Beginning of Modern Astronomy

• Copernicus

  Made Ptolemy's system simpler by putting the sun at the center of the solar system. Retained epicycles in a modified form.

• Tycho Brahe

  Improved observational data to the point where Copernicus' epicycle scheme was clearly insufficient.

• Kepler

  Found the key to the planetary motions; planetary paths are ellipses, not combinations of circles. His system of prediction is the basis of that in use today.

  We will repeatedly make use of his second and third laws as they were confirmed by Newton. Kepler's third law relates each planet's distance from the sun to the time it takes that planet to go once around. It says 'divide the cube of the distance by the square of the time, and the number you get is the same for one planet as it is for every other one.' The number, Newton showed, is the mass of the sun. The law applies to all satellites of any central object and is the primary way of measuring masses of celestial objects, from planets to clusters of galaxies. Kepler's second law describes how the speed of a planet changes as it moves around the sun. The planet goes faster when it's close and slower when farther away. It's sometimes phrased 'the planet sweeps equal areas n equal times'; in physics it is known as 'conservation of angular momentum'; we will apply it to other kinds of angular motion besides planetary.

• Galileo

  1) Broke with the tradition of Aristotle's word as law, and actually did experiments. (Most famous: Aristotle said that (absent air) heavy things fall faster. Galileo tried it and found that heavy things and light things fall equally fast.)

  2) Made the first important astronomical use of the telescope, and with it, made many new observations.

  3) Elucidated the law of inertia.

• Newton
  • Proposed a general theory of mechanics, which applied to all phenomena, earthly (``terrestial'') as well as celestial, and showed that Kepler's laws were only a special case of this general theory.

  • He suggested that gravity was universal, and that it was the force responsible for holding the planets near the sun.

  • He recognized that artificial satellites were possible and that the motions of satellites could be thought of as ordinary falling.

  • Other implications of universal gravitation:

    1) tides are caused by the moon pulling on the earth and the oceans, and the two responding differently. (Test this idea: compare lunar and tidal periods.)

    2) the earth wobbles like a top; this causes the direction in which the axis points to change slowly but regularly. The wobble manifests itself as the "precession of the equinoxes": the place where the ecliptic intersects the celestial equator slowly moves around the equator. One "wobble" takes 26,000 years and it is caused by the moon pulling on the non-spherical earth's equatorial bulge.