Astronomy Textbook Notes - Astronomy Textbook Notes Chapter 1 The Shores Of Light Ancient Greeks thought that the earth a sphere was at the center of

Astronomy Textbook Notes - Astronomy Textbook Notes Chapter...

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Astronomy Textbook Notes:Chapter 1: The Shores Of LightAncient Greeks thought that the earth, a sphere, was at the center of the universe orbited by crystalline spheres which represented suns, moons and stars. Known as the heliocentric cosmology developed mostly by Aristotle and Eudaoxus.oModern theory wasn’t possible to comprehend back thenoPlato and many others thought that the planets were perfect geometric shapesoThe Greeks were very skeptic – made good scientists.oIn the long run we see that this isn’t a very accurate representation of our universe, but was at the time a beautiful theory.In the 2ndCentury AD, Ptolemy at Alexandria came up with another model – a much more complicated one. In his universe each planet orbited in an epicycle, a small circle, centered on a point in its orbit around the earth or even on a point in another epicycle.oGood theory, but highly abstract. Reigned in the West for 1400 years, but was regarded as unduly elaborate or insufficiently physical.oInherently small because the sphere of stars that enclosed it had to spin once around every day and if the starry sphere was very big it would have to rotate at tremendous speed.Then Copernicus came along with the model we still use today. oHe believed the orbits of the planets were circular (epicycles) oBigger universe because if the stars in the universe stayed in the same place while the earth moves through orbit they must be very far away.Opened up a vast universe beyond.oTwo major problems with it thoughPlanetary orbits are not circular but elliptical so predicting planetary motions on circular orbits it like learning how a football bounces by bouncing a basketball. If the earth rotates why don’t jumpers land to the west of their starting pointGreeks couldn’t figure this outJohannes KepleroLooked at the stars little himselfoHad three laws which wereThe first law was that planets orbit in elliptical shapes rather than perfect circles with the sun located at one focus of each ellipseEveryone before him believed that they orbited in circles
The second law shows that the orbital velocity of each planet increases when close to the sun and decreases when far away from it. Where the rate that the area is swept within its orbits isequal during equal intervals of time.Made sense because it was symmetrical and divineHis third law stated that the cube of the semimajor axis (half the long axis) of each planets orbit is proportional to the square of the planet’s orbital periodHelps map the solar system because it allowed to solve the relative size of each planet compared to other planetsGalileo – from Pisa, worked in Venice.

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