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Principle of Mediocrity the assumption that the laws of nature we see at work here on Earth apply throughout the universe.
"Organizing the view" Constellations We group stars in the same direction in the sky into constellations. Technically, a constellation is an officially recognized grouping of stars and the region of sky around it. There are 88 constellations. An asterism is a group of stars resembling some object or figure. (like Orion's Belt, the Big Dipper, etc.)
Celestial Sphere We can imagine the stars, planets, etc. are "stuck" on the inside of a very large ball with the Earth at the center. This is the celestial sphere.
The north & south celestial poles are the points on the sky directly above Earth's north & soute poles, respectively. (The sky appears to "spin" around the N & S celestial poles.) The celestial equator is the circle on the sky directly above Earth's equator.
Definitions: rotation the moving of an object about an axis inside the objectspinning. (Earth's rotates once a day.) revolution the moving of an object about an axis outside the objectorbiting. (Earth revolves (around the Sun) once a year.)
Daily Motion of the Sky The general, daily motion of the entire skythe stars, Sun, Moon, planets, etc., is to rise in the E & set in the W, traveling in arcs arcs across the sky. The arcs are centered on a celestial pole. (Some stars that are close enough to the celestial pole will never drop below the horizon. These are called circumpolar stars.)
Sun's Motion: Daily: same motion as everything else: arcs across the sky form E to W.
Now, if we look at a "day" in detail, then we find there are different "days" depending on our perspective:
solar day the time for Earth to rotate once relative to the Sun. (24 hours) sidereal day the time for Earth to rotate once relative to the stars. (about 23 h 56 m)
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