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Natural Science - Year I

Unit 5: Timekeeping and Planetary Motions

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History Weblecture for Unit 5


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History Lecture for Unit 5: Science in the Civilizations of the Ancient World II

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Outline/Summary

Ancient Observations

Stonehenge and Avebury

We've already talked a bit about how using the sun's motion in the sky and the pattern of stars rising and setting helped ancient cultures determine when to plant and harvest crops at proper seasons. But many cultures, even the fairly primitive stone age cultures of ancient Britain and Mayan MesoAmerica, created ways of tracking the exact time of the first day of spring and fall, and the days of the summer and winter solstice. Monuments like Stonehenge on the Salisbury Plains of England and Avebury in Wiltshire allowed them to make observations that identified the important days of the solar year. If these interpretations are correct, Stonehenge could have been used to identify the key points of the solar year: the first days of our modern seasons of spring, summer, fall, and winter.

There are many theories about how Stonehenge was built and how long it took: the most commonly accepted explanation today is that the stones were assembled in phases over almost 1300 years. Although there is evidence that the site was used for burials starting in the seventh century bce, the embankments and the earliest stones were set up around 3000 bce Later wooden buildings no longer exist although we can still see the post holes used to anchor the foundations. The trilithons of dressed sarsen stone that make up the huge "doorways" were probably erected around 2600 bce The stones were arranged so that if you stand at the center of the circle, you will see the sun rise directly above a particular marker called the Heelstone on "Midsummer Day", the day of the Summer solstice. This marks the northernmost rising position of the sun.

One of the most puzzling aspects of Stonehenge are the 56 "Aubrey" holes (named after the man who discovered them in the 1700s). These were probably added to the Stonehenge circle many years after the original stones were set up. They were made by driving wooden posts into the ground, then burning the wood.

Using computer simulations of observations one could make from the Aubrey hole points of reference, some modern historians concluded that these holes were meant to mark positions of the moon, namely, its most northernly and southernly points. While there is no way now to be certain, it may be that Stonehenge allowed its builders to track solar and lunar positions and predict eclipses as well as seasonal dates through the year. Even in a society without writing, people found a way to communicate the important patterns they observed in nature!

Mesopotamia

Old Babylon (Hammurabi and Melishipak, 1800 bce - 1200 bce) The Babylonians and their successors, the Assyrians and Persians, used planetary positions to accurately date events, recognizing that the patterns of motions were regular enough to calculate the passage of time over the reigns of many kings. Early observations of the seventeenth century bce record the rising times of Venus over a 21-year period. Elaborate stele or stones citing specific astronomical events were used to date legal transactions, such as this stone, in which the king Milishipak records the formal presentation of his daughter to her patron Goddess Nannaya, as witnessed by the god Sin (moon), the sun Shamas, and the goddess Ishtar (the star Venus).

© 2006 Christe Ann McMenomy

Persia, Artaxerxes and Darius II (450 bce - 400 bce). Tablet #21 below records an event on the 24th day, in the 10th month of the thirteenth year of the reign of Darius II, and Tablet #22 states that in the forty-second year of the reign of Artaxerxes II, Jupiter became stationary in the fourth month, reached opposition in the sixth month, and entered the constellation of Pisces in the eighth month, and finally was in conjunction in Aries in the twelfth month of that year [see the Science unit for more information on opposition and conjunction]. The precision of these observations turns out to be very useful to us. This kind of information allows us to date the events in the reign of Artaxerxes with great precision, based on our own current knowledge of Jupiter's orbit. We can run a planetarium program "back" to the period around the time of Artaxerxes' reign and identify exactly which periods that Jupiter was in the constellation Aries. We can then map our modern calendar date for this period to the "forty-second year" of Artaxerses reign.

© 2006 Christe McMenomy

The Babylonians were even able to use their record-keeping and calculating skills to determine that some of the phenomena in the sky had periods of more than a man's normal lifetime that repeated over hundreds of years. Cuneiform tablets from the second century before Christ preserve the prediction for, and observation of, a returning comet, which the Babylonians figured out came once every 76 years or so. These two cuneiform tablets record the sighting of the comet in 164 B. C. Because of such records, the seventeenth century Astronomer Royal of England, Edmund Halley, was able to show that the same object had a repeating orbit and could be identified with the comet depicted as appearing in 1066 in the Bayeux Tapestry, as well as this Babylonian record. The comet was named for him, and last appeared in Earth's skies as a naked-eye object in 1910 and 1986.

© 1986, 2005 Christe A. McMenomy

Egypt

As in Babylon, astronomical bodies and events were identified with the local pantheon of gods and goddesses. The constellation Orion represented Osiris, the Milky Way was the sky goddess Nut, the sun was the god Re, the Moon was Thoth, the god of wisdom. While astronomical observation was necessary for determining the calendar, and the cardinal directions for properly orienting temples and pyramids to the rising sun, the Egyptians focussed more identifying the appropriate divine forces behind natural events, and predicting the future than on looking for periodic patterns in the motions of the planets, sun, or moon. For nearly two millenia, the calendar cycled through all the seasons because the Egyptian year did not recognize the extra "quarter day" of the solar cycle required a leap year until Ptolemy III suggested the practice in 238 bce

The Egyptians depended on Babylonian astronomy. In the early 19th Century, Napoleon Bonaparte invaded Egypt and brought back to France a number of Egyptian artifacts, including a carved stone ceiling from a temple at Hathor with a diagram of the heavens called the Zodiac of Denderah (or sometimes Dendera). Historians have dated the zodiac carving to about 50 bce — very late in Egyptian history, during the period of Hellenistic influence under the reign of the Ptolemy dynasty that included Cleopatra. The zodiac is now displayed in its appropriate overhead position in its own nook in the Antiquities section of the Louvre, so you can observe it as the ancient Egyptians would have seen it.

This first picture shows the layout of the zodiac with its multiple rings of figures, and four great figures of goddesses at each corner holding up the circle of the sky as you might observe it from below. Around this circle march 36 figures, each representing 10° sections or decans of the sky, allowing the Egyptians to plot positions of stars quickly and accurately.

© 2006 Christe McMenomy

The next picture shows one corner of the Zodiac. Look carefully above the disk with 8 small seated figures, and you can see the goat-fish figure associated with the constellation Capricorn. This particular figure is interesting because it is nearly identical to representations of suhurmashu, the Mesopotamian figure for this constellation, found on boundary stones marking the edges of fields in Sumeria. This would indicate that the astronomers of Hathor had contact with and shared information with their Mesopotamian counterparts and were checking their observations with the records available in Babylon and Assyria — or perhaps simply copied the Mesopotamian results!

Standing on the back of Capricorn is a human figure holding a hooked staff. This is supposed to be Mars. To the left of this figure is Aquarius, who is pouring water from a jug onto the tail of Capricorn. To the left of Aquarius is another figure with a hooked staff, the representation of the planet Venus, and behind Venus, you can see the two fish of the constellation Pisces swinging at the ends of long ropes.

© 2006 Christe McMenomy

The number of identifiable objects — constellations, planets, and individual stars — give us some idea of how the Egyptian astronomers recorded the motions of the heavens, although the use of Mesopotamian figures gives rise to doubts that the Egyptian astronomers were making their own careful observations.

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