Apparent solar time or true solar time is given by the daily apparent motion of the true, or observed, Sun. It is based on the apparent solar day, which is the interval between two successive returns of the Sun to the local meridian. Solar time can be crudely measured by a sundial.
The length of a solar day varies through the year, and the accumulated effect of these variations produces seasonal deviations of up to 16 minutes from the mean. The effect has two main causes. First, Earth's orbit is an ellipse, not a circle, so the Earth moves faster when it is nearest the Sun (perihelion) and slower when it is farthest from the Sun (aphelion) (see Kepler's laws of planetary motion). Second, due to Earth's axial tilt (known as the obliquity of the ecliptic), the Sun's annual motion is along a great circle (the ecliptic) that is tilted to Earth's celestial equator. When the Sun crosses the equator at both equinoxes, the Sun's daily shift (relative to the background stars) is at an angle to the equator, so the projection of this shift onto the equator is less than its average for the year; when the Sun is farthest from the equator at both solstices, the Sun's shift in position from one day to the next is parallel to the equator, so the projection onto the equator of this shift is larger than the average for the year (see tropical year). Consequently, apparent solar days are shorter in March and September than they are in June or December.
| Date | Duration in mean solar time |
|---|---|
| February 11 | 24 hours |
| March 26 | 24 hours − 18.1 seconds |
| May 14 | 24 hours |
| June 19 | 24 hours + 13.1 seconds |
| July 26 | 24 hours |
| September 16 | 24 hours − 21.3 seconds |
| November 3 | 24 hours |
| December 22 | 24 hours + 29.9 seconds |
These lengths will change slightly in a few years and significantly in thousands of years.
Read more about this topic: Solar Time
Famous quotes containing the words apparent, solar and/or time:
“Charity is a cop-out so traditionally female in its apparent self-effacement that there seems resonant comfort in it. We’re no longer supposed to serve the imaginations of men who have dominated us. We are to give up ourselves instead to those whose suffering is greater than our own. Looking down is just as distorting as looking up and as dangerous in perpetuating hierarchies.”
—Jane Rule (b. 1931)
“The solar system has no anxiety about its reputation, and the credit of truth and honesty is as safe; nor have I any fear that a skeptical bias can be given by leaning hard on the sides of fate, of practical power, or of trade, which the doctrine of Faith cannot down-weigh.”
—Ralph Waldo Emerson (1803–1882)
“There are times when words seem empty and only actions seem great. Such a time has come, and in the Providence of God America will once more have an opportunity to show the world that she was born to save mankind.”
—Woodrow Wilson (1856–1924)