The phenomena known as the equation of time describes the differences in time between the noon passages of the apparent sun and the mean sun or apparent time minus mean time. The apparent motion of the sun as it crosses the sky is mostly affected by the earth's rotation. The speed that sun travels across the sky from day to day is not an exact constant, and varies some. This variation is affected by the following 2 reasons: 1. Differences in the earth's motion around the sun in its slightly elliptic orbit at its farthest and closest distance from the sun; and 2. By the tilt of the earth's equator, 23 degrees and 27 minutes, to the plane of its orbit.

Since it is impossible to build a clock that accounts for these variations, of the apparent sun across the sky day to day, our clocks assume a mean sun. A mean sun assumes a scenario where the sun comes across the sky at an equal speed every day, crossing the local meridian at the highest point in the sky at exactly noon every day. The apparent sun and mean sun reach the local meridian at noon 4 times a year; then at other times of the year they differ. The difference in the noon passages of the apparent sun and mean sun is called the equation of time.

Return to Spring Weather over South Central Texas