1. Astronomy
Astronomy is the study of the planets and other objects in space.
The “Golden Age of Astronomy” occurred during 600 – 150 B.C. when the ancient Greeks used observations, instruments, and geometry and trigonometry to make the first discoveries of space.
During this time, Aristotle (384 – 322 B.C.) was the first person to determine that Earth was round based on the curved shadow left on the surface of the moon when Earth fell between the sun and the moon. His belief was abandoned in the Middle Ages.

2. Models of the Solar System
Geocentric Model – The Greeks determined that the moon, the sun, and the known planets at that time (Mercury, Venus, Mars, and Jupiter) orbited Earth.
Heliocentric Model – Aristarchus ( B.C.) was the first Greek to believe in a sun-centered universe.
Ptolemaic System – Claudius Ptolemy published his 13-volume work in 141 A.D. He used the geocentric model, but was the first to explain retrograde motion (the planets move eastward in the sky, appear to stop, move backwards, and then resume its eastward movement).

3. The Birth of Modern Astronomy
Nicolaus Copernicus ( ) – The first major astronomer to make advancements since Ptolemy. He was the first to determine that Earth is a planet and that the sun is the center of the universe.
Tycho Brahe ( ) – He convinced King Frederick II to build an observatory in Copenhagen. Here, Brahe invented measuring devices (telescopes had not yet been invented) and made the most accurate observations of several planets, especially Mars.

4. The Birth of Modern Astronomy (2)
Johannes Kepler ( ) – Studied under Brahe and continued his work by using math to develop 3 laws of planetary motion.
The path of each planet around the sun is an ellipse. There are 2 “centers” – 1 at the sun and 1 at the other end of the ellipse.
There is an imaginary line over which the planet travels. This ellipse is traveled in the same time interval every time it orbits and the planet moves more quickly when traveling around the sun and slower at the other end of the ellipse.
P2 = a3 (P = the time it takes the planet to complete 1 orbit; a = distance of the planet to the sun)

5. The Birth of Modern Astronomy (3)
Gallileo Galilei ( ) – was the great Italian astronomer and was the first to use the telescope. He made 5 major discoveries.
The discovery of 4 satellites (moons) around Jupiter.
The discovery that planets are circular disks, not just points of light.
Venus has phases (just like our moon does).
The sun has sunspots (dark spots).

6. The Birth of Modern Astronomy (4)
Sir Isaac Newton ( ) – developed the Law of Universal Gravitation.
The gravitational pull is a result of the mass of the object and the distance between 2 objects.
The greater the mass of an object, the greater the gravitational pull. (Weight does not mean the same as mass – the mass of an object will never change; however, the weight of an object is a result of the mass and the gravitational pull of the planet. This is why your weight will change when you are on another planet.)
The force of gravity and the tendency to orbit on a straight-line is the result of the elliptical orbits that were discovered by Kepler.

7. Assignment
Solar System Introduction Worksheet