A solar year and a sidereal year both refer to the amount of time it takes Earth to revolve about the Sun. The difference between the two measures is in the reference point for one revolution. The Latin root of sidereal is sidereus, “starry,” which itself comes from sides, “star, constellation.” The Latin root of solar is solis, “sun.” Thus, the difference between a solar year and a sidereal year is the difference in time between one complete revolution of Earth relative to the Sun, and one complete revolution of Earth relative to the constellations.
1
Consider the solar year. In order to establish the reference points for the motion of Earth relative to the Sun, we need to examine closely the orbit of Earth. The orbit of Earth about the Sun is not a perfect circle; it is an ellipse and the distance between Earth and the Sun varies over the course of a year. When Earth is nearest to the Sun, this point in Earth’s orbit is called the perigee. When Earth is farthest from the Sun, this point in Earth’s orbit is called the apogee. The time from perigee to perigee, or from apogee to apogee, is one solar year.
2
Suppose that at successive occurrences of apogee (to the minute), you observed the position of Earth with respect to the constellations. You would notice a slight difference from year to year. You would need to wait an extra 20 minutes after reaching apogee for Earth to be in the same position with respect to the constellations that it was at the previous apogee. Obviously, this variation of Earth’s orbit about the Sun with respect to the stars is slight. Since the constellations do not affect seasons on Earth to the extent that the Sun does, the difference between the solar year and the sidereal year is not of immediate importance. However, one direct result of the difference between a solar year and a sidereal year is the shifting of the North Star. When the Sumerians lived in the fourth millennium B.C., the North Star was Thuban in the constellation Draco. Nowadays, the celestial North Pole is near the star Polaris, on the handle of the Little Dipper. By A.D. 14,000 the North Pole will be near the star Vega. It will be 26,000 years before the North Star is near Polaris again.
