solar time

solar time time defined by the position of the sun. The solar day is the time it takes for the sun to return to the same meridian in the sky. Local solar time is measured by a sundial . When the center of the sun is on an observer's meridian, the observer's local solar time is zero hours (noon). Because the earth moves with varying speed in its orbit at different times of the year and because the plane of the earth's equator is inclined to its orbital plane, the length of the solar day is different depending on the time of year. It is more convenient to define time in terms of the average of local solar time. Such time, called mean solar time, may be thought of as being measured relative to an imaginary sun (the mean sun) that lies in the earth's equatorial plane and about which the earth orbits with constant speed. Every mean solar day is of the same length. The difference between the local solar time and the mean solar time at a given location is known as the equation of time. Tables used by navigators list the equation of time for different times of year so that an observer can calculate his mean solar time from his local solar time (found by determining the sun's hour angle ). Mean solar time is the basis for civil time and standard time .

solar time Time with respect to the Sun; technically, the hour angle of the Sun plus 12 hours, added to make the day begin at midnight rather than noon. The true Sun is the basis of apparent solar time, as shown on a sundial, but this runs irregularly because the Earth's orbit is not circular and the Sun's path (the ecliptic) is inclined to the celestial equator. For accurate timekeeping purposes mean solar time is used, based on the hour angle of the fictitious mean sun. Solar time, which is the basis of all civil timekeeping, loses about 4 min a day against sidereal time, so that a star will rise about 4 min earlier each night.

mean solar time The time shown on a clock; technically, it is the time since the mean sun crossed the meridian with 12 hours added to make the day begin at midnight rather than noon. As the variations of apparent solar time due to the equation of time have been averaged out, mean solar time proceeds uniformly except for small irregularities in the gradual slowing of the Earth's rotation. Mean solar time is closely related to mean sidereal time, and one can be calculated if the other is known. The mean solar time on the Greenwich meridian is Universal Time.