Chapter 1: Chapter 1: Origins of Modern Origins of Modern

AstronomyAstronomy

22.1: Early Astronomy22.1: Early Astronomy

Ancient Greeks and Ancient Greeks and AstronomyAstronomy

• AstronomyAstronomy is the is the science that studies science that studies the universe.the universe.

• The first accurate The first accurate European European astronomers were astronomers were the ancient Greeks.the ancient Greeks.

Aristotle (384 - 322 B.C.E.) concluded that the earth was round because it always cast a curved shadow when it passes between the sun and the moon.

Aristotle’s belief that the Earth is round was abandoned by the Middle Ages.

Eratosthenes (276 – 194 B.C.E.), an ancient Greek mathematician, calculated that the earth’s circumfer-ence is 39,000 kilometers - very close to our own modern measure-ment of 40,075 km.

The Greeks thought that the Earth was a sphere that stayed motionless at the center of the universe. The other planets and stars revolved around the Earth on their own hollow spheres.

The Geocentric ModelThe Geocentric Model

• In this model, the moon, sun, and the In this model, the moon, sun, and the known planets – Mercury, Venus, Mars, known planets – Mercury, Venus, Mars, Jupiter and Saturn – orbit earth.Jupiter and Saturn – orbit earth.

• Every other body in space circled this Every other body in space circled this system on their own transparent, system on their own transparent, hollow sphere.hollow sphere.

• This was called the This was called the celestial spherecelestial sphere..

A A modern modern version version

of an of an earth-earth-

centered centered system.system.

The Heliocentric ModelThe Heliocentric Model

• Aristarchus (312-230 Aristarchus (312-230 B.C.E.) was the first B.C.E.) was the first Greek to believe in a Greek to believe in a sun-centered, or sun-centered, or heliocentricheliocentric, universe., universe.

• In the heliocentric In the heliocentric model, Earth and other model, Earth and other planets orbit the sun.planets orbit the sun.

Aristarchus was condemned by his Aristarchus was condemned by his own religious leaders for his theory.own religious leaders for his theory.

Modern Greece, however, honors him, and Modern Greece, however, honors him, and even puts his theories on stamps…even puts his theories on stamps…

The Ptolemaic SystemThe Ptolemaic System

• Claudius Ptolemy (c. 90 – 168 C.E.) is our main source for what the ancient Greeks knew about astronomy.

• He also attempted to explain retrograde motion, or how each planet appears sometimes to stop in the night sky, reverse direction, and then resume eastward motion.

Ptolemy was wrong – the planets do not orbit Earth.

Yet although he used the geocentric model, he did try explain the planets’ apparent motions.

The Birth of Modern Astronomy

• The first great astronomer after the The first great astronomer after the Greeks was Greeks was Nicolaus CopernicusNicolaus Copernicus (1473-1543) of Poland.(1473-1543) of Poland.

• Copernicus proposed Copernicus proposed the heliocentric the heliocentric modelmodel of the solar system: Earth is a of the solar system: Earth is a planet and all planets of the solar planet and all planets of the solar system revolved around the sun at its system revolved around the sun at its center. center.

Heliocentric Model of the Solar SystemHeliocentric Model of the Solar System

Tycho Brahe (1546-1601) had the King of Denmark build him an observatory.

Although the telescope had not been invented, Brahe’s findings were extremely precise for the time.

Johannes KeplerJohannes Kepler (1571-1630) was (1571-1630) was Brahe’s assistant, Brahe’s assistant, and later the first and later the first important modern important modern astronomer. astronomer.

Kepler applied Kepler applied mathematics to mathematics to Brahe’s findings Brahe’s findings and discovered and discovered Three Laws of Three Laws of Planetary MotionPlanetary Motion..

• First Law: The path of each planet around the sun is an ellipse, with the sun at one focus.

How is an ellipse different from a circle?

• A circle is a closed curved shape that is flat. In a circle, all points on the circle are equally distant from the center of the circle.

•An ellipse is also a closed curved shape that is flat.

•Instead of having all points the same distance from the center (like a circle), an ellipse has two focus points.

• Second Law:Second Law: Kepler determined that a Kepler determined that a planet travels most rapidly when it comes planet travels most rapidly when it comes closest to the Sun and moves slowest closest to the Sun and moves slowest when farthest away.when farthest away.

Third LawThird Law: Keppler’s third law gives the : Keppler’s third law gives the precise relation between the distance of a precise relation between the distance of a planet from the Sun and how fast it planet from the Sun and how fast it completes an orbit, using Astronomical completes an orbit, using Astronomical Units (AU).Units (AU).

One AU equals 150 million km,

the average distance of the Earth from the

Sun.

Galileo Galilei Galileo Galilei (1564-1642)(1564-1642)

The greatest Italian

scientist of the

Renaissance

One of the most brilliant humans who ever lived.

• Galileo not only provided

evidence that proved the

heliocentric model accurate,

but he also invented

pendulum clocks and the modern

thermometer.

• He also created one of the first telescopes, without anything but a written description to guide him. He even ground glass for its lenses himself.

Galileo used his telescope to view the universe in a new way. He made important discoveries that supported Copernicus’ heliocentric model of the universe.

1. The discovery of the largest four moons of Jupiter.

(He also discovered Saturn’s rings.)

2.Planets are NOT pinpoints of light.

They are actually spheres, like Earth.

3. Venus has phases, just like the moon.

Therefore, Venus circles the Sun.

4, The moon’s surface is not smooth.

(Galileo thought the moon’s dark areas might be seas.)

5. The sun has sunspots, or dark regions.

Galileo tracked the movement of these sunspots and estimated the rotational period of the sun.

Sir Isaac Newton (1642-1727)Sir Isaac Newton (1642-1727)

Mathematician, Inventor, General GeniusMathematician, Inventor, General Genius

•The first one to explain that planets are held in their orbit by a force:

Universal Gravitation

Universal GravitationUniversal Gravitation

1. Every body in the universe attracts every other body with a force directionally proportional to their masses.

2. Gravitational forces decrease with distance.

3. The greater the mass of the object, the greater its gravitational force.

Newton also improved on

Galileo’s original telescope by adding

mirrors, allowing it to be much shorter.