What is the exact time takn by the earth to revolve around the sun and at what speed?
sidereal time? 365.25636 mean solar days In the study of astronomy, keeping accurate time is critical. Astronomical time keeping has to do with the celestial meridian. Every point on earth has a celestial meridian. It is a line on the celestial sphere, an imaginary sphere containing the earth, which goes from the north celestial pole to the south celestial pole and intersects the zenith, the point directly above a point on earth. The celestial poles are extensions of the terrestrial poles into space. A picture of the zenith and celestial meridian for a spot on earth is shown below. Figure 1: This image shows the zenith and celestial meridian for a spot on earth. There are two common ways of keeping time, in relation to the rotation of the earth, and in relation to a phenomenon which has a constant period. The first method is not completely accurate because of the variations in the speed of the earth’s rotation, but is useful for astronomical purposes. Solar and sidereal times fall into
