Evolution of Astronomy Part 1 What did ancient

Evolution of Astronomy

Part 1

What did ancient civilizations achieve in astronomy? • Daily timekeeping, calendars • Tracking the seasons • Monitoring lunar cycles • Monitoring planets and stars • Predicting eclipses • navigation

• Egypt: Shadows of obelisk tell time of day.

England: Stonehenge (completed around 1550 B. C. ), seasons

Mexico: model of the Templo Mayor New Mexico: Anasazi kiva aligned north-south

SW United States: “Sun Dagger” marks summer solstice Scotland: 4, 000 -year-old stone circle; Moon rises as shown here every 18. 6 years.

Peru: Lines and patterns, some aligned with stars. Macchu Pichu, Peru: Structures aligned with solstices.

South Pacific: Polynesians were very skilled in art of celestial navigation France: Cave paintings from 18, 000 B. C. may suggest knowledge of lunar phases (29 dots)

Ancient people could predict seasons from the orientation of the crescent moon

Planets Known in Ancient Times • Mercury – difficult to see; always close to Sun in sky • Venus – very bright when visible; morning or evening “star” • Mars – noticeably red • Jupiter – very bright • Saturn – moderately bright

Days of week were named for Sun, Moon, and visible planets

Modern science traces its roots to the Greeks • They were the first people known to make models of nature. • They tried to explain patterns in nature without resorting to myth or the supernatural. Greek geocentric model (c. 400 B. C. )

Geocentric model • Earth at center of Universe • Heavens must be “perfect”: Objects moving on perfect spheres or in perfect circles. Plato Aristotle

The most sophisticated geocentric model was Ptolemy’s - the Ptolemaic model: It was accurate enough to remain in use for 1, 500 years. Ptolemy (A. D. 100 – 170)

Ptolemaic model explained retrograde motion with epicycles.

Copernicus (1473 -1543): • Proposed Sun-centered model • Used model to determine layout of solar system But. . . • no more accurate than Ptolemy’s for predicting planetary positions.

Ancient Greeks considered, but rejected the sun- centered model • Their inability to observe stellar parallax was a major factor.

Tycho Brahe • Compiled most accurate naked eye measurements ever made of planetary positions. • Still couldn’t detect stellar parallax, so still thought Earth must be at center of solar system • Hired Kepler, who used Tycho’s observations to discover the truth about planetary motion.

• Kepler tried to match Tycho’s observations with circular orbits • But an 8 -arcminute discrepancy led him eventually to conclude planets move in elliptical orbits. Johannes Kepler (1571 -1630) “If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy. ”

Part 2

Galileo (1564 -1642) overcame major objections to Copernican view- these were: 1. Earth could not be moving because objects in air would be left behind. 2. Non-circular orbits are not “perfect”. 3. If Earth were really orbiting Sun, we’d detect stellar parallax.

Overcoming the first objection (nature of motion): • Aristotle thought that all objects naturally come to rest. • Galileo showed that objects will stay in motion unless a force acts to slow them down (Newton’s first law of motion).

Overcoming the second objection (heavenly perfection): • Tycho’s observations of comet and supernova already challenged this idea. • Using his telescope, Galileo saw: • Sunspots on Sun (“imperfections”) • Mountains and valleys on the Moon (proving it is not a perfect sphere)

Overcoming the third objection (parallax): • Tycho thought he had measured stellar distances, so lack of parallax seemed to rule out an orbiting Earth. • Galileo showed stars must be much farther than Tycho thought — in part by using his telescope to see the Milky Way is countless individual stars. ü If stars were much farther away, then lack of detectable parallax was no longer so troubling.

Galileo also saw four moons orbiting Jupiter, proving that not all objects orbit the Earth

Galileo’s observations of phases of Venus proved that it orbits the Sun and not Earth.

The idealized scientific method • Based on proposing and testing hypotheses • hypothesis = educated guess

Hallmarks of Science: #1 Modern science seeks explanations for observed phenomena that rely solely on natural causes. (A scientific model cannot include divine intervention)

Hallmarks of Science: #2 Science progresses through the creation and testing of models of nature that explain the observations as simply as possible. (Simplicity = “Occam’s razor”)

Hallmarks of Science: #3 A scientific model must make testable predictions about natural phenomena that would force us to revise or abandon the model if the predictions do not agree with observations.

What is a scientific theory? • A scientific theory must: —Explain a wide variety of observations with a few simple principles, AND —Must be supported by a large, compelling body of evidence. —Must NOT have failed any crucial test of its validity.

• Astronomy is a science focused on learning about how stars, planets, and other celestial objects work. • Astrology is not a science. It’s the belief that the positions of planets and stars in the sky have influences on human lives.

• Scientific tests have shown that astrological predictions are no more accurate than we should expect from pure chance.