True solar time

The true solar time is the time observed by the daily movement of the Sun in the firmament. It is based on the true solar day, which is the interval between two successive returns of the Sun to a local meridian. True solar time can be measured with a sundial.

The duration of a solar day is not uniform and varies throughout the year and the cumulative effect of these variations produces seasonal deviations of up to 17 minutes from the mean. The effect has two main causes. First, the Earth's orbit is an ellipse and not a circle, so the Earth's translation is fastest when it is closest to the Sun (Perihelion) and slowest when it is farthest from the Sun (Aphelion) (see Kepler's laws of planetary motion). Second, because of the Earth's axial tilt (known as the obliquity of the ecliptic), the Sun's yearly motion is along a great circle (the ecliptic) that is tilted toward the equator. celestial from Earth. When the Sun crosses the equator at both equinoxes, the daily change of the Sun (relative to the background stars) is at an angle to the equator, so the projection of this change on the equator is less than its average for the year, when the Sun is furthest from the equator at both solstices. Consequently, the true solar days are shorter in March and September than they are in June or December.