Measuring time - Time Standards
“The solar system has no anxiety about its reputation.” Ralph Waldo Emerson
There are many natural phenomena at work about us that we have combined in a way to help us tell time. Many, such as the earth' s orbit about the sun or the spin of the earth about its axis, are irregular and beyond our control. This makes the job of measuring time all the more complicated and, in the past, imprecise. Up to now, as long as there have been calendars based on the location and movement of the earth, moon, sun, and stars, there have been problems recording accurate time that could not be overcome. In an attempt to make order out of this chaos, many different time scales, or time standards, have been created.
Sidereal time is a time standard that is based upon the length of time it takes the earth to complete one rotation on its axis relative to a given star. That is, it is the interval between the moment when a star appears at a point in the sky
and the moment when it appears at the very same spot once again. This time interval measures approximately 4 minutes less than a normal 24-hour day.
Solar time, on the other hand, takes note of the position of the sun rather than the position of the stars to tell the time. Under this standard, a day has passed when the sun returns to the same point in the sky two consecutive times. The length of a solar day is not always constant, but varies. This is due, in part, because the earth's rotation about the sun follows an elliptical orbit rather than a circular one. This means the earth travels faster when it is closer to the sun in its orbit than it does when it is further away.
Greenwich, England is the starting point from which we define both time and place. Place because it is the town that the Prime Meridian goes through. The Prime Meridian is the imaginary, 0 degree longitude line that goes from the North Pole to the South Pole separating East from West.
Greenwich is also the origin of time as we know it because it is the point on the earth where all other time zones are measured from. This established time scale is called Greenwich Mean Time (GMT) and is based on the mean, or corrected time with respect to the Prime Meridian. In 1928, the name GMT was changed to Universal Time (UT) in order to better reflect its universal acceptance as the world’s civil-time keeping standard. Today both GMT and UT are used interchangeably today.
Because UT is affected by the rotation of the earth, UT and its variants are not uniform time scales. To compensate for this, we now base our world time on Atomic time. Atomic time is not directly related to the movement of the earth or moon, but rather is measured in oscillations of the element cesium and is accurate down to a nanosecond, or one billionth of a second.
Since atomic time is exact, and earth time is not, a new time scale called Coordinated Universal Time (UTC) was designed to meld the two. UTC is based on Atomic time but is constantly being adjusted to stay within .9 second of UT. Occasionally, we have to add a leap second to keep the UTC fully in synch with the UT. UTC is presently used as the basis for global time scales, the one that the world sets its clocks to.
Unlike standards which are based on the rotation of the earth about its axis, there are other standards based on the rotation of the earth around the sun. In the early 1950’s, people began to realize that a day measured by the earth’s rotation about its axis was too inaccurate to be used any longer. They figured that the earth’s rotation was neither smooth nor consistent and any sort of time unit derived from it could not possibly be accurate. To resolve this issue, the International Astronomical Union adopted the Ephemeris Time scale in 1952. This standard measured a second as a fraction of a year (as a fraction of the earth’s rotation about the sun) rather than of a fraction of a day (as a fraction of the earth’s rotation about its axis). The only problem with this is that the gravitational pull of the sun causes the earth’s orbit to shrink. This makes the earth go around the sun a bit faster each year which means that each consecutive year is shorter than the last. So by the late 1970’s the timekeepers once again realized that the current system in place could not be maintained.
Therefore, in 1979, the Ephemeris Time standard w replaced with 2 other standards: Barycentric Dynamical Time and Terrestrial Dynamical Time. Terrestrial Dynamical Time (TDT) takes into consideration Einstein’s Theory of relativity and measure’s time based on both Earth’s position and motion. Barycentric Dynamical Time (TDB) on the other hand bases its measurement on time at the center of our solar system. Terrestrial Dynamical Time is nearly synonymous with International Atomic Time (TAI); they’re only 32 seconds apart from one another. International Atomic Time is calculated using hundreds of atomic clocks in over fifty different laboratories, striving to stay in sync with the Atomic second (SI) and not the rotation of the earth. UTC is calculated from TAI, as are many other time standards of today. The main difference between UTC and TAI is that occasionally, UTC has a leap second added on to it when it’s deemed appropriate by the International Earth Rotation Service.
Time standards will continue to evolve as our technology evolves and keeps pace with man’s seemingly unquenchable desire for more accurate timekeeping. Just as calendars and clocks continue to become more accurate, so to do the standards by which such devices are set – the current atomic time is merely the next step in a long series of strides toward a perfected system of global time measurement.