ARCHAEOASTRONOMY Thursday meeting January 25 2007 Roberta Zanin

ARCHAEOASTRONOMY Thursday meeting – January 25 2007 Roberta Zanin 1

Archaeoastronomy: the study of the practice of astronomy using both the written and unwritten records. It began as a meeting ground for three established disciplines: (A. Aveni-Journal of Archaeological Research, Vol. 11, No. 2, 2003) • Astroarchaeology: a methodology for retrieving astronomical information from the study of alignments associated with ancient architecture. (Hawkins, 1966) • History of Astronomy: it is concerned with acquisition of precise knowledge by ancient cultures. (Crowe and Down, 1999) • Ethnoastronomy: a branch of cultural anthropology that develops an understanding of cultural behavior as gleaned from indigenous perceptions of events in 2 the heaven. (Fabian, 2001) 1

Outline • Astroarchaeonomy two examples of building alignments: 1. Stonehenge 2. Chichén Iztá (Mexico), the Caracol and El Castillo as proof of the perfect astronomical knowledge of these two ancient cultures. • History of astronomy how this knowledge could be obtained without any modern instruments 1. how to predict an eclipse 2. how to measure the cycle of celestial bodies • Conclusions Ethnoastronomy 3 2

Stonehenge • Phase I (2950 -2900 BC): a circular bank with a ditch, inside the bank a circle of the 56 Aubrey holes. An earthwork, called Avenue, along which the Heel Stone was located. • Phase II (2900 -2400 BC): Aubrey holes partially filled, wooden settings in the center and at the eastern entrance. • Phase III (2550 -1600 BC): a circle of Sarsens within a horseshoeshaped arrangement of Trilithons and four great stones as stations. 4 3

Stonehenge • Phase I (2950 -2900 BC): a circular bank with a ditch, inside the bank a circle of the 56 Aubrey holes. An earthwork, called Avenue, along which the Heel Stone was located. • Phase II (2900 -2400 BC): Aubrey holes partially filled, wooden settings in the center and at the eastern entrance. • Phase III (2550 -1600 BC): a circle of Sarsens within a horseshoeshaped arrangement of Trilithons and four great stones as stations. 5 3

Stonehenge alignments Heel Stone 6 4

The Caracol: Maya observatory (Chichén Itzá, Yucatan-Mexico) viewing shaft A. Aveni, Tropical Astronomy, Science 1981 These windows align with some astronomical sightlines: Venus rising at its northernmost southernmost positions, as well as the equinox sunset 7 5

The Caracol: Maya observatory (Chichén Itzá, Yucatan-Mexico) These windows align with some astronomical sightlines: viewing shaft Venus rising at its northernmost southernmost positions, as well as the equinox sunset Since Venus’s orbit is tilted 4º with respect to the ecliptic, its position shifts against the horizon, the northernmost and the southernmost positions correspond to the farthest northern and southern points above the celestial equator. A. Aveni, Tropical Astronomy, Science 1981 8 5

The Caracol: Maya observatory (Chichén Itzá, Yucatan-Mexico) N. Strobel, Astronomy without a telescope These windows align with some astronomical sightlines: Venus rising at its northernmost southernmost positions, as well as the equinox sunset Since Venus’s orbit is tilted 4º with respect to the ecliptic, its position shifts against the horizon, the northernmost and the southernmost positions correspond to the farthest northern and southern points above the celestial equator. 9 5

The Caracol: Maya observatory (Chichén Itzá, Yucatan-Mexico) Staircase almost perfect match with Venus setting at its northernmost position The building diagonal is aligned with winter and summer solstices 10 5

El Castillo: Pyramid of Kukulkán (Chichén Itzá, Yucatan-Mexico) 1. At the equinox sunsets, a play of light and shadow creates the appearance of a snake that gradually undulates down the stairway of the pyramid. 11 6

El Castillo: Pyramid of Kukulkán (Chichén Itzá, Yucatan-Mexico) 1. At the equinox sunsets, a play of light and shadow creates the appearance of a snake that gradually undulates down the stairway of the pyramid. this sinuous shadow joins with one of the snake-head sculpture carved into the base of the monument 12 6

El Castillo: Pyramid of Kukulkán (Chichén Itzá, Yucatan-Mexico) 1. At the equinox sunsets, a play of light and shadow creates the appearance of a snake that gradually undulates down the stairway of the pyramid. this sinuous shadow joins with one of the snake-head sculpture carved into the base of the monument 2. It was used as calendar: each of the 4 stairways has 91 steps + 1 step on the top = 365 steps 13 6

El Castillo: Pyramid of Kukulkán (Chichén Itzá, Yucatan-Mexico) 1. At the equinox sunsets, a play of light and shadow creates the appearance of a snake that gradually undulates down the stairway of the pyramid. this sinuous shadow joins with one of the snake-head sculpture carved into the base of the monument 2. It was used as calendar: each of the 4 stairways has 91 steps + 1 step on the top = 365 steps 3. the west plane faces the zenith passage with a precision within 1º 14 6

Maya astronomy Maya were skilled observers of the sky: they calculated the complex motions of the Sun, the stars and planets and recorded this information in their codices (“Dresden Codex”). From this information, they developed calendars to Keep track of celestial movements: their solar calendar was more precise than the present Gregorian calendar. Modern (day) Maya (day) Lunar period 29. 53059 29. 53086 Solar period 365. 2420 365. 2466 Mars period 779. 94 780 Venus period 583. 93 583. 9203 Venus had been recognized as morning and evening star! Greek astronomers had recorded Venus as two different stars. 15 9

