Temple IV at Tikal, built in about 470AD, rises to a height of 212 feet above the jungle floor. (G. Rieke)

The Maya achieved impressive accuracy in measuring the repetitions of celestial events (synodic periods are the times required for the body to come back to the same position relative to the sun):

  Modern (days) Maya in days
Lunar (synodic) month 29.53059  29.53086 
Synodic period of Venus 583.93 583.92027 
Synodic period of Mars 779.94 780
Solar (tropical) year 365.24198 365.242

However, this accomplishment was addressed more toward prediction of propitious times for rulers to take action, that is, predicting the future. It belongs more in the realm of a very sophisticated astrology than to that of astronomy. The failure of the Mayas to seek underlying causes for the processes they measured so precisely meant that their approach was a dead end scientifically. To be a bit brutal, we should call their work pseudo-science because of its shortcomings as pure investigation and its use for non-scientific goals.

Where it is a duty to worship the sun it is pretty sure to be a crime to examine the laws of heat.

- Viscount Morley John, Voltaire

Not all ancient societies were as accomplished in astronomy as the Maya. However, virtually all had myths associated with the sky and earth, particularly creation mythsbuttonex.jpg (1228 bytes) In general, such myths are not refined through detailed measurements (such as those in the table above by the Maya) and they may not even be updated as new astronomical knowledge is acquired. Thus, they are basically not examples of a scientific approach.

planetariumcave.jpg (16993 bytes)Star petroglyphs, in "Planetarium Cave," Canyon de Chelly, from Gary Tepfer, http://www.wlotus.com/GaryTepfer/default.htm

sirtflaunch.jpg (4413 bytes)

uraniaa.jpg (8276 bytes)

 

 

 

 

Urania, Greek muse of astronomy, by Simon Vouet, http://www.luminarium.org/sevenlit/james/urania.htm

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hypertext copyright.jpg (1684 bytes) G. H. Rieke

 

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