The Celestial Sphere

• What is it? An imaginary sphere surrounding the Earth on which celestial objects appear to be located; thought to really exist in antiquity, today used as tool to identify locations in the sky.
• Special places: Celestial Equator, North and South Celestial Poles (Polaris or the North Star); the Ecliptic (and the Zodial constellations); Solstices and equinoxes.
• Question: In Oxford, MS, 34°20' N, where would you look for the North celestial pole, and for the celestial equator?

  Locating Objects

• The rotation problem: The altitude and azimuth of objects change as the whole sky moves from E to W (Why, is it because stars and planets are moving?), as well as with location on Earth.
• What can we do? Use coordinates on the celestial sphere, as opposed to the local sky; right ascension RA ("longitude" around the equator, in hours, minutes, seconds) and declination ("latitude" above the equator, in degrees).

Motions on the Celestial Sphere

• Apparent vs proper motion: Subtract the effect of Earth's daily rotation to find actual displacements on the celestial sphere.
• Do objects move? On the celestial sphere, only the Sun, the Moon, the 5 naked-eye planets, and other "nearby" objects appear to; the Sun along the ecliptic, the Moon on a tilted orbit that produces different "phases", the planets in somewhat stranger paths near the ecliptic, sometimes with retrograde (East to West) motion!
• Telescope view: Nearby stars show a slight back and forth motion every six months (parallax), or move in other ways over periods of years; to the naked eye, they don't seem to move at all. (Forget completely about seeing galaxies move, although most of them are moving at millions of mph, at least!)
• Fast-moving objects: They must be near us, either right outside the Earth (shooting stars and fireballs, spacecraft), or in our part of the solar system (comets, asteroids).
• What have humans made of all this? They identified constellations and assigned meanings to their patterns; tried to interpret or understand the motions of planets; took the apparent lack of stellar parallax to mean that the Earth doesn't move...