Caption: An animation illustrating the celestial sphere, the equatorial coordinate system, right ascension (RA), declination (Dec or δ), the north celestial pole (NCP), the south celestial pole (SCP), the celestial equator, the vernal equinox (0 h RA) (AKA First Point of Aries), the ecliptic, and the locations of stars Aldebaran and Sirius.

Features:

1. The equatorial coordinate system is essentially a projection (as viewed from the center of the Earth) of the geographic coordinate system onto the celestial sphere.

   Longitude transforms to right ascension (RA) and latitude transforms to declination (Dec or δ), mutatis mutandis.

2. Right ascension (RA) is measured eastward from the vernal equinox in hours (h) = 15°, minutes (m) = 1/4 °, and seconds (s) = 1/60 m = 1/240 °.

3. Declination (Dec or δ) is measured from celestial equator in degrees (°), arcminutes (') = 1/60 °, and arcseconds ('') = 1/60 ' = 1/3600 °. Declination to the north/south is counted as positive/negative.

4. The equatorial coordinate system is tied to location of the Earth and the Earth's rotation axis and the Earth's equator.

5. Why is equatorial coordinate system good?
   1. The Earth is our observation platform: so it should be at the center---any other center for an angular coordinate system would be very awkward for locating astronomical objects for observations.

   2. The Earth's rotation spins us around once per sidereal day = 86164.0905 s = 1 day - 4 m + 4.0905 s (on average), and so all points on the celestial sphere move X in units of right ascension in X units of sidereal time.

      For example, if an astronomical object (one unmoving on the celestial sphere) is 3 hours east of the meridian, it will transit the meridian in 3 sidereal hours (which is just a bit less than 3 metric hours).

   3. EOF

   4. The horizontal coordinates of an object vary with time and location on the Earth, and so CANNOT be used for standard locations in catalogs, etc.

6. So the equatorial coordinate system is observationally convenient and that's why it's good.