Difference Between Day And Date
A day is a period of 24 hours. The length of the hours is determined by the length of the day, and the length of the day is defined in astronomical terms. Because of the Earth’s orbital motion, the Sun appears to move eastward nearly 1° (about 0.986°) each day, on the average, with respect to the stars. This means that the apparent solar day averages about 3 min 56 sec longer than the sidereal day.
Apparent solar days are not of equal length on Earth, however, because of the tilt of the Earth’s axis and the Earth’s variable orbital speed. Two successive apparent solar days can vary by as much as 30 sec. For that reason, when clocks and watches came into general use, mean solar time was introduced for civil purposes. A mean solar day has the average length of all the apparent solar days that make up a year. The difference between apparent and mean solar time, known as the equation of time, reaches a maximum of about 16.3 min on or about November 1.
The international date line is the imaginary line on the Earth where a calendar day ends and the next one begins. It extends from the North Pole to the South Pole along or close to the 180th meridian, one of the longitudinal lines that are drawn on maps and globes. The date on the western side of the line is one full day ahead of the date on the eastern side of the line. For example, when it is 6 P.M. Friday on the western side of the line, it is 6 P.M. Thursday on the eastern side. Why is this so?
The Earth is divided into 24 standard time zones. Each zone covers 15 degrees longitude, extending 71/2 degrees east and 71/2 degrees west from its central meridian of longitude. This is a convenient measure, because the Earth rotates eastward at the rate of 15 degrees longitude an hour. Moving westward, therefore, each succeeding time zone is one hour earlier than the zone before it. Moving eastward, each succeeding zone is one hour later. The starting zone is labeled by the prime meridian (0 degrees longitude), which runs through Greenwich, England—the site of the world’s first astronomical observatory.
Suppose it is 6 A.M. Friday at the prime meridian. What time is it in the standard time zones of the “later half” of the world, moving east from the prime meridian to the 180th meridian (zones +1 through +12)? The time in each eastward zone is 1 hour later than the time in the preceding zone (zones −1 through −12). Therefore, in this half of the world (representing the 12 hours from 6 A.M. Friday to 6 P.M. Friday), the latest time on the globe (6 P.M. Friday) is found at the 180th meridian. What time is it in the “earlier half” of the world, moving west from the prime meridian to the 180th meridian? Each westward zone is 1 hour earlier than the preceding one. Therefore, in this half of the world (representing the 12 hours from 6 A.M. Friday to 6 P.M. Thursday), the earliest time on the globe is also found at the 180th meridian. This example means that on opposite sides of the 180th meridian, the time differs by a full day (24 hours). And since the western side of the 180th meridian’s zone always has the latest time on the Earth, this is where each new date begins.
Nations of the world selected the 180th meridian as the dividing line between the Earth’s earliest and latest hours because it runs largely through ocean rather than land. And by zigzagging away from the 180th meridian in a few places, the international date line avoids land entirely. This prevents nearby villages and towns from having two different dates.