APPARENT SUN TIME

Apparent sun time is measured by placing the 1200 position of the time ring at the meridian of the sun's current position in the sky. This position is given in the graphical ephemeris for the current date, or can be estimated by interpolating for the current date between the dates shown along the ecliptic on the Uniglobe. Local Apparent Sun Time (LAT) can be directly read forr any longitude at its intersection with the time ring. Figure 21 shows the 1200 position set at the position of the sun for November 22, and the earth globe oriented for 1100 hours in Rio de Janeiro. The local Apparent Sun Time in Dakar, Senegal is 1245 when it is 1100 hours in Rio. The Local Apparent Sun Time is 1200 or noon for any obervation oint when the sun is due north or south of that point.

Greenwich Apparent Time (GAT) is the Local Apparent Sun Time at the Greenwich meridian. When apparent sun time at Rio de Janeiro is 1100, GAT is 1350.

The apparent sun day differs from the sideral day by 3 1/2 to 4 1/2 minutes. This is due to the fact that the sun's position in the sky moves about 1° during the time that it takes the earth to rotate once. The precise difference depends upon two factors. One is the earth's varying speed in its elliptical orbit around the sun during the year. The other is due to the inclination of the ecliptic to the equator. Near the solstices, the sun moves to the east, during a day, a greater amount than when it is near the equinox.

To have a unit of time which is approximately equal to the apparent sun day and of the same length throughout the year, the concept of the mean sun is introduced.

MEAN SUN TIME

The mean sun is an imaginary point that moves uniformly along the celestial equator with a speed equal to the average speed of the apparent sun as it moves through the year. Since the mean sun day constitutes one complete rotation with respect to a reference point that is also moving at the rate of approximately 1 degree per day in the same direction of rotation, the mean sun day is approximately 1 degree or 4 minutes longer than the sidereal day. (See Figures 22 and 23).

Mean Sun Time is measured with the 1200 position on the time ring set at the meridian of the mean sun. Since the average apparent sun speed is used, there is no variation in the length of the mean sun day.