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# 12 - Semi-Major Axis(a = Distance from the planet to the...

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- We measure the size of a circle using: diameter or radius - But how would you measure the size of an ellipse? - We can use its longest and shortest “diameters” - The major axis of an ellipse is its “long diameter” - The major axis passes through the two foci (focal points) - (the minor axis is perpendicular to the major axis.) Keplers First Law - The semimajor axis is half the length of the major axis - In astronomy, the semi major axis is designated: “a” Keplers Second Law - When a planet is at its greatest distance from the sun it moves the slowest - When a planet is at its smallest distance from the sun it moves the fastest - The point of closest approach to the sun is called perihelion - The point of farthest distance from the sun is called aphelion. Keplers 3 rd Law [P2=a3] …relates a planets orbit period to its semi major axis

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Unformatted text preview: Semi-Major Axis (a) = Distance from the planet to the sun (in AU) The Astronomical Unit (AU) is a unit of distance It is used to measure distances to planets. 1 AU = Earths distance from the sun =93,000,000 miles P = orbital period in years A = semi major axis in AU An example: the planet Uranus orbits the sun at a distance of 19 AU So its semi major axis is a = 19 AU Question: how long does it take Uranus to orbit the sun? P2=a3 A3 = a x a x a = 19 x 19 x 19 = 6859 P2 = a3 = 6859, so p= the square root of 6859 = 83 years-Keplers third law also applies to satellites orbiting earth. -The faster the orbital period (p), the smaller the orbit (a)-A close satellite orbits in just 90 minutes-A satellite at about 6R earth orbits in 24 hours...
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