1. Early Astronomers Tycho Brahe Galileo Galilei Johannes Kepler Nicholas Copernicus Edmund Halley Sir Isaac Newton

2. Nicholas Copernicus Polish – 1473-1543 Concluded Earth was a planet Developed the theory that Earth moves around the Sun with the other planets. Model of the solar system with the Sun at the center He couldn’t prove his theory Used circles to predict the orbits of the planets, so the planets orbits strayed from predicted positions

3. Tycho Brahe Danish 1546-1601 Persuaded King Fredrick the II to build an observatory in Copenhagen before telescopes were invented (used his eyesight) Designed and built instruments (astrolabes and quadrants) to accurately measure locations of heavenly bodies. Observed a supernova (exploding star) Observations, especially of Mars, were far more precise than any previously made.

4. Galileo Galilei Italian 1564-1642 Made improvements to the telescope 3 times magnification to look at the moon, sun and planets out to Saturn Discovered the moon has mountains and craters, the Sun has sunspots, planets were circular discs, not points of light, Venus has phases like the moon and that 4 moons (satellites) orbit Jupiter Catholic church believed his theories went against the bible.

5. Johannes Kepler German 1571-1630 Discovered 3 laws of planetary motion using Brahe’s data of Mars while working as his assistant. 1.The path of each planet around the sun is an ellipse 2.Planets travel more rapidly as they near the Sun and slow as they become more distant 3.Planets orbital period squared is equal to it’s mean solar distance cubed (T2 = d3). Solar distances of planets can be calculated

6. Sir Isaac Newton English 1642-1727 Used Kepler’s work to formulate and test the universal theory of gravity  Each one of every pair of bodies pulls on the other, the strength of the pull (gravity) depends on the amount of matter in the objects and the distance between them – This explained how planets and their moons could remain in space while revolving around the Sun.

7. Edmund Halley English 1656-1742 Calculated the orbit of a comet observed in 1682 and proved it was the same comet observe in 1531 and 1607. Predicted the comet would again return in 1758. Mapped the stars of the southern hemisphere Studied the Moon’s effect on tides Measured the distance from Earth to the Sun Convinced Sir Isaac Newton to publish his findings