Caption: The equatorial coordinate system is explicated and illustrated.

Features:

1. The equatorial coordinate system is the main celestial coordinate system used for locating astronomical objects on celestial sphere.

2. The equatorial coordinate system is analogous to geographic coordinate system of longitude and latitude.

   In fact, it is essentially a projection of the geographic coordinates from the Earth's center onto the celestial sphere.

   The equatorial coordinate system and the geographic coordinate system are comparably ancient in their earliest versions. Hipparchus (c.190--c.120 BCE) may have invented the first version of the equatorial coordinate system (see Wikipedia: Right ascension: History). Eratosthenes (c.276--c.195 BCE) may have invented the first version of the geographic coordinate system (see Wikipedia: Geographic coordinate system: History).

3. The equatorial coordinate system coordinate names are different from those of the geographic coordinate system. Right ascension (RA) replaces longitude and declination (Dec or δ) replaces latitude.

   Often people identify the equatorial coordinate system by just saying "right ascension and declination" or "RA and Dec" rather than say "equatorial coordinate system".

4. The units of declination are normal: degrees (°), arcminutes ('), and arcseconds (''),

   As foreshadowed above, declination is measured is measured north and south of the celestial equator.

   But one does NOT say north and south declination.

   North is positive declination. South is negative declination.

   To be concise: 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.

   The full range of declination is [-90°,90°]. The south celestial pole (SCP) is at -90° and the north celestial pole (NCP) is at 90°.

5. Right ascension is measured always eastward from the zero-point the vernal equinox on the celestial equator. The vernal equinox (AKA First Point of Aries) is 0h RA (where h stands for hour of right ascension: see below).

6. The units of right ascension are UNIQUE to right ascension.

   To be brief: hours, minutes, and seconds used so that the celestial sphere westward rotates on the celestial axis relative to the Earth 1 hour per hour (more precisely sidereal hour), etc.

7. To be detailed: The angle units for right ascension (RA) are 1 hour RA (h) = 15°, 1 minute RA (m) = 1/4 °, and 1 second RA (s) = 1/60 m = 1/240 °. The following table gives their conversions among themselves and to degrees.

   ______________________________________________________________________
   
   Table:   Conversions for the Angle Units Hours, Minutes, Seconds
   ______________________________________________________________________
    unit          hours RA    minutes RA   seconds RA      degrees
   ______________________________________________________________________
   
    1 hour RA         1            60         3600            15
    1 minute RA     1/60            1           60        1/4 = 0.25
    1 second RA    1/3600         1/60           1      1/240 = 0.00625    
   ______________________________________________________________________

   Why are these strange angle units used for right ascension? First note that there is a thing called sidereal time. It and its units are set by the Earth's rotation relative to the observable universe (in older astro-jargon relative to the fixed stars). The Earth rotates westward relative relative to the observable universe at the rates of:

   1. 24 hours RA per sidereal day.
   2. 1 hour RA per sidereal hour.
   3. 1 minute RA per sidereal minute.
   4. 1 second RA per sidereal second.

   The sidereal-time units are all 0.99726958 times their metric system analogs or about 0.3 % shorter. For example, the sidereal day = 86164.0905 s = 1 day - 4 m + 4.0905 s (on average) while the metric day =24 h = 86400 s exactly by definition.

   You can see now that the angle units of right ascension were chosen to be the natural units given the nature of sidereal time and its natural units.

   The units of right ascension make calculations easy.

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

8. The full range of right ascension is from 0 h RA at the vernal equinox to 24 hours RA back at the vernal equinox.