Introductory Astronomy: The Celestial Sphere Motions Visible Without Optical Aid Daily Motion Long-term Motion Sun E to W in about 12 hours from sunrise to sunset. Length of day varies from season to season and with latitude. W to E along the ecliptic 1 degree per day. The height of the sun in the sky at noon is at maximum in the summer, minimum in winter (excludes tropical regions). The sun returns to the same constellation in 1-year intervals. Moon E to W in about 12 hours, 25 minutes from moonrise to moonset. Moonrise is about 50 minutes later each day. Like the sun, this timing is modulated by the season and your latitude on Earth. W to E within 5 degrees of the ecliptic. It takes 27.3 days to travel 360 degrees with respect to the stars, but phases re peat on a 29.5 -day cycle. Planets E to W in about 12 from rising to setting (again modulated by season and latitude). Additional, very small variations are caused by the planets's own motions against the background stars. W to E within 7 degrees of the ecliptic. The average speed varies according to planet. It is fastest forMercury and slowest for Saturn (slower for Uranus, Neptune, Plut o but we can 't see them with our eyes). All the planets have periods where they go retrograde (E to W) with timing different for each planet. Stars E to W in about 12 hours (modulated by season, by latitude, and by where the star is on the sky: circumpolar stars, for instance, do not rise and set, but they wi ll travel in a circle aroun d the pole, 180 degrees in 12 hours minus about 4 minutes.) Star rise is about 4 minutes (3m 56s) earlier each day. The stars remain fixed on the celestial sphere with respect to themselves. The earth's pole describes a circular wobble of 23.5 degree amplitude centered on a point in the constellation of Draco every 26,000 years (often called "precession of the equinoxes"). Long time exposures taken at night illustrate daily motion: South Pole Star Trails 1 | South Pole Star Trails 2|North Pole St ar Trail s The Celestial Sphere Astronomy: The Celestial Sphere http://astro.wsu.edu/worthey/astro/html/lec-celestial-sph.html 1 of 10 1/1/2014 6:25 PM
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