solar time 
figure 20: 
Solar and sidereal time.

1.  The Sun is said to be at perigee, R, when it is nearest to the Earth. This occurs once per year around January 1st. A fictitious body called the dynamical mean sun, D, is introduced which starts off from perigee with the Sun, moves along the ecliptic with the mean angular velocity of the Sun and consequently returns to perigee at the same time as the Sun. 
2.  When this dynamical mean sun, moving in the ecliptic, reaches the first point of Aries, , the mean sun, M, starts off along the celestial equator with the Sun's mean angular velocity, returning to at the same time as the dynamical mean sun. 
figure 21: 
The positions of the Sun, the mean sun, the dynamical mean sun and the
perigee on the celestial sphere. 
figure 22: 
The annual variation of the equation of time.

figure 23: 
A photograph of an equatorial sundial, clearly showing the
shadow cast by the gnomon. The equation of time is marked
on the stone slab at the foot of the sundial.
