ISP205 Section 2
Lecture#2: Thursday, January 11

Handouts:

Topics:

  1. Review
  2. Motion of Sun and Stars - Celestial Sphere
  3. Motion of Planets
  4. Astronomy and Astrology - The concept of science

 

  1. Review
    1. Light travels at a fixed speed of 300,000 km/s
    2. Looking far away is like looking back in time
    3. Astronomical distances
      1 AU = distance sun-earth (average) (1.5x1011 m)
      1 light year = distance light travels in a year (9.5x1015 m)
      size of the universe: ~14,000,000,000 light years
  2. Celestial Sphere
    1. The celestial sphere is an imaginative sphere surrounding the earth with the
      stars attached to it.
    2. The concept works because the stars are so far away.
      Therefore Practically no changes in relative positions of stars from earth rotation

      (there is a very small parallax for stars not too far away from the rotation
      of the earth around the sun - only noticable with large telescopes,
      will be discussed in next lecture)
    3. The Motion of the stars as seen from earth can be described as the rotation of
      the celestial sphere (clockwise if seen from above celestial north pole).
      (Demo: celestial sphere) (figure 1.2)
    4. Celestial north and south pole: continuation of earth's rotation axis through celestial sphere
      Celestial equator                   : Cut of earth's equatorial plane through celestial sphere
      Zenith                                   : point on celestial sphere directly above observer
    5. The stars move on circular orbit around celestial north (or south) pole.
      A star directly at the pole is not moving at all. At north pole there is one: Polaris
      Figure 1.4. (Redshift Demo)

      Observer on northern hemisphere at different latitude sees celestial north pole at different
      altitudes in the sky. Altitude = Latitude.
    6. Ecliptic: Path of the Sun (not fixed to celestial sphere because its not far away -
      we see earths rotation around sun as a movement  of the sun on the celestial sphere.
      (Demo: celestial sphere) (see Figure 1.6)
    7. Only stars in opposite direction of suns position are visible because sun is so bright
      (we see the stars only at night). Therefore we see different stars at different times
      of the year (winter sky, summer sky)

      The sun moves through a sequence of constellations called the zodiac (old sequence used for
      Astrology - discussed later).
      (see Figure)
    8. Points on the path of the sun on the celestial sphere:
      Summer solstice : Sun highest in the sky (different times for northern and southern hemisphere !)
      Winter   solstice : Sun lowest in the sky  (different times for northern and southern hemisphere !)
      Vernal equinox  : Sun crosses equator from southern to northern hemisphere (everywhere same time)
      Atumnal equinox: Sun crosses equator from northern to southern hemisphere (everywhere same time
           - even though its called Autumnal equinox it is always at the same time - in September, which is spring
              on the southern hemisphere !)
    9. Precession: the celestial north and south poles move slowly because of the precession of earths
      rotational axis. One rotation 26000 years.
      (see Figure)
      (Demo: precession of spinning top)

      Polaris will move away from north pole. Then no star pointing to north available anymore until Vega
      will moves in around 14000 AD (see Figure)

      Therefore also the Zodiac signs change. Signs used in Astrology were determined by Ptolemy around
      150 AC when he wrote his Tetrabiblios). So in early January the Sun is in Sagittarius, not in
      Capricorn
      (Redshift Demo)
  3.  Motion of Planets
    1. Complicated as motions of planets and earth around the sun add together.
    2. Always close to ecliptic as planets rotate in one plane (except Pluto)
    3. Retrograde motion: sometimes Planets change direction in their movements on celestial sphere
      (Redshift Demo) (Redshift Presentation)
  4. Astrology, Astronomy, Science

    Astronomy is Science (taught in this class)
    Astrology is not Science and sometimes even Pseudo Science (not taught in this class)
    1. The scientific method:
      1. Arrive at a Hypothesis
        For example by Induction from observations with some creativity. Different
        philosophical schools disagree on how this is done)
      2. Deduce as many testable predictions as possible
        (for complicated hypothesis - or several a model might be needed)
      3. Compare them with observations (reproducable by many scientists)
      4. If they agree - go back to 2
        If sufficient evidence is accumulated the hypothesis becomes a theory.
      5. If they don't agree refine Hypothesis until the do - go then back to 2 using the
        refined hypothesis.
        If refinement is not possible - reject Hypothesis and create new one
        (go back to 1)

       

    2. The main characteristics of the scientific method are:
      1. Many competing scientists try to carry out the experiments or observations that
        provide the most critical tests for a hypothesis or a theory.
      2. Scientists are ready to reject any hypothesis or  theory that does not agree with the facts
      3. Only observed facts are important and all observed facts are taken into account.
    3. Characteristics of Pseudo science are for example:
      1. Pseudo scientists avoid experiments that could disprove their theory
      2. Pseudo scientists are never willing to reject their theory
      3. Pseudo scientists ignore facts that speak against their theory and emphasize
        facts that seem to prove it.
    4. Astrology is not a Science. If scientific vocabulary is used or testable predictions are made
      it is a Pseudo science.

      Examples: Ptolemys Tetrabiblios is still used by Astrologers. Scientists have used his Almagest
                      for centuries, but were ready to reject it when evidence against it was found (earth orbits the sun).