Venus: morning and evening star Superior conjunction Venus is an inferior planet: it has phases as the Moon Dresden Codex (day) Modern (day) Morning star (after heliacal rise) 236 263 Invisible (superior conjunction) 90 50 Evening star 250 263 Invisible (inferior conjunction) 8 8 Total 584 Skywatchers Inferior conjunction Heliacal rise=Sun and Venus rise together. After heliacal rise, Venus rises before the sunrise: morning star. After superior conjunction, Venus rises after the sunrise, so set after the sunset: evening star. 16 8

Maya astronomy Maya were skilled observers of the sky: they calculated the complex motions of the Sun, the stars and planets and recorded this information in their codices (“Dresden Codex”). From this information, they developed calendars to keep track of celestial movements: their solar calendar was more precise than the present Gregorian calendar. Modern (day) Maya (day) Lunar period 29. 53059 29. 53086 Solar period 365. 2420 365. 2466 Mars period 779. 94 780 Venus period 583. 93 583. 9203 It seems incredible! But we have forgotten what can be achieve by careful naked eye observation using simple instruments. 17 7

Marking time without instruments To determine the solstice day is rather easy only by studying shadows: at summer solstice the Sun is at its highest point and the shadows it casts are the shortest; vice versa at winter solstice. Gnomons, simple long sticks located on a plate, were already used by Greek astronomers. no thickness gnomon At the beginning, gnomons were used as sundials (by dividing the plate into equal intervals), as well as to establish cardinal directions (south=the position of the shortest shadow of a day) and as calendar (by dividing the period between two solstices into intervals, each of one characterized by a particular shadow length) Maya used the zenith passage which are characterized by shadowless moments as reference day. 18 11

The zenith-horizon system The horizon functions as fundamental reference line, together with the zenith. Here star motion is vertical. The sun can be observed at zenith at the equinoxes. At temperate zones, the observer views circulatory motion. In this case it is simpler using the celestial pole and the celestial equator as reference lines. Tropics are the maximum latitudes at which the Sun can be observed at zenith: ZENITH PASSAGES. N. Strobel, Astronomy without a telescope 19 10

Zenith Tubes There is no evidence that gnomons were used by Maya, but they used “zenith tubes” to identify the shadowless moment. These tubes admit the Sun’s image to pass vertically into a darkness chamber At the ruins of Xochicalco, Mexico, a 8 m long perfect straight tube, which opens into a roundish chamber (10 m diameter), was found. The cross section of this tube is hexagonal with a 2. 5º of FOV. A. Aveni, Tropical Astronomy, Science (1981) 20 12

When an eclipse occurs? Sun-Moon angle 0º (new phase) SOLAR ECLIPSE Sun-Moon angle 180º (full phase) LUNAR ECLIPSE AND Moon at the line of nodes Moon’s orbit precesses intersection of the Moon’s orbit with the ecliptic twice a year at different dates 21 13

Stonehenge: an eclipse predictor only when, not where + Full moon 4 markers G. Hawkins, The Stonehenge Decoded, Nature 1963 22 14

Stonehenge: an eclipse predictor only when, not where 56 a perfect number! + Full moon 56/2=28 Moon’s orbit is 27. 322 days Moon marker – twice a day and skip one each cycle 4 markers G. Hawkins, The Stonehenge Decoded, Nature 1963 23 14

Stonehenge: an eclipse predictor only when, not where 56 a perfect number! + 56/2=28 Full moon Moon’s orbit is 27. 322 days Moon marker – twice a day and skip one each cycle 4 markers 56*6. 5=364 Earth’s orbit is 365. 25 days Sun marker – every 6. 5 days and during solstices half more G. Hawkins, The Stonehenge Decoded, Nature 1963 24 14

Stonehenge: an eclipse predictor only when, not where 56 a perfect number! + 56/2=28 Full moon Moon’s orbit is 27. 322 days Moon marker – twice a day and skip one each cycle 4 markers 56*6. 5=364 Earth’s orbit is 365. 25 days Sun marker – every 6. 5 days and during solstices half more 56/3=18. 66 Orbit of Nodes is 18. 61 y Node markers – every four months G. Hawkins, The Stonehenge Decoded, Nature 1963 25 14

Stonehenge: an eclipse predictor only when, not where + Full moon 4 markers Solar eclipse: Moon, Sun and a node in the same position Lunar eclipse: the Sun and a node opposite to the Moon and the other node G. Hawkins, The Stonehenge Decoded, Nature 1963 26 14

Conclusions Ancient astronomers were surely skilled sky observers They knew the precise cycle of many celestial objects, they were able to predict important astronomical events such as eclipses. This whole knowledge had been obtained without any modern instruments. If this appear us incredible is only because we are no more accustomed to take simple measurements. • It is not necessary to invoke the help of gods coming from the tenth planet of the solar system! (A. Alford, Gods of the new millennium) • Astronomical knowledge in ancient cultures was used for religious (to time rituals, to decide where to build a temple) and civil purposes (when to sow and harvest) Ethnological implications. They were not interested in accuracy! We should remember this difference with the modern science. 27 15

THE END 28

Backup Slides 29

When the gnomon has a finite thickness, there will be two dial centers and double noon lines. The double noon lines are spaced a distance equal to the thickness of the gnomon and this space is known as the noon gap. 30

Precession of Moon 31

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