The Origin of Modern Astronomy

Chapter 4

The preceding chapters gave you a modern view of Earth. You can now imagine how Earth, the moon, and the sun move through space and how that produces the sights you see in the sky. But how did humanity first realize that we live on a planet moving through space?

That required revolutionary overthrow of an ancient and honored theory of Earth’s place. By the 16th century, many astronomers were uncomfortable with the ancient theory that Earth sat at the center of a spherical universe. In this chapter, you will discover how a Polish astronomer named Nicolaus Copernicus changed the old theory, how a German astronomer named Johannes Kepler discovered the laws of planetary motion, and how the Italian Galileo Galilei changed what we know about nature.

Guidepost

Here you will find answers to four essential questions:

• How did classical philosophers describe Earth’s place in the Universe?

• How did Copernicus revise that ancient theory?

• How did astronomers discover the laws of planetary motion?

• Why was Galileo condemned by the Inquisition?

Guidepost (continued)

This chapter is not just about the history of astronomy. As they struggled to understand Earth and the heavens, the astronomers of the Renaissance invented a new way of understanding nature – a way of thinking that is now called science.

I. The Roots of AstronomyA. ArchaeoastronomyB. The Astronomy of GreeceC. Aristotle and the Nature of EarthD. The Ptolemaic Universe

II. The Copernican RevolutionA. The Copernican ModelB. De Revolutionibus

III. Planetary MotionA. Tycho BraheB. Tycho Brahe's LegacyC. Kepler: An Astronomer of Humble OriginsD. Joining TychoE. Kepler's Three Laws of Planetary MotionE. The Rudolphine Tables

Outline

IV. Galileo GalileiA. Telescopic ObservationsB. Dialogo and Trial

V. Modern Astronomy

Outline (contd.)

The Roots of Astronomy• Already in the stone and bronze ages, human

cultures realized the cyclic nature of motions in the sky.

• Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance.

• Those monuments were probably used as

calendars or even to predict eclipses.

Newgrange, Ireland, built around 3200 B.C.:

Sunlight shining down a passageway into the central chamber of the mount indicates the day of winter solstice.

Stonehenge

• Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices

Summer solstice

Heelstone

• Constructed: 3000 – 1800 B.C.

• Probably used as calendar

Other Examples All Around the World

Chaco Canyon, New Mexico

Slit in the rock formation produces a sunlit “dagger” shape,

indicating the day of summer solstice

Other Examples All Around the World (2)

Mammoth tusk found at Gontzi, Ukraine: Inscriptions probably describing

astronomical events

Ancient Greek Astronomers (1)

• 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:

1. Geocentric Universe: Earth at the Center of the Universe

2. “Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles

Greeks assumed the Earth was not moving because they did not observe parallaxes in the sky.

Ancient Greek Astronomers (3)• Eudoxus (409 – 356 B.C.):

Model of 27 nested spheres

1. Imperfect, changeable Earth,

• He expanded Eudoxus’ Model to use 55 spheres.

2. Perfect Heavens (described by spheres)

• Aristotle (384 – 322 B.C.), major authority of philosophy until the late middle ages:

Universe can be divided in 2 parts:

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

Later refinements (2nd century B.C.) • Hipparchus: Placing the Earth away from the centers of the

“perfect spheres”

• Ptolemy: Further refinements, including epicycles

Epicycles

The Ptolemaic model was considered the “standard model” of the Universe until the

Copernican Revolution.

Introduced to explain retrograde (westward)

motion of planets

The Copernican Revolution

Nicolaus Copernicus (1473 – 1543):

Heliocentric Universe (Sun in the Center)

Copernicus’ New (and Correct) Explanation for the Retrograde Motion of the Planets

This made Ptolemy’s epicycles unnecessary.

Retrograde (westward) motion of a planet occurs when the Earth passes the planet.

Tycho Brahe (1546 – 1601)• High precision observations of the

positions of stars and planets

• Evidence against Aristotelian belief of

“perfect”, unchangeable

heavens

• Measurement of the nightly

motion of a “new star” (supernova)

showed no parallax

Tycho Brahe’s Legacy

New World model

• Sun and Moon orbit Earth;

Planets orbit the sun.

• Still geocentric (Earth in the center of the sphere of stars)

Johannes Kepler (1571 – 1630)

• Used the precise observational tables of Tycho Brahe to study

planetary motion mathematically.

1. Circular motion

• Planets move around the sun on elliptical paths, with non-uniform velocities.

• Found a consistent description by

abandoning both

2. Uniform motion

Kepler’s Laws of Planetary Motion

1.The orbits of the planets are ellipses with the sun at one focus.

Eccentricity e = c/a

c

Eccentricities of Ellipses

e = 0.02 e = 0.1 e = 0.2

e = 0.4 e = 0.6

1) 2) 3)

4) 5)

Eccentricities of Planetary OrbitsOrbits of planets are virtually

indistinguishable from circles:

Earth: e = 0.0167Most extreme example:

Pluto: e = 0.248

Planetary Orbits (2)

2. A line from a planet to the sun sweeps over equal areas in equal intervals of time.

Planetary Orbits (3)

3. A planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed:

Py2 = aAU

3(Py = period in years; aAU = distance in AU)

Galileo Galilei (1594 – 1642)

• Invented the modern view of science: Transition from a faith-based “science” to an observation-based science

• Greatly improved on the newly invented telescope technology, (But Galileo did NOT invent the telescope!)

• Was the first to meticulously report telescope observations of the sky to support the Copernican Model of the Universe

Major Discoveries of Galileo• Moons of Jupiter (4 Galilean moons)

• Rings of Saturn

(What he really saw)

Major Discoveries of Galileo (2)• Surface structures on the moon; first estimates

of the height of mountains on the moon

Major Discoveries of Galileo (3)

• Sun spots (proving that the sun is not perfect!)

Major Discoveries of Galileo (4)• Phases of Venus (including “full Venus”),

proving that Venus orbits the sun, not the Earth!

Historical Overview