time-1 Indefinite, unlimited duration in which things are considered as happening in the past, present, or future; continuance. 2 Finite duration, as distinguished from infinity. 3 The hour of the day reckoned by the position of a celestial reference point relative to a reference celestial meridian. 4 A system of measuring duration; solar, lunar, or sidereal time, when the reference is the sun, moon, or vernal equinox, respectively. Solar time may be further classified as mean time if the mean sun is the reference; or as apparent, if the apparent sun is the reference. 5 Time classified according to the reference meridian, either as the local or Greenwich meridian or, additionally, in the case of mean solar time, as the designated zone meridian. Standard, daylight saving or summer, and war time are variations of zone time. 6 Time designated according to the time piece used, e.g., chronometer time or watch time. 7 An elapsed interval.

time, apparent solar—See time, solar (definition 3); day.

time, astronomical—Solar time in a day (astronomical day) that begins at noon. Astronomical time may be either apparent solar time or mean solar time. It is counted from noon in a single series of 24 hours. Astronomical time appears in the American Ephemeris and Nautical Almanac prior to 1925; in the volumes for 1925 and later, civil time is used instead.

time, civil—Solar time in a day (civil day) that begins at midnight. Civil time may be either apparent solar time or mean solar time; it may be counted in two series of 12 hours each, beginning at midnight, marked “a.m.” (ante meridian), and at noon, marked “p.m.” (post meridian), or in a single series of 24 hours beginning at midnight.

time, daylight-saving—A substitute for standard time; exactly 1 hour ahead of that time. Clocks showing standard time are changed to daylight-saving time by moving the hands ahead exactly 1 hour. The time in a given time belt is identified by the particular designation of the belt: Eastern Daylight Saving Time; Central Daylight Saving Time, etc.

time, equation of—The algebraic difference in hour angle between apparent solar time and mean solar time. The sun is sometimes before and sometimes behind the mean sun by an amount that ranges from 0 to approximately 16 minutes. As the equation of time may be expressed as a correction to either apparent solar time or to mean solar time, its sign must be carefully observed.

time, Greenwich mean—Mean solar time of the meridian of Greenwich, used by most navigators, and adopted as the prime basis of standard time throughout the world. Also called by the military “z-time” or “zulu time.”

time, Greenwich sidereal—The sidereal time from the Greenwich meridian.

time, local-1 Time based upon the local meridian as reference, as contrasted with that based upon a time zone meridian, or the meridian of Greenwich. 2 Any time kept locally.

time, sidereal-1 Time measured by the apparent diurnal rotation of the (true) vernal equinox. At any instant, sidereal time is the hour angle of the vernal equinox. It is counted from 0 hour, when the vernal equinox is on the meridian, through 24 hours. Naming the meridian of reference is essential to its complete identification; for example, 75th meridian sidereal time; Greenwich sidereal time; and local sidereal time for the meridian of the observer. Because of small differences between the positions of the true and mean equinoxes, sidereal time, like the sidereal day, is subject to slight irregularities. These irregularities are absent from (uniform) sidereal time, which is measured by the motion of the mean equinox and is used in rating clocks of the highest precision. 2 The basic interval in the sidereal time system; mean sidereal day. It is defined as the interval between two successive upper transits of the mean vernal equinox over some meridian (corrected for polar motion and short periodic irregularities in the rate of rotation of the earth). The mean sidereal day is conventionally divided into 24 mean sidereal hours, 1,440 mean sidereal minutes, and 86,400 mean sidereal seconds. The interval between two successive upper transits of the true vernal equinox over some meridian is referred to as the apparent sidereal day. See also day.

time, solar-1 Time based upon the rotation of the earth relative to the sun. Solar time may be classified as mean if the mean sun is the reference; or as apparent if the apparent sun is the reference. The difference between mean and apparent time is called equation of time. Solar time may be further classified according to the reference meridian, either the local or Greenwich meridian or additionally in the case of mean time, a designated zone meridian. Standard, daylight saving or summer, and war time are variations of zone time. Time may also be designated according to the timepiece, as chronometer time or watch time, the time indicated by these instruments. 2 Time on the sun. 3 The basic interval in the mean solar time system is the mean solar day, being the interval between two consecutive transits of the fictitious sun over a meridian corrected for the motion of the pole. The mean solar day is divided into 24 mean solar hours, 1,440 mean solar minutes, and 86,400 mean solar seconds. The time interval between two successive transits of the true sun across the same meridian is the apparent solar day. The time required by the sun (or by the fictitious sun) to make two consecutive passages on the mean vernal equinox is termed a tropical year (or mean solar year). See also day.

time, standard—Mean solar time for a selected meridian adopted for use throughout a belt (zone). In the continental United States, the meridians of reference for standard time are multiples of 15° from the initial meridian, Greenwich Meridian. The standard time for each belt is designated by the number of its meridian, and also by some name of geographic significance: 75th meridian or Eastern Standard Time; 90th meridian or Central Standard Time; 105th meridian or Mountain Standard Time; and 120th meridian or Pacific Standard Time. The standard time meridians for Alaska are 150° and 165° west longitude; for Hawaii, 150° west longitude; and for the Philippine Islands, 120° east longitude. Standard time was established in 1883 to correlate train schedules of various railroads over the same areas. The standard time belts were planned to be roughly symmetrical with respect to the meridians of reference and to extend 7.5° to either side thereof Practical reasons such as the need of time consideration of cities outside the original boundaries of a time belt with cities within the belt, have caused a gradual shifting of those boundaries until some of them now exhibit large irregularities.

Time, Coordinated Universal (UTC)—A time system determined b atomic clocks and not by the rotation of the earth. To coordinate the time with the rotation of the earth, leap seconds are added as necessary.

time, Universal (U.T.)—The same as Greenwich mean time (G.M.T.). The first use of Universal time as a reference in the American Ephemeris and Nautical Almanac is in the volume for the year 1939.

timeshare ownership—See ownership, timeshare.

time signals—The U. S. Naval Observatory broadcasts time signals at regular intervals. Schedules of these broadcasts ran be obtained from the U. S. Naval Observatory, in Washington, D.C. The time signal of the National Bureau of Standards (WWV) is broadcast continuously.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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