Introduction to Astrophysics Aristotle, Kepler, Brahe, Newton and the other guys

Bibliography The History of Modern Astronomy http://csep10.phys.utk.edu/astr161/ Cavendish Experiment http://en.wikipedia.org/wiki/Cavendish_experiment Brief History of the Universe http://filer.case.edu/~sjr16/advanced/cosmos_history.html Electroweak Theory http://www.answers.com/topic/electroweak-force Newton’s Law of Universal Gravitation http://www.slideshare.net/tufdaawg/universal-gravitation, www.nd.edu/~dhoward1/30389s06%20Fox%20Gravitation.ppt, bohr.winthrop.edu/faculty/mahes/link_to_webpages/courses/phys201/lecture4.2.ppt Newton’s Secret http://www.pbs.org/wgbh/nova/elegant/media2/3012_q_03.html

But how did we get here? What we know now Four types of forces Sun-centred solar system Many other solar systems, galaxies, nebulae Big Bang theory Theories of special and general relativity But how did we get here?

Types of Forces: Fundamental forces: Gravitational force Strong nuclear force Weak nuclear force-----| Electromagnetic force--|—Electroweak force Non-fundamental forces: Pushing, Pulling, Friction, Tension, etc…. These are related to the electromagnetic force. They arise from the interactions between the electrically charged particles that comprise atoms and molecules.

Fundamental Forces Fundamental Force Example Particles Affected Relative Strength Strong nuclear Nuclear fission & fusion Nuclear 1 Electromagnetic Static Electricity, chemical reactions, friction Charged 10-2 Weak nuclear Radioactivity 10-15 Gravitational Your weight All 10-38

Unification of Fundamental Forces (. ) http://hyperphysics. phy-astr

Motion in the Heavens and on Earth We know how objects move on Earth. We can describe and even calculate projectile motion. Early humans could not do that, but they did notice that the motions of stars were quite different. Stars moved in regular paths. Planets moved in more complicated paths. Many ancient cultures had explanations for their observations about the sky, often combining their observations with religious rites and ideas.

The Celestial Sphere Pre 1500: Succession of giant spheres with all bodies in sky stuck on them The heavens are “perfect” The Earth is at the centre of the universe Aristotle & Ptolemy: http://csep10.phys.utk.edu/astr161/lect/retrograde/aristotle.html Problems: Planets have varying brightness and retrograde motion

Tycho Brahe Age 14 observed an eclipse of the sun August 21, 1560. The date of the event was off by two days as predicted in all of the books of the time, so Brahe decided to become an astronomer to make accurate observations and predictions.

Brahe’s Interesting life story… Became royal astronomer of Denmark Given the island of Hven (now Swedish) for observatory, Uraniborg Lost nose in a duel, wore gold or silver prosthesis Designed extremely accurate astronomical instruments Made incredibly accurate observations, later used to formulate several important theories

Brahe’s story (cont’d) In 1597, after falling out of favor with his sponsor, Brahe moved to Prague. Became the astronomer to the court of Emperor Rudolph of Bohemia. Johannes Kepler became one of his assistants.

Brahe and Kepler New ideas: Copernicus’ sun-centred universe (!) Scientific community and public divided Brahe: Aristotelian, geocentric (Earth-centred) model Kepler: interested in Copernican, heliocentric model

Brahe’s conclusions Brahe made excellent observations of parallax for stars, and could find none. He thus concluded that either “the earth was motionless at the center of the Universe, or the stars were so far away that their parallax was too small to measure.”

Brahe’s conclusions Proposed intermediate model of solar system, between the Ptolemaic and Copernican models (geocentric). Was widely accepted for a time, although ultimately proved incorrect Thus, Brahe's ideas about his data were not always correct, but the quality of the observations themselves was central to the development of modern astronomy.

Kepler’s laws Kepler was convinced that geometry and mathematics could be used to explain the number, distance, and motion of the planets. He used Brahe’s data to formulate his three laws, which apply to ALL planets, satellites, heavenly bodies and are used to this day.

Kepler’s laws of planetary motion 1. The paths of the planets are ellipses with the sun at one focus. This was a very big deal at the time!

Kepler’s laws of planetary motion 2. An imaginary line from the sun to a planet sweeps out equal areas in equal time intervals. Thus, the planets move faster when closer to the sun. … also kind of a big deal

Kepler’s Second law: another view

Kepler’s laws of planetary motion 3. The square of the ratio of the periods of any two planets revolving about the sun is equal to the cube of the ratio of their average distances from the sun. (Hunh?)

On your formula sheet, Kepler’s constant

Note that the first two laws apply to each planet, moon, or satellite individually. The third law, however, relates the motion of several satellites about a single body.