1. Chapter 1: Origins of Modern Astronomy
22.1: Early Astronomy

2. Ancient Greeks and Astronomy
Astronomy is the science that studies the universe.
The first accurate European astronomers were the ancient Greeks.

3. Aristotle ( 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.

4. 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.

5. 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.

6. The Geocentric Model
In this model, the moon, sun, and the known planets – Mercury, Venus, Mars, Jupiter and Saturn – orbit earth.
Every other body in space circled this system on their own transparent, hollow sphere.
This was called the celestial sphere.

7. A modern version of an earth-centered system.

8. The Heliocentric Model
Aristarchus ( B.C.E.) was the first Greek to believe in a sun-centered, or heliocentric, universe.
In the heliocentric model, Earth and other planets orbit the sun.

9. Aristarchus was condemned by his own religious leaders for his theory.
Modern Greece, however, honors him, and even puts his theories on stamps…

10. The 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.

11. Ptolemy was wrong – the planets do not orbit Earth.
Yet although he used the geocentric model, he did try explain the planets’ apparent motions.

12. The Birth of Modern Astronomy
The first great astronomer after the Greeks was Nicolaus Copernicus ( ) of Poland.
Copernicus proposed the heliocentric model of the solar system: Earth is a planet and all planets of the solar system revolved around the sun at its center.

13. Heliocentric Model of the Solar System

16. 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.

18. Johannes Kepler (1571-1630) was Brahe’s assistant, and later the first important modern astronomer.
Kepler applied mathematics to Brahe’s findings and discovered Three Laws of Planetary Motion.

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

20. 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.

21. 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.

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

23. Third Law: Keppler’s third law gives the precise relation between the distance of a planet from the Sun and how fast it completes an orbit, using Astronomical Units (AU).
One AU equals 150 million km, the average distance of the Earth from the Sun.

24. Galileo Galilei ( )
The greatest Italian scientist of the Renaissance
One of the most brilliant humans who ever lived.

25. Galileo not only provided evidence that proved the heliocentric model accurate, but he also invented pendulum clocks and the modern thermometer.

26. 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.

27. Galileo used his telescope to view the universe in a new way
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.)

28. Planets are NOT pinpoints of light.
They are actually spheres, like Earth.

29. Therefore, Venus circles the Sun.
3. Venus has phases, just like the moon.
Therefore, Venus circles the Sun.

30. 4, The moon’s surface is not smooth.
(Galileo thought the moon’s dark areas might be seas.)

31. 5. The sun has sunspots, or dark regions.
Galileo tracked the movement of these sunspots and estimated the rotational period of the sun.

33. Sir Isaac Newton ( )
The first one to explain that planets are held in their orbit by a force:
Universal Gravitation
Mathematician, Inventor, General Genius

34. Universal Gravitation
Every body in the universe attracts every other body with a force directionally proportional to their masses.
Gravitational forces decrease with distance.
The greater the mass of the object, the greater its gravitational force.

35. Newton also improved on Galileo’s original telescope by adding mirrors, allowing it to be much shorter.