1. The Solar System Explorers

2. Ancient Greeks The ancient greeks noticed that the stars would move across the sky, but would stay in the same position relative to each other. The ancient greeks noticed that the stars would move across the sky, but would stay in the same position relative to each other. They came up with the word planet which means “wandering star”. They came up with the word planet which means “wandering star”. Many greek scientists, such as Aristotle, believed that the universe is perfect and finite and that Earth is stationary at its center. Many greek scientists, such as Aristotle, believed that the universe is perfect and finite and that Earth is stationary at its center.

3. Geocentric System “geo”= greek word for Earth “geo”= greek word for Earth In a geocentric system, Earth is at the center of the revolving planets. In a geocentric system, Earth is at the center of the revolving planets.

4. Ptolemy (A.D. 140) Ptolemy was a greek astronomer who, like the earlier greeks, thought that the Earth was at the center of the universe. Ptolemy was a greek astronomer who, like the earlier greeks, thought that the Earth was at the center of the universe. He agreed that the other planets rotated around Earth, and explained that the planets move in little circles that move in bigger circles. He thought this explained why the planets seem to move at different speeds and even backwards. He agreed that the other planets rotated around Earth, and explained that the planets move in little circles that move in bigger circles. He thought this explained why the planets seem to move at different speeds and even backwards. People believed Ptolemy's theory for the next 1400 years. People believed Ptolemy's theory for the next 1400 years.

5. Copernicus (early 1500's) Copernicus was a polish astronomer who thought that the sun was at the center of the system of planets. Copernicus was a polish astronomer who thought that the sun was at the center of the system of planets. In a heliocentric system, Earth and the other planets (Mercury, Venus, Mars, Jupiter and Saturn) revolve around the sun. In a heliocentric system, Earth and the other planets (Mercury, Venus, Mars, Jupiter and Saturn) revolve around the sun. Helios is greek for “sun”. Helios is greek for “sun”.

6. Checkpoint How is Copernicus's description of the system of planets different from Ptolemy's description? How is Copernicus's description of the system of planets different from Ptolemy's description?

7. Galileo An Italian astronomer who supported Copernicus's heliocentric explanation and disproved Ptolemy's theory. An Italian astronomer who supported Copernicus's heliocentric explanation and disproved Ptolemy's theory. Was the first to use a telescope to look at objects in the sky and made 2 discoveries with the telescope that helped support the heliocentric theory. Was the first to use a telescope to look at objects in the sky and made 2 discoveries with the telescope that helped support the heliocentric theory. 1. He saw four moons revolving around Jupiter, which proved that not everything in the sky revolves around Earth. 1. He saw four moons revolving around Jupiter, which proved that not everything in the sky revolves around Earth. 2. He discovered that Venus goes through phases similar to moon phases which could not be explained if Earth were at the center of the universe. 2. He discovered that Venus goes through phases similar to moon phases which could not be explained if Earth were at the center of the universe.

8. Checkpoint What 2 discoveries made by Galileo supported the heliocentric description of the solar system? What 2 discoveries made by Galileo supported the heliocentric description of the solar system?

9. Circles? Copernicus and Galileo were both right, the Sun is at the center of our solar system, but Copernicus still falsely believed the orbits of the planets were circles just like Ptolemy believed. Copernicus and Galileo were both right, the Sun is at the center of our solar system, but Copernicus still falsely believed the orbits of the planets were circles just like Ptolemy believed. Tycho Brahe, a Danish astronomer observed the positions of the planets for about 20 years and gathered a lot of data. Tycho Brahe, a Danish astronomer observed the positions of the planets for about 20 years and gathered a lot of data.

10. Johannes Kepler (1600) Kepler, a German mathematician, used Brahe's data to study the planet's orbits thinking at first that the orbits were all circles. Kepler, a German mathematician, used Brahe's data to study the planet's orbits thinking at first that the orbits were all circles. He discovered, through trying to figure out the exact orbit of Mars, that the orbit of each planet is an ellipse. He discovered, through trying to figure out the exact orbit of Mars, that the orbit of each planet is an ellipse. Ellipse=an elongated circle or oval shape Ellipse=an elongated circle or oval shape ellipse

11. Isaac Newton Newton was an English scientist who concluded that 2 factors combine to keep the planets in orbit: inertia and gravity. Newton was an English scientist who concluded that 2 factors combine to keep the planets in orbit: inertia and gravity.

12. Inertia and Gravity Galileo had discovered inertia, which is the tendency of a moving object to continue in a straight line or a stationary object to stay in place. (more mass=more inertia) Galileo had discovered inertia, which is the tendency of a moving object to continue in a straight line or a stationary object to stay in place. (more mass=more inertia) Newton while watching an apple fall from a tree discovered gravity which causes objects in space to pull toward one another. Newton while watching an apple fall from a tree discovered gravity which causes objects in space to pull toward one another.

13. Planets are in orbit around the sun because the sun's gravity pulls on them while their inertia keeps them moving ahead. Therefore, the planets keep moving around the sun and end up in orbit. Planets are in orbit around the sun because the sun's gravity pulls on them while their inertia keeps them moving ahead. Therefore, the planets keep moving around the sun and end up in orbit.

14. Checkpoint What shape are the orbits of the planets? How was the discovery of this orbit shape made? What shape are the orbits of the planets? How was the discovery of this orbit shape made? What 2 factors work together to keep the planets in orbit around the sun? What 2 factors work together to keep the planets in orbit around the sun?

15. Other discoveries... Ancient greeks only knew of Mercury, Venus, Earth, Mars, Jupiter and Saturn, we now know of Uranus, Neptune and a handful of dwarf planets including Pluto. Ancient greeks only knew of Mercury, Venus, Earth, Mars, Jupiter and Saturn, we now know of Uranus, Neptune and a handful of dwarf planets including Pluto. As technology is advancing, our understanding of the solar system continues to change thanks to the use of telescopes and space probes to explore the unknown. As technology is advancing, our understanding of the solar system continues to change thanks to the use of telescopes and space probes to explore the unknown.