Does Anybody Really Know What Time It is?

As the Earth rotates (spins), it creates the illusion that the Sun is moving across the sky. This apparent movement of the Sun relative to the Earth determines the length of day and night, and all other divisions of time that derive from the length of day (seconds, minutes, hours, weeks, months, years). This is solar time. Solar time is simply the time indicated by the Sun.

The Earth rotates in a west-to-east motion (or counterclockwise, as seen if you were looking down on the Earth from over the North Pole). Because of this spin, our position under the Sun is constantly changing. The key to understanding time is that it is truly only the same time on the Earth along the same lines of longitude. For example, it is Noon in Greenwich when the Sun is directly, vertically, over the Prime Meridian. At that moment, the Sun is at its highest point in the sky over Greenwich. When it is Noon in Greenwich, it is truly Noon only along the entire Prime Meridian. Only one longitude line can be directly under the Sun at that same moment.

But we cannot have 360 (or more) different times - one for each degree of longitude (or each minute, or each second). So we must simplify. Since the Earth makes one full rotation of 360 in about 24 hours, the Earth turns 15 of longitude every hour (360 divided by 24 hours = 15 per hour)*. For this reason, the Earth is divided into 24 hourly time zones. In theory, each time zone consists of 15o of longitude, but in practice, actual time zone boundaries zig and zag around major cities, states, and countries. This is necessary to avoid major time conflicts. There are also variations and regional exceptions, actually creating more than 24 time zones.

The time that is used in each time zone is essentially the solar time along a Standard Meridian that passes roughly through the middle of the time zone. Standard Meridians are at 15 intervals from the Prime Meridian. So the first time zone, called Greenwich Mean Time (GMT; or Zulu Time, a military/nautical designation; or Universal Time, UT), extends 7 1/4 degrees on each side (east and west) of the Prime Meridian (0 Longitude). In North America, our Standard Meridians are at 75 West (Eastern Standard Time), 90 West (Central Standard Time), 105 West (Mountain Standard Time), and 120 West (Pacific Standard Time) (Figure 4). Standard time zones were first used in United States by the railroads in 1883, and were established as law on March 19, 1918 (Standard Time Act). Niceville, Florida, is in the Central Standard Time Zone, which is 6 hours behind Greenwich Mean Time. In Niceville we use solar time at 90 West, the standard meridian for Central Standard Time.

But it gets more complicated. The solar-based GMT, also called Universal Time (UT or UT1) is variable due to irregularities in the Earth's orbit, and the length of day is slowly increasing due to a gradual slowing of the spin of the Earth. This variation is only in seconds, and may not seem to make much of a difference in normal day-to-day clock time around the world. But the differences do add up quickly. To avoid the complications of natural, astronomical variations of solar time, more stable atomic clocks were developed.

For most applications around the world, time is based on Coordinated Universal Time, or UTC, which is a calculated time based on averaging the time of many atomic clocks around the world. Greenwich Mean Time is based on solar time at the Prime Meridian, which varies slightly due to irregularities in the Earth's orbit and spin. Of course, atomic-clock-based UTC is almost the same as the solar-based GMT, but there are slight differences that do make a difference in some scientific applications. By international agreement, UTC is not allowed to differ from GMT by more than 0.9 seconds. To keep these two times in-sync, a leap second is added to UTC about every 1.5 years.

*Or, the Earth turns one degree of longitude every 4 minutes (60 minutes divided by 15 degrees = 4 minutes per degree). In Niceville, you are moving eastward on the Earth about 1/4 mile every second. This is nearly 900 mph!


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