The Origin of Modern Astronomy (Write Only What is In Green)

Guidepost This chapter describes how astronomers learned to understand what they saw in the sky and how that changed humanity’s understanding of what we are. In learning to interpret what they saw, Renaissance astronomers invented a new way of knowing about nature, a way of knowing that we recognize today as modern science. This chapter tells the story of heavenly motion from a cultural perspective.

Outline I. The Roots of Astronomy A. Archaeoastronomy B. The Astronomy of Greece C. The Ptolemaic Universe II. The Copernican Revolution A. Copernicus the Revolutionary B. Galileo the Defender III. The Puzzle of Planetary Motion A. Tycho the Observer B. Tycho Brahe's Legacy C. Kepler the Analyst D. Kepler's Three Laws of Planetary Motion E. The Rudolphine Tables IV. Modern Astronomy

The Roots of Astronomy A. In the stone and bronze ages (circa 2000-1000BC), ancient human cultures realized the cyclic nature of motions in the sky. B. Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance. C. Those monuments were probably used as calendars or even to predict eclipses. 1. Examples: Stonehenge, Big Horn and Caracol

Stonehenge Constructed: 3000 – 1800 B.C. Summer solstice Heelstone Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices Probably used as calendar.

More on Stonehenge NatGeo Discovery Channel

Other Examples All Over the World Big Horn Medicine Wheel (Wyoming)

Big Horn Video https://www.youtube.com/watch?v=xPhDlsWGplA

Other Examples All Over the World (2) Caracol (Maya culture, approx. A.D. 1000)

Ancient Greek Astronomers Unfortunately, there are no written documents about the significance of stone and bronze age monuments. First preserved written documents about ancient astronomy are from ancient Greek philosophy. Greeks tried to understand the motions of the sky and describe them in terms of mathematical (not physical!) models.

Ancient Greek Astronomers (2) Models were generally wrong (because they were based on wrong “first principles”, believed to be “obvious” and not questioned): Geocentric Universe: Earth at the Center of the Universe. “Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles.

Ancient Greek Astronomers A. Eudoxus (409 – 356 B.C.): 1. The earth is the center of the universe. 2. All celestial motion is circular. 3. All celestial motion is regular. 4. The center of all celestial motion is the center of the universe.

B. Aristotle (384 – 322 B.C.), major authority of philosophy until the late middle ages: Universe can be divided in 2 parts Imperfect, changeable Earth, Perfect Heavens (described by spheres)

C. Eratosthenes (~ 200 B.C.): Calculation of the Earth’s radius Angular distance between Syene and Alexandria: ~ 70 Linear distance between Syene and Alexandria: ~ 5,000 stadia  Earth Radius ~ 40,000 stadia (probably ~ 14 % too large) – better than any previous radius estimate.

The Copernican Revolution Nicolaus Copernicus (1473 – 1543): Heliocentric Universe (Sun in the Center)

Copernicus’ new (and correct) explanation for retrograde motion of the planets Retrograde (westward) motion of a planet occurs when the Earth passes the planet. https://www.youtube.com/watch?v=LSBrj-DBXr4

Galileo Galilei (1594 – 1642) A. Invented the modern view of science: Transition from a faith-based “science” to an observation-based science. B. Greatly improved on the newly invented telescope technology. (But Galileo did NOT invent the telescope!- Hans Lippershey did) C. Was the first to meticulously report telescope observations of the sky to support the Copernican Model of the Universe.

Major Discoveries of Galileo 1. Moons of Jupiter (4 Galilean moons) (What he really saw) 2. Rings of Saturn (What he really saw)

Major Discoveries of Galileo (2) 3. Surface structures on the moon; first estimates of the height of mountains on the moon

Major Discoveries of Galileo (3) 4. Sun spots (proving that the sun is not perfect!)

Major Discoveries of Galileo (4) 5. Phases of Venus (including “full Venus”), proving that Venus orbits the sun, not the Earth!

Johannes Kepler (1571 – 1630)

Kepler’s Laws of Planetary Motion The orbits of the planets are ellipses with the sun at one focus. c Eccentricity e = c/a

Eccentricities of Ellipses 1) 2) 3) e = 0.02 e = 0.1 e = 0.2 5) 4) e = 0.4 e = 0.6

Eccentricities of Planetary Orbits Orbits of planets are virtually indistinguishable from circles: Most extreme example: Pluto: e = 0.248 Earth: e = 0.0167

Planetary Orbits (2) 2. As a planet revolves in orbit, a line from a planet to the sun sweeps over equal areas in equal intervals of time.

Tycho Brahe https://www.youtube.com/watch?v=H4tZ9tyd_MM

Historical Overview

Question! 1. Why are Stonehenge and The Big Horn Medicine Wheel thought to be ancient astronomical observatories? a. Petroglyphs at each site describe how they were used to make observations. b. Ancient Greek writings list the important discoveries made at each of these two sites. c. Stones at each site aligned with significant rising and setting positions. d. Both a and c above. e. All of the above.

Question! 2. Who accurately determined the size of Earth by considering Sun angles at Syene and Alexandria? a. Thales of Miletus (c. 624-547 BC) b. Pythagoras (c. 570-500 BC) c. Eudoxus (409-356 BC) d. Aristotle (384-322 BC) e. Eratosthenes (c. 200 BC)

Question! 3. One of the first principles of ancient astronomy is that the heavens beyond _____ are perfect, and the Earth is corrupt. a. the atmosphere b. the Sun c. the Moon d. Saturn e. Pluto

Question! 4. What does Kepler's second law indicate about the orbital speed of a planet? a. The orbital speed of each planet is constant. b. A planet moves at its slowest when it is closest to the Sun. c. A planet moves at its fastest when it is closest to the Sun. d. The orbital speed of a planet varies in no predictable way. e. None of the above.