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Lecture 5

Astronomy 1021 Lecture Notes - Lecture 5: Nicolaus Copernicus, Deferent And Epicycle


Department
Astronomy
Course Code
ASTR 1021
Professor
Sarah Gallagher
Lecture
5

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sAstronomy- October 8th 2015
Nicolas Copernicus
Proposed a heliocentric universe (sun centered) rather than a geocentric
universe (earth centered)
PROS
Its simple
It naturally explains why Mercury and Venus never travel far from the sun
It reproduces much better than observed change in brightness of planets, as
consequence of changing distance from the earth
Its provides a natural explanation for the seasons
Its provides a natural explanation of retrograde motions without relying on
epicycles
CONS
Copernicus still assumed that the motion occurs in circles
Since true orbits are not circles, he needed to offset toe centres of circles and
add more circles to the model
In essence, his model was no more accurate that the Ptolemaic model
So why change the paradigm?
Tycho Brahe
Compiled the most accurate naked eye observations ever over a period of 3
decades
Observed a “new star”(nova) in 1572 measured its parallax, and found it was
much further away than the moon
Similarly, found that a comet was much further away than the moon
Parallax- is the difference in the apparent position of an object viewed along
two different sightlines
Nearby objects have a larger parallax than more distant objects
Lunar Parallax- Instead of using the left/right eye, use different locations on
earth and look at the moon at the time measure the lunar parallax
Stellar Parallax- instead of using; left/right eye, use two sides of the earths
orbit
Observe apparent positions of stars 6 months apart, if different it must be
much closer than background stars and we can calculate the distance
Johannes Kepler- Analyzed Tycho’s careful measurements of planetary
motions in context of heliocentric models with circular orbits
Trust data over preconceived beliefs
Discovered that planetary orbits are not circles but ellipses
First Law: orbits are ellipses
Second Law: planets move faster in their orbit when their closer to the sun
Third Law: more distant planets move more slowly
First Law- Basically tells us that a planets distance from the sun varies
during its orbit. It is closest at the point called the perihelion and farthest at
the point called aphelion
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