CONCEPT SUMMARY- 2

  1. Cosmology based on science vs myth Early cosmologies were based on myths. Greek astronomers built a cosmology based on rational argument and observations - they built (mental) models based on mathematics (geometry).

  2. Lack of Parallax caused conviction that Earth could not be moving Despite very careful observations, the bright (presumably near stars) could not be detected to move relative to the background stars. Since it seemed unlikely that the stars could really be so very far away, the logical conclusion was that the Earth had to be stationary.

  3. Apparent vs. actual motion Copernicus argued that a heliocentric model could explain all the apparent motions of the Moon, Sun and planets by having the Sun at the center and describing the actual motion of Earth, Moon and planets. Kepler proposed that the actual motions were along ellipses instead of circles.

  4. For example, Retrograde motion of planets. Planets = "wanderers", appear to move relative to stars. Occasionally, planets appeared to move backwards across the sky = retrograde. Explaining retrograde motion of planets - particularly Mars was key to the revolution.

    5.

  5. Ptolemy (140 AD) - Geocentric system of epicycles. Earth at center - geocentric. Planets follow circles on circles - epicycles
    6.

  6. Copernicus (1473-1543) - Heliocentric system (still circles). Argued for heliocentric system with Sun at the center. More rational explanation and philosophy than hard evidence. Claimed motions of stars, Sun & planets have apparent motion which is mostly due to the actual motion of the Earth.

  7. Tycho Brahe (1571 - 1630) . Good observations -> better data. Set up large observatory in Denmark and made accurate measurements of the retrograde motion of Mars. Lost nose in duel, got silver false nose, died due to burst bladder - no, you will not be asked about this on the test.

  8. Kepler (1571 - 1630) - 3 Laws of planetary motion

    9.

    1st - Planets move along Ellipses rather than circles (with Sun at one focus)

    2nd - Planet moves faster when closer to the Sun

    3rd - Orbital Period (in years) related to semi-major axis (in AU) of orbit P2 = a3

  9. Observational evidence separates models With inaccurate data, either geocentric (epicycles) or heliocentric (circles or ellipses) could fit the data equally well. After Tycho Brahe made more accurate measurements of the retrograde motion of Mars, it became clear that the geocentric model only works if you have many epicycles. The heliocentric model, using Kepler's 3 laws of planetary motion, was able to match the data with a relatively simple model.

  10. Galileo (1564- 1642) - Used observations of the phases of Venus to prove the heliocentric layout of the solar system (Fig. 6.13). Used a telescope to look at astronomical objects and detected: "blemishes" the Moon, rotation of sunspots on the Sun, the 4 large moons of Jupiter. Showed that celestial objects are not "perfect" spheres and that at least some objects (moons of Jupiter, Venus) orbit objects other than the Earth - arguing against geocentric system.

  11. Newton (1642-1727) - Devised his Laws of Motion and of Gravity to explain WHY planets followed Kepler's laws. Newton's Laws of Motion apply to ALL objects under ALL forces (all 4 of them, not just gravity) - nearly always*

  12. Definitions
    13.

      Speed = Distance / Time

      Speed & Direction -> Velocity

    Change in velocity = Acceleration = change in speed, or direction, or both

    Inertia = reluctance of object to change motion

    Mass = measure of inertia -> more mass, greater inertia

  13. Newton's 3 Laws of Motion

    14.

    1st - A body at rest or in uniform motion tends to stay so
        Think of a stone skipping across ice (where there is little frictional forces)
    2nd - A body accelerates when a force is applied
        Acceleration = Force / Mass of bodyOr Force = Mass x Acceleration The greater the force, the greater the acceleration. Double the force and the acceleration is twice as big. For a given force, the acceleration will be greater for a smaller mass and less for a greater mass.
    3rd - For every action there is an equal and opposite reaction

Key Words: Cosmology, geocentric, heliocentric, retrograde motion, epicycle, ellipse, focus, semi-major axis, orbital period, speed, velocity, acceleration, inertia, mass, force, laws of motion, action, reaction.

*Einstein discovered some circumstances under which Newton's Laws do not work - but these are extreme conditions.