Chapter 3: Motion of Astronomical Bodies

1. Chapter 3:Motion of Astronomical Bodies

2. Observed Motion of the Planets At its core, astronomy is an observational science. Thus, we must start by simply observing. After that, we can try to explain why things move as they do.

3. The ancient Babylonians were the first to make records of celestial motions

4. The Ancient Greeks were the first to try to Explain Celestial Motions Sun, Moon and stars are no problem. Each rotates around Earth on a giant crystal sphere. This is known as the Geocentric model. The Physics of Aristotle All celestial motions must be circular

5. The motions of the planets were difficult to explain The Ptolemaic (ancient Greek) Model Earth is at the center of everything. All celestial motions are circular (as per Aristotle). Circles on top of circles gives a reasonably close description of planetary motions. Watch ClassAction Renaissance Astronomy module Animation Ptolemaic Orbit of Mars

6. Nicolaus Copernicus 1453 – 1543 Moved Earth out of the center and placed the Sun there instead. The Earth was just another planet moving around the Sun.

7. The Copernican Solar System De RevolutionibusOrbiumCoelestium was published in 1543, the year of his death. The orbits were still circles so he still needed epicycles but they were very small. Still, there were problems, it wasn’t perfect. Watch Museo Galileo Copernican System video

8. The Copernican system explained retrograde motion in a much simpler way Watch ClassAction Renaissance Astronomy module Animation Retrograde Motion

9. Tycho Brahe: The Father of Observational Astronomy 1546 – 1601

10. The Observatory at Uraniborg In the late 1500’s, if you wanted to study astronomy with the best, you went to Uraniborg on the Danish island of Hven

11. Johannes Kepler 1571 – 1630 Tycho hired Kepler to come work for him as an assistant. Kepler couldn’t see very well so his job was to make calculations. When Tycho died Kepler took the data from Tycho’s heirs and used them to develop his Laws of Planetary Motion.

12. Kepler had pre-existing ideas about how the solar system worked Kepler’s planetary spheres were nested inside the five perfect geometrical solids Once he started his work on the data, he let the data lead him to the answer rather than trying to force it to his pre-existing idea

13. Kepler’s Laws of Planetary Motion are empirical laws The laws are based on the observational data of Tycho Brahe. They were derived to fit the data. They do not try to explain why the planets move as they do.

14. Kepler’s 1st Law The Law of Ellipses The planets move in elliptical orbits with the Sun located at one focus Check out Eccentricity Demonstrator In ClassAction Renaissance Astronomy module Animations This was a major change since everyone that came before had used circles, including Copernicus

15. Kepler’s 2nd Law The Law of Areas A line drawn from a planet to the Sun will sweep out equal areas in equal time periods Check out Planetary Orbit Simulator in ClassAction Renaissance Astronomy module Animations

16. Kepler’s 3rd Law:The Law of harmonies The ratio of the square of the orbital period to the cube of the orbital semimajor axis (the radius) is the same for all the planets

17. Galileo Galilei was a contemporary of Kepler’s Galileo preformed physics experiments and developed new Physics to replace the old physics of Aristotle

18. Galileo discovered the Law of Falling Bodies Watch Apollo 15 Feather and the Hammer video

19. Astronomical Discoveries of Galileo Lunar Observations Sunspots Saturn Drawing Phases of Venus Watch ClassAction Renaissance Astronomy Phases of Venus animations

20. Galileo sent his observations of Jupiter to Kepler to verify Kepler’s Laws The ratio of the square of the orbital period to the cube of the orbital radius was the same for all four bodies orbiting Jupiter but it wasn’t the same constant as the planets orbiting the Sun

21. It took 50 years after Galileo and Kepler before Isaac Newton explained why things moved as they did on Earth and in the heavens 1642 – 1727

22. Newton’s 1st Law of Motion The Law of InertiaAn object in straight line uniform motion will continue that motion unchanged unless some external force acts on it This law was based entirely on the work of Galileo

23. Newton’s Second Law The Force Law: F = maThe acceleration a body experiences is directly proportional to the net force acting on it and inversely proportional to its mass

24. There can be an acceleration even when there isn’t a change in speed

25. Mass plays an important part in Newton’s 2nd Law

26. Newton’s Third Law The Action-Reaction LawFor every force there is an equal and opposite reaction force

27. The Action-Reaction forces always apply to different objects While the force on each object may be the same, the acceleration (and thus damage) each experiences depends inversely on their mass according to Newton’s 2nd Law