TIME

Physicists agree that time is one of the most difficult properties of our universe to understand. Although scientists are able to describe the past and the future and demarcations such as seconds and minutes, they cannot define exactly what time is. The scientific study of time began in the 16th century with the work of Italian physicist and astronomer Galileo Galilei. In the 17th century English mathematician and physicist Sir Isaac Newton continued the study of time. A comprehensive explanation of time did not exist until the early 20th century, when German-born American physicist Albert Einstein proposed his theories of relativity. These theories define time as the fourth dimension of a four-dimensional world consisting not just of space but of space and time.

Several ways to measure time are in use today. Solar time is based on the rotation of Earth on its axis. It makes use of the Sun’s apparent motion across the sky to measure the duration of a day. Sidereal time is also based on Earth’s rotation, but uses the apparent motion of the “fixed” stars across the sky as Earth rotates as the basis for time determination. Standard time, the familiar clock time most people use in everyday life, is based on the division of Earth’s sphere into 24 equal time zones. Dynamical time—formerly called ephemeris time—is the timescale of astronomy. Astronomers use the orbit of Earth around the Sun, as well as the orbital motions of the Moon and the other planets, to determine dynamical time. Atomic time is based on the frequency of electromagnetic waves that are emitted or absorbed by certain atoms or molecules under particular conditions. It is the most precise method for measuring time.

The measurement of time passage probably began with the concepts of past, present, and future. Throughout history humans have used various celestial bodies—that is, the Sun, the Moon, the planets, and the stars—to measure the passage of time. Ancient peoples used the apparent motion of these bodies through the sky to determine the seasons, the length of the month, and the length of the year. See also Calendar. Humans created the sundial and the hourglass to measure time. The first mechanical clocks were invented in the 14th century. The use of the pendulum clock became popular in the 1600s when Dutch astronomer Christiaan Huygens applied the pendulum to regulate the movement of clocks (see Clocks and Watches). At this point, clocks became accurate enough to record minutes as well as hours.

The use of chronometers (precision timepieces) for precise measurement of time played an important role in navigation from the mid-18th century to the 1920s by helping to determine longitude. Prior to the invention of an accurate chronometer in the mid-18th century, navigators could easily determine their latitude, but determining longitude was more difficult. If a reading of the Sun’s position was not made at precisely the noon hour, great errors in longitude could result. For example, an error of a second of longitude, for a ship at Earth’s equator, produces an error in longitude position of about 400 m (about 1,300 ft). Precise time measurement gained further importance with the evolution of modern industrial societies. During the late 18th century, the Industrial Revolution prompted factory work to start and stop at appointed times, thus changing the tempo of life. The growth of railroads and the use of train schedules in the mid-19th century further emphasized the need for precise timekeeping.

The apparent motion of the Sun across the sky has long been used as a basis for measuring time. Under solar time, at any given locality it is noon—twelve o’clock in the daytime, or midday—when the Sun reaches the highest point in the sky. Noon at any place on the surface of Earth is when the Sun's direct rays pass over the meridian of that particular place (See also Prime Meridian). A meridian is an imaginary line that stretches from pole to pole on Earth's surface. A meridian is also known as a line of longitude (see Latitude and Longitude). The interval between successive passages of the Sun across the same meridian is one day, and this day is by custom divided into 24 hours. The amount of daylight in a day varies throughout the year, based on the tilt of Earth’s axis and its orientation to the Sun as the seasons change (see Season). For the same reasons, a day in solar time is not always 24 hours long. The difference in the length of the 24-hour day during different seasons of the year can amount to as much as 16 minutes. With the invention of accurate timepieces in the 17th century, this difference in the length of the day became significant. To overcome this problem scientists invented mean solar time, which is based on the motion of a hypothetical sun traveling at an even rate throughout the year.

Universal time is simply the mean solar time measured at the Greenwich meridian, which is designated 0° longitude and from which the longitude of all points on the surface of Earth are measured. The meridian passing through the original site of the Royal Greenwich Observatory in Greenwich, England, has been recognized by international agreement since 1884 as the prime meridian. Universal time was originally called Greenwich Mean Time (GMT) but replaced that designation in 1928. Universal time is used to denote solar time when an accuracy to about one second suffices.

Because the basis of mean solar time relates to the motion of a hypothetical sun, scientists established a base position from which the mean time is calculated. This base position is the vernal, or spring, equinox (see Ecliptic), an imaginary point in the sky that is, nevertheless, calculated with great accuracy by astronomers (see Astronomy). Practically, scientists define the location of the vernal equinox by reference to the position of the “fixed” stars.

Sidereal time is based on the apparent motion of the distant, “fixed” stars across the sky. It has various astronomical purposes, such as predicting locations of objects in outer space. The primary unit of sidereal time is the sidereal day, which is subdivided into 24 sidereal hours. Each sidereal hour is subdivided into 60 minutes, and each minute into 60 seconds. Astronomers rely on sidereal clocks because any given star will cross the same meridian, or line of longitude, at the same sidereal time throughout the year.

According to convention, each sidereal day begins at the instant the vernal equinox crosses the prime meridian. The vernal equinox is the point on the celestial sphere at which the sun crosses the plane of the equator, moving from south to north. The celestial sphere is the apparent surface of the heavens, on which the stars appear to be fixed.

The U.S. Naval Observatory in Washington, D.C., uses mathematical tables to calculate mean solar time from mean sidereal time. The sidereal day is almost four minutes shorter than the mean solar day, so a discrepancy exists between the total number of hours in a mean solar year and in a mean sidereal year. This discrepancy arises because Earth rotates on its axis at the same time that it revolves around the Sun. According to mean sidereal time, Earth returns to the vernal equinox every 365 days 6 hours 9 minutes 9.54 seconds. According to mean solar time, Earth returns to the vernal equinox every 365 days 5 hours 48 minutes 45.5 seconds. The difference between the two is 20 minutes 24.04 seconds.