DSST Astronomy notes 1

# Ci2 according to keplers first law the path that the

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(c.i.2) According to Kepler's first law, the path that the planet follows will be an ellipse (not a circle as assumed by Copernicus!), and the sun will be located at one of the foci of that elliptical orbit: c.ii) Kepler’s Second Law states that the straight line joining a planet and the sun sweeps out equal areas in space in equal intervals of Time. (c.ii.1) This one can be difficult to understand without an illustration; take a look at the graphic below. The point of this law is that when a planet is closer to the sun, it moves more quickly. (c.ii.2) Because planetary orbits are elliptical, the distance between a given planet and the sun varies. (c.ii.3) When a planet is nearest to the sun (this point is called perihelion) it is at its fastest speed; when it's at the furthest point of its orbit (aphelion), it is slowest. c.iii) Kepler's Third Law states that the squares of the sidereal periods of the planets are in direct Proportion to the cubes of the semi-major axes of their orbits. (c.iii.1) This law sounds a lot more complicated than it is. A "sidereal period" is the amount of time it takes for a planet to do one complete orbit around the sun. (c.iii.2) The "semi-major" axis of an ellipse is almost like the "radius" of an ellipse. (c.iii.3) The point of this law is that the period of time it takes a planet to orbit the sun increases rapidly with the radius of its orbit; i.e. Mercury takes 88 days while Pluto takes 248 years. (c.iii.4) To arrive at his third law, it was not necessary for Kepler to know the actual distances of the planets from the sun, but only the distances in units of the Earth's distance, the astronomical unit. 145) Astronomical Unit (AU) – is approximately equal to the distance from the Earth to the sun. a) Currently the AU is 93 million miles 146) Sidereal Period – is the period of revolution of one body about another with respect to the stars a) How long it takes to complete one orbit (planets around the sun with respect to the stars) b) True time it takes for a planet to complete an orbit 147) Synodic Period – period of revolution about the sky with respect to the sun of a planet a) Time required for the planet to return to the same configuration again b) Time it takes from the planet to complete one orbit from the perspective of an observer 148) Measuring gravitational influences on other objects or on the stars within them, the masses of galaxies can be determined 149) Velocity of the stars depends upon the gravitational attraction from them within their galaxy and upon the mass of the star a) Consists of both the magnitude and direction of a star’s movement and denotes both the speed and direction a body is moving 150) Galaxies – majority fall into two categories a) Spirals (a.i.1) Consist of nucleus, disk, corona, and spiral arm

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(a.i.2) More than 2/3s of the most conspicuous galaxies (a.i.3) Our own galaxy (the Milky Way) and Andromeda Galaxy (M31) are spiral galaxies, Bodes Galaxy, and M33 – Traingulum Galaxy b) Ellipticals – dwarf galaxies (b.i.1) Are spherical or ellipsoidal, consist of stars with no spiral arms (b.i.2) Has many stars concentrated toward the center
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ci2 According to Keplers first law the path that the planet...

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