Terms: ecliptic, vernal equinox

The stars we see at night have fixed positions on the celestial sphere. Not so the Sun. The Sun moves only about 1° per day, but over a year it completes a great circle on the celestial sphere. The path of the Sun on the celestial sphere is called the ecliptic. On your SC001 you should find dates near the ecliptic that report the date the Sun was at that location on the ecliptic.

On about March 21 the Sun moves (following the ecliptic right in the middle of the SC001) from the southern hemisphere to the northern hemisphere thus crossing the celestial equator. This intersection point is called the vernal equinox, and is by definition RA=0h.

About 3 month later on about June 21, the Sun reaches its maximum declination of 23½°. This point is called the summer solstice. Note that it has RA=6h.

About 3 months after that on about September 21, the Sun crosses from the northern hemisphere to the southern hemisphere, thus crossing the celestial equator. This point is called the fall equinox. Note that it has RA=12h.

About 3 months after that on about December 21, the Sun reaches its most southerly declination: -23½°. This point is called the winter solstice. Note that it has RA=18h.

Thus in its yearly journey on the ecliptic, the Sun travels through 24h of RA in 12 months: and average rate of 2h/month. (Don't forget that as viewed from the Earth, the celestial sphere is rotating at a rate of 1h or RA per hour of time.)

Significant Occasions on the Ecliptic
OccasionApprox. DateSun's
RAdec
vernal equinoxMarch 210h0°
summer solsticeJune 216h23½°
fall equinoxSeptember 2112h0°
winter solsticeDecember 2118h-23½° 


 For Folks without an HTML2 browser:
 Significant Occasions on the Ecliptic
                                   
                   Approx.          Sun's
   Occassion        Date          RA   dec

 vernal equinox    March 21       0h     0°
 summer solstice   June 21        6h   23½°
 fall equinox      September 21  12h     0°
 winter solstice   June 21       18h  -23½°

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