163 the sun also produces tides on the earth although

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163) The sun also produces tides on the Earth, although it is less than half as effective a tide-raising agent as the moon. a) Actually, the gravitational attraction between the sun and the Earth is about 150 times as great as that between the Earth and the moon. a.i) However, the sun is so distant that it attracts all parts of the Earth with almost equal strength. a.ii) The moon is close enough for its attraction on the near side of the Earth to be substantially greater. 164) When the sun and moon are lined up (at a full moon), the tides produced are greater than normal and these are called spring tides. a) The spring tides are the highest tides that the Earth experiences. b) Of these high tides, the highest occur at those times during a full moon when the moon is closest to the center of the Earth. 165) When the moon is at its first or last quarter, the tides produced by the sun partially cancel out the tides of the moon and these are called Neap tides. a) The neap tides are lower than usual on the Earth. 166) The Earth is not perfectly spherical and its equatorial diameter is twenty-seven miles greater than its Polar diameter. a) A perfect sphere would have the same diameter at all points. 167) Precession is a slow, conical motion of the Earth's axis of rotation caused by the gravitational torque of the moon and the sun on the Earth's equatorial bulge. a) The Earth's rotation axis is not fixed in space; it is moving like the top of a wobbly top a.i) By human standards, this conical motion is imperceptible--with a precession period of 26,000 years, it takes 26,000 years for the Earth's axis to do a full rotation. a.ii) Another way to look at this is that right now the North Star is Polaris, but in 13,000 years the star Vega will be approximately at the North Celestial Pole. a.iii) Then in 26,000 years, the pole will be pointing in the same direction again--back at Polaris. 168) The twenty brightest stars as seen from Earth are classified in that manner because they have high Luminosity and/or are nearby. a.i) Those stars are: Sun, Sirius, Canopus, Arcturus, Alpha Centauri, Vega, Rigel, Procyon, Achernar, Betelgeuse, Hadar (Agena), Capella A, Altair, Aldebaran, Capella B, Spica, Antares, Pollux, Fomalhaut, and Deneb.
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a.ii) Sirius is the brightest star after the sun. 169) A visual grouping of five galaxies of which four form the first compact galaxy group ever discovered is called the Stephan's Quintet. a) This group was discovered by Edouard Stephan in 1877 and is one of the most studied of all the compact galaxy groups. 170) The length 206,265 AU (astronomical units) is defined as a Parsec. a) A parsec is 206,265 AU, which is about 30 trillion kilometers (19 trillion miles) or 3.262 light years. b) The name parsec stands for "parallax of one second of arc" and the concept is based on trigonometry.
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