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Observing the Solar System


Key Terms:

Notes: (26-2)

Early astronomers were fascinated with the movements of stars across the sky.  They grouped the stars into constellations (groups of stars) that periodically became visible in the night sky.  These constellations became the basis for the celestial calendar.  

(www.branches.com/ calendar_zodiac.gif)






This movement across the night sky led observers to believe that the universe was geocentric (Earth centered) and that at the edge of the universe there was a tapestry with holes in it.  The stars were holes in the tapestry where an incredible light shown through.

(www.astro.columbia.edu/.../images/ celestial-sphere.jpg)

The early astronomers were also interested in periodic stars that appeared to move within the tapestry.  The called these eastward moving stars wanderers (planets).  They also noticed that one planet appeared to periodically move westward for a short period of time before continuing to the east.

retrograde motion - apparent backward motion of the planets

retrograde motion

(physics.unl.edu/.../lee/astroed/ applets2/cop/retrograde.gif)


Ptolemy (~200AD)
predicted the first planetary model to predict the location of the planets
model was geocentric
described the retrograde motion as epicycles
Copernicus (1473 - 1543)
proposed the heliocentric model of the universe
explained retrograde motion 
imprisoned for his beliefs on the center of the universe
Tycho (late 1500s - 1601)
made the most complete star charts without the aid of the telescope
father of modern astronomy
Tycho's assistant
explained the motion of the stars by applying elliptical orbits to them
developed the 3 laws of astronomical motion
  1. The planets move in elliptical motion with foci(csep10.phys.utk.edu/astr161/ lect/history/kepler1.gif)
  2. Planets move around the sun in increments of equal time (much like a clock)(drumright.ossm.edu/astronomy/ images/ellipse2.gif)
  3. The time it takes a planet to travel around the sun in its period.  It can be predicted by the formula P2 = D3


(www.shef.ac.uk/.../vdhillon/ teaching/phy105/kepler3.gif)

Newton (1642 - 1727)
used the observations of Kepler, Tycho, Copernicus and Galileo to formulate his three laws of motion
from his three laws Newton formulated the basis of calculus
used calculus to describe the Universal Law of Gravitation
all objects are attracted to each other by the virtue of their masses 
the amount of gravity is directly proportional to the masses of two objects and inversely related to the distance between their masses
used this law to explain Kepler's 2nd law












Last modified: February 23, 2004