Spherical coordinate systems
     On the Earth (Terrestrial)
     Latitude (measured N and S from the Equator)
     Longitude (measured E and W from the Prime Meridian -- politically decided, in early 1900's, as going through Greenwich, England)

(PRESENT DIAGRAMS SHOWING/COMPARING: Terrestrial coordinate system; Terrestrial vs Equatorial; Terrestrial vs Ecliptic; Ecliptic vs Equatorial; Horizon; Horizon vs Equatorial; Galactic?)

     In the Sky (Celestial)
     Several coordinate systems, each named by the circle which corresponds to the Equator, in the Earth-based system.

     Equator system is based on the Celestial Equator (and the Celestial Poles)
     Circles parallel to the Equator, are like parallels of latitude, on the Earth, and we measure N and S from the Equator, to the ‘parallel’ that a star is on, to measure its DECLINATION (from zero, at the Celestial Equator, to N or S 90 degrees, at the Celestial Poles).
     Circles perpendicular to the Equator, are like meridians of longitude on the Earth, and we measure from the 'Prime Meridian , to the 'meridian' that a star is on, to measure its RIGHT ASCENSION. EXCEPT -- we don’t measure E and W, but only TO THE EAST, and we measure it in time units, not degrees.
     We measure right ascension to the East, in TIME units, so that as the stars move to the West, they can serve as a clock. If a star with a right ascension of 6 h 45 m is on “the Meridian” (the arc running from the North point on the horizon, through the Celestial Pole, through the Zenith, through the South point on the horizon), it is 6:45 on a star clock. If a star with a right ascension of 12 h is on the Meridian, it is 12:00 on a star clock. And if a star with a right ascension of 18 h 40 m is on The Meridian, it is 18:40 on a star clock.
     IN THIS SYSTEM, every star has a particular declination, and right ascension, and we could, on a globe (celestial globe) plot the positions of all the stars, in the sky, and use that, to see where they are, relative to each other.
     These numbers -- RA (right ascension) and Dec (declination) -- are almost constant for a given star, because the stars are so far away, that any motion that they have, relative to us (or vice-versa) is too small to see, without tremendous effort, over times as short as a human lifetime. As a result, the positions of the stars relative to each other seem absolutely fixed, to a casual observer (leading to the term, the ‘fixed’ stars).
     (There are very small changes, over long periods of time, because of proper motion (second half of semester), and precession (beyond scope of introductory class))
     However, there are seven objects -- the planeteV (‘planetes’), or “wanderers” -- which MOVE relative to the stars, in ‘short’ periods of time: the Moon, the Sun, Mercury, Venus, Mars, Jupiter, Saturn (leads into another page; not in quite the right place)