# Time - Time Measurement Systems

### sun hours based motion

Our time scale is based upon the movement of the Sun. Since the Sun appears to move from east to west in the sky, when it is 12 noon in New Jersey, people in Seattle, Washington, say that the Sun has not yet reached its apex, or noontime position. In fact, if you say that the Sun has attained its noontime position, someone who is west of you by one degree of longitude will not see the Sun in its noontime position for approximately four minutes.

In October of 1884, an international agreement divided the planet into 24 time zones (longitudinally into 15° segments), allowing us to standardize time keeping. The starting point, or zero meridian, was the longitude that ran through the Royal Observatory in Greenwich, England. This is the common position from which we measure our time and is generally referred to as Greenwich Mean Time (GMT).

As you move westward from Greenwich, you add one hour for every 15° meridian you cross. As you move eastward from Greenwich, you subtract one hour for every 15° meridian you cross. (On land the time zones are not always straight lines; this is sometimes done to keep countries, states, cities, etc., all within the same time zone.) It should also be noted that these time zones are also listed by letters of the alphabet as well as 15° increments: Greenwich is designated Z for this purpose, the zones to the East are designated A through M, omitting the letter J, and the zones to the West are N through Y.

In North America, most people measure time based upon two 12 hour intervals. To distinguish the first 12 hours from the second, we label the time. The first 12 hours are denoted by the letters A.M. (for the Latin ante meridiem, meaning before noon–before the Sun has reached its highest point in the sky). The second 12 hours is denoted by P.M. (for post meridiem, which means after noon).

The International Date Line is near the 180° longitude in the Pacific Ocean. When you cross it going west you add 24 hours (one day), and when you cross it going east you subtract 24 hours (one day).

If you ask someone from Europe, or someone in the armed services, what time it is, he or she will answer you using a 24–hour system. Their time starts at midnight of every day, which they call 0000 hours (or 2400 hours of the previous day). Their time then proceeds forward for 24 hours, at which point it starts again. If someone tells you it is 0255 Zulu time, it means that it is 2:55 A.M. in Greenwich, England. This type of time reckoning is extremely important to synchronizing clocks for navigation, etc.

Astronomers found GMT to be awkward to work with in their calculations, because its measurements were based upon the Sun's actual motion and period between successive occurrences at its noontime position. Therefore, in 1972 they defined a Universal Time, which starts at midnight. They also realized that the solar day, the time from noon to noon, is not necessarily constant because of the irregular motion of the earth. To compensate for this, they have adopted a mean (average) solar day, time from noon to noon, upon which to base their measurements.

Both GMT and Universal Time are solar-based time systems, which means that they are based upon the apparent movement of the Sun. This method is not accurate enough for all scientific measurements because the earth's orbit around the Sun is not circular (it is an ellipse), earth's rotation rate (how fast it spins on its axis) is not constant, and its rotation axis is nutating (wobbling). Astronomers compensate for these "imperfections" by using a Sidereal Time measurement, a measurement system based upon the repetitive motion of the stars. This motion is, in fact, due to the earth's motion. Ephemeris time is a Sidereal Time based on the apparent repetitious motion of the Moon and the planets.

During World War II a "war time" was instituted to save electricity. Later, this became daylight saving time. However, some states and cities no longer use daylight saving time because they see it as a needless complication.

Einstein's Theory of Relativity prompted scientists to revise the idea of time as an absolute. For example, the German mathematician Hermann Minkowski (1864-1909) described time as the fourth dimension of space, postulating a space-time continuum, which scientists, including Einstein, accepted. Essentially, time cannot be a basis for absolute measurement. Because we are all in motion, in one form or another, our time measurements are dependent upon how we are moving. Relativistic time measurements depend upon who is measuring it, how fast the person is moving, and whether or not they are undergoing an acceleration.