Since the dawn of civilization, humans have struggled to make sense of the complex motions of celestial objects,

and countless ancient monuments and artifacts reflect their fascination.

THE BEGINNINGS OF

ASTRONOMY

THE BABYLONIAN TRADITION

• Stonehenge in England and the Pyramids of Egypt, both dating from around 2500 BC, embody astronomically significant alignments based on knowledge of the skies,but the true birthplace of astronomy was in the Middle East.

• Two baked clay tablets produced around 700 BC by the Babylonians of present-day Iraq summarize information on the motions of stars and planets.

• The list of stars and constellations known to the Babylonians is clear evidence of a long-standing tradition of celestial observation. Some constellations, such as Leo and scorpius, have come down to us virtually unchanged.

THE BABYLONIAN TRADITION

THE BABYLONIAN TRADITION

• the Babylonians made another lasting contribution to astronomy: having measured the length of the year as approximately 360 days, they divided the circle of the sky into 360 degrees, subdivided each degree into 60 parts, and introduced the 24-hour day, with each hour also divided into 60 parts.

GREEK ASTRONOMY

• the Greeks sought to understand the physical principles on which the universe worked, thus initiating the separation of science from superstition.

• Eudoxus

• 4th century BC, developed Celestial Model• a scheme of 27 crystalline spheres all

nested within each other, rotating on different axes and at different speeds, which carried the celestial bodies around the spherical Earth.

• Hipparchus

• compiled the first accurate catalog of the naked-eye stars in the 2nd century BC.• classified stars into six categories of

brightness

• Ptolemy (second century AD)

• presented a summary of Greek astronomical knowledge in a work usually known as the Almagest, meaning “greatest,” a name given to it by later Arabic astronomers.

• This included an updated version of Hipparchus’s catalog, expanded from 850 stars to over 1,000 and arranged into 48 constellations— the foundation of our present-day constellation system.

• Ptolemaic Model

ARABIC ASTRONOMY

• al-Sufi (before 1000 AD) • produced a revised version of Ptolemy’s

star catalog, called the Book of the Fixed Stars.• Also contained drawings of each

constellation.

THE REBIRTH OF WESTERN ASTRONOMY

• Nicolaus Copernicus (1473–1543)• revived the sun-centered or

heliocentric theory proposed by the Greek philosopher Aristarchus in the 3rd century BC.

• Tycho Brahe (1546–1601), a Danish nobleman• Between 1576 and 1586, he built two

observatories, called Uraniborg and Stjerneborg, on the island of Hven, between Denmark and Sweden• Model: Earth remained stationary at the

center, orbited by the Moon and sun, while the planets orbited the moving sun.

•Johannes Kepler (1571–1630)•discovered that the planets do indeed orbit the sun

GALILEO’S DISCOVERIES• greatest pioneer of telescopic astronomy

• Moon’s surface was not a smooth, polished sphere but was scarred by craters and mountains

• Jupiter was orbited by four moons (Galilean satellites)

• Venus shows phases

• glimpsed the rings of saturn

• swept away the old Earth-centered view of the universe for good

• Acceleration due to gravity

NEWTON AND GRAVITY

• deduce his law of gravity, publishing it in 1687 in Principia Mathematica.

• Edmond Halley (1656–1742)• calculated that comets move around

the sun on highly elliptical orbits.• Convinced that the comets seen in

1531, 1607, and 1682 were one and the same, Halley predicted that the comet would return around 1758