ANCIENT ASTRONOMY

Long before the number of days in a year was known and the calendar was invented, people recognized that the stars rise and set at the same point on the horizon each night, but not the Sun--the rising and setting point of the Sun depend on the season.

Various monuments such as Stonehenge (in Britain) and medicine wheels in North America are aligned with features such as the northernmost and southernmost rising or setting points of the Sun. These sites are thus astronomical observatories that may have been used to determine planting seasons. The approach of planting time can thus be predicted without having a calendar, without knowing the number of days in a year, without even knowing how to count.

A year is one complete cycle of the seasons, i.e. a complete cycle of apparent movement by the Sun. Thus our calendar year is based on the movement of the Sun, and is called a solar calendar. The year based on the seasons is called a tropical year.

EGYPT

2800 B.C. Some pyramids are aligned with significant orientations of the bright stars. Imhotep, the architect of the first pyramid, enters mythology as the first astronomer.

Calendars

• Planting seasons governed by annual flooding of the Nile. Egyptians observe that the star Sirius ("Sothis") rises just before dawn ("heliacal rising") coincides with the rising of the Nile. This defines the Sothic year, which the Egyptians observe to be about 365 1/4 days long. (The year based on motions of the stars is called a sidereal year.)
• The Egyptians also used a lunar calendar based on the motions of the Moon.
• For numerical convenience they had a third, civil calendar of 365 days.

The task is to reconcile these calendars. (12 x 29 ¹/₂ 365 ¹/₄) Until late in their history, the ancient Egyptians did this empirically, that is, by observation, without the aid of mathematics.

MESOPOTAMIA (Assyria and Babylonia)

ca. 700 B.C., Mesopotamian astronomers start recording data to synchronize calendars (perhaps partly also for astrological purposes)

ca. 480 BC, develop a systematized cycle (Metonic): 19 years=235 months to great accuracy. This discovery was possible because of the long period over which observations had been made.

Other Babylonian achievements

• predicted eclipses of the Moon, and to a lesser extent, the Sun.
• made detailed observations of the apparent motion of the Sun, Moon and planets with respect to the stars, and were particularly good at predicting the motion of the Moon and Venus.
• constellations of the zodiac (Constellations are groups of stars that are associated because they happen to lie in the same direction from Earth; but they may lie at very different distances.)

New features of Babylonian astronomy: it was both mathematical and predictive. As they had no calculus, algebra or trigonometry, and very little geometry, the calculating techniques of the Babylonians were strictly based on arithmetic--very complicated motions had to be described with very simple mathematics!

THE GREEKS

Archaic period

Early philosophers (e.g. Pythagoras, Thales, 6th C BC) began to speculate about causes and origins of natural phenomena, reasoning from observation

• Parmenides (b. ca. 540 BC) is said to have argued that the Earth is round, at the center of the Universe, and that the Moon is illuminated by the Sun.
  
• Empedocles (490? - 430 BC) and Anaxagoras (500? - 428 BC) give explanation of solar eclipses; Anaxagoras suggests that the Sun is a hot glowing rock, and the Moon made of earth.

Classical period

A period of remarkable achievements in all branches of the arts and sciences occurred in Greece beginning roughly in 479 BC with the victory over the Persian Empire at Thermopylae.

PLATO (427? - 347 BC) describes the stars as being attached to a celestial sphere centered on the Earth. He also describes the ecliptic and the celestial equator.

• Plato introduces an unfortunate prejudice in favor of circles and spheres and a tendency to value theory over observation.

EUDOXUS (408 - 355 BC) introduces a 4-sphere model to account qualitatively for more complicated motions of the planets. Model fails to account for brightness variations

ARISTOTLE (384 - 322 BC) describes and enlarges the model of Eudoxus to 55 spheres. The Earth is stationary at the center, and the Sun, Moon and planets go around the Earth (geocentric model).

Among his many good ideas:

• Argues that the Earth is spherical from the shape of its shadow on the Moon during eclipse

• the heavens are unchanging and eternal
• physics of the heavens is different from that of the Earth
• circular motion is natural to the heavens

Aristotle's system was very complete and persuasive, though not universally accepted even in antiquity

HERACLEIDES (388? - 315 BC) proposes that the Earth rotates, not the celestial sphere. He may also have suggested that Mercury and Venus orbit the Sun, not the Earth.

ARISTARCHUS (320? - 250? BC) proposes, nearly 2000 years before Copernicus, that all the planets orbit the Sun, including the Earth (heliocentric model)

The heliocentric model greatly simplifies the description of the motions of the planets, but there was no direct evidence for it.

One objection to the heliocentric model is that we don't observe parallax of the stars. Aristarchus recognized the objection and correctly argued that this must mean that the stars are very far away.

• The heliocentric model explains brightness variations (but so can other systems)
• There was a religious objection (anticipating Galileo's troubles by 2000 years)

Another remarkable achievement of Aristarchus was his attempt to measure the relative distance and sizes of sun and moon. His results were poor, because he used erroneous measurements as input data, but his methods were correct.

Hellenistic period.

By the time Alexander the Great died in 322 BC, he had conquered many of the great civilizations of the world, including Egypt, Persia and even part of India. Although his empire fell apart after his death, his generals established kingdoms where Greek culture blended with the native cultures. Greek became the international language, and the language of learning.

ERATOSTHENES (276? - 196? BC):

measures radius of earth with about 20% error.

HIPPARCHUS (190?- 120 BC):

• star catalogue with celestial coordinates
• discovers precession of the equinoxes
• length of year = 365 + ¹/₄ - ¹/₃₀₀ days. Most accurate measurement until Gregorian calendar, instituted 1582 AD
• lunar period to w/in 1 sec from data
• minimum distance to sun estimated from failure to observe parallax
• distance to moon from eclipse data; cites 59 - 67 ¹/₃ times radius of Earth. Actual value 60. Note estimate of uncertainty.

APOLLONIUS OF PERGA (261? - 190? BC): introduces epicycles and eccentric circles

PTOLEMY (b. ca. 75 AD): Megiste Mathematike Syntaxis (handed down in Arabic as the Almagest).

• Geocentric model with epicycles.
• quantitative; parameters adjusted to fit observation
• comprehensive model
• provides tables for ease of computation

The Ptolemaic system was a great astronomical achievement unequalled until the Renaissance.

Further reading about ancient astronomy.

• Marshall Clagett, Greek Science in Antiquity (New York: Collier, 1963).
• Sir Thomas L. Heath, Greek Astronomy (New York: Dover, 1991).
• David C. Lindberg, The Beginnings of Western Science (Chicago: University of Chicago, 1992).
• John North, The Norton History of Astronomy and Cosmology (New York: Norton, 1995).

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