Understanding of Gravity

  • What did the ancient Greeks think?
      - It was obvious that objects fell towards the Earth
      - The quantity they thought was important was "heaviness"
      - Thus, they thought (wrongly) that heavier objects fell faster
      - This quantity was particular to the Earth and most believed that it was only important here (thus, the heavens did not follow the same rules).
  • Copernicus
      - first to study the positions of the planets with enough accuracy that we could start understanding the underlying physical model. His work highlights the start of the modern view of the scientific method
      - Sun at center (1507) and Earth rotated on its axis
      - wrote in scholarly language (Latin, mathematics, philosophy) so it was not very well read
      - his basis for Sun-centered was to produce a very simple explanation for retrograde motion
      - however, not very accurate since it still relied on circular orbits
      - thus, his impact was limited at the time
  • Kepler
      - worked 1 year with Tycho Brahe (until Tycho died)
      - used observational data to establish three laws (sun centered)
        1) planets orbit on an ellipse
        2) equal areas in equal time
        3) P2 = a3
      - this leads to a relationship of decreasing velocity with distance from Sun: planet velocities
      - published in 1627 (died in 1630)
      - predicted accurate positions (100 times more accurate) with Sun at center
      - Church blacklisted his books, but he was never punished
      - his three laws led the way eventually for Newton to provide a physical model for gravity
  • Galileo
      - first to use telescope on the sky
      • sunspots
      • phases of Venus
      • moons of Jupiter
      • craters on Moon
      • Milky Way made up of faint stars
      - strong evidence that Sun is at center
      - wrote in Italian, so his works were widely read
      - published in 1632, and placed under house arrest in 1633 (dies 1642)
      - Church ended its persecution of Galileo in 1992 (359 years later)
      - Galileo's crime was based on societal reasons and not scientific ones
      - his popularity with scientists and the public was part of the problem
  • Newton
      - in the late 1600's, Newton came up with his three laws and the Universal Law of Gravitation
      - his accomplishments are many including: laws of gravity, built the first reflecting telescope, studied the nature of light, invented calculus
      - he is also mainly responsible for instilling that physical laws determined here on Earth happened everywhere in the heavans. This basically started the idea of the Universe.
        The Three Laws of Motion:
      • In the absence of a net force, an object moves with constant velocity.
      • The force that acts on an object equals mass times acceleration.
      • For any force, there always is an equal and opposite reaction force.
      - these three laws lead to the Universal Law of Gravitation, which is the gravitional force between two objects of masses M1 and M2, separated by distance D is : Force=G M1 M2 /D^2

      There are many good intro Astronomy books that cover details of the lives for many great astronomers. These often include accounts of the reaction that the public has had on them.

    • Gravity using Uranus and Neptune
        - Uranus was discovered in 1781 by Herschel during a star mapping campaign
        - It was first thought to be a comet, but when its orbit was determined to be circular it was clearly a planet.
        - Uranus' orbit had irregular motion and that led people to suggest another planet beyond it. They based their calculations on Newton's Laws of gravity; it was the first use of the laws used to predict an observation.
        - Neptune was discovered in 1846 (by Adams and Leverrier) after using detailed calculations of its expected position based on Uranus' orbit. Both had trouble trying to convince anyone to look for it, and it took almost a year to get an astronomer to point a telescope at it. After that happened, it took one night to make the discovery.
        - Adams had done the calculation first but couldn't get observing time. This was seen poorly in the eyes of Britain since France had actually beaten them to the discovery.
        - But then Neptune's orbit was not where it was supposed to be as well, and we begun a search for another planet....
    • The mistake with Pluto
        - discovered in 1930 by Clyde Tombaugh (died 1997) after an aggressive observing campaign
        - the estimated mass of Pluto has undergone enormous changes (see plot )
        - when it was first discovered it was not called a planet, but there was huge public interest which might be partly why it was finally called a planet
        - in 1996, Pluto's planet status was challenged
        - Pluto may just be the largest Kuiper Belt object
        - as of Aug 2006, Pluto is not a planet
        - Here is a current diagram of the known Kuiper Belt objects, including Pluto
        - one of the most exciting missions in the past year is the flyby to Pluto with New Horizons
    • The 9th Planet?
        - the recent discussion of a large planet just beyond Pluto is due to ordered motion of some Kuiper Belt objects
        - We still have quite a ways to go for this interpretation to be considered real