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OU ASTR 1514 - Keplers Laws

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ASTR 1514 1st Edition Lecture 5Objectives: - Learn Kepler’s three laws- Learn how to use Kepler’s three laws to understand and predict the motions of the planetsin our solar system. Key terms- Orbital period – How long it takes a planet to orbit once around the sun, earth’s period is 1 year.- Geocentric – Universe is centered around the Earth- Retrograde: To move in a backwards direction.- Heliocentric – Solar system is centered around the Sun- Ellipse – elongated circle. 1.) Early Astronomy:- Early astronomers noticed that the stars showed no parallax, effect of the position or direction of an object appears to differ when viewed from different positions, while observing the sky. - They conclude that the Earth does not move, and the stars orbit the Earth on a crystal sphere.- Retrograde motion – Directed or moving backwards. - The combination of these observations with the belief that motion should be in perfect circles, produced the Ptolemaic Universe: Geocentric, centered around the Earth, with uniform circular motion. These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.2.) Copernicus:- Copernicus realized that the solar system is actually Heliocentric, centered on the sun.- But like early astronomers he too believed the orbits of the planets were perfectly circular.- Copernicus Universe: Centered around the sun with uniform circular motion. 3.) Kepler’s Laws- Tycho Brahe: Used his observatory to make very precise measurements of plants- Kepler used Tycho Brahe’s data to figure out the laws that govern the planet’s motion. - Kepler’s laws, however, where empirical, meaning they describe the planetary motion but do not explain why they have the motion. - Ellipse i. Size described by the semimajor axis (longest length is twice the length of the semimajor axis)ii. Shape of ellipse specified by eccentricity, how elongated an ellipse is, how far apart the foci are. Nearly circular ellipses - have very small eccentricity, close tozero and the foci are very close together. Long thin ellipses – Eccentricity closer to 1, foci are far apart. Eccentricity of Earth = 0.0167iii. Every ellipse contains two foci, one being the sun and the other being empty space.Kepler’s First Law- Planet orbits are ellipses, with the sun being at one focus point. Kepler’s Second Law: Law of Equal Areas- The line between the sun and each planet “sweeps” out equal areas in equal times. - Applies to only one planet at a time- Planets move faster when closer to the sun. Kepler’s Third Law- A planet’s orbital period squared is proportional to its average distance from the sun cubed. - Planets with largest average distance take longest to


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