Time Measurement, Time Zones, and the International Date Line
The two natural cycles on which time measurements are based are the year and the day. The year is defined as the time required for Earth to complete one revolution around the Sun, while the day is the time required for Earth to complete one turn upon its axis. Earth needs 365 days plus about six hours to go around the Sun once, so a year does not consist of a round number of days; the fractional day has to be taken care of by an extra day every fourth year.
But because Earth, while turning upon its axis, also moves around the Sun, there are two kinds of days. A day may be defined as the interval between the highest point of the Sun in the sky on two successive days. This, averaged out over the year, produces the customary 24-hour day. But one might also define a day as the time interval between the moments when a certain point in the sky, say a conveniently located star, is directly overhead. This is called:
Sidereal time. A sidereal day is the time that it takes the Earth to complete one rotation on its axis so that a particular star can be observed twice at the meridian that runs directly overhead. Because the Earth is moving around the Sun as it rotates on its axis, the sidereal day is about four minutes shorter than the solar day, being equivalent to 23 hours, 56 minutes, and 4 seconds in mean solar time. As a result, a star will appear to rise about four minutes earlier every night, and different stars will be visible at different times of the year. Astronomers use a point that they call the “vernal equinox” to determine local sidereal time.
Apparent solar time is the time based directly on the Sun’s position in the sky. In ordinary life the day runs from midnight to midnight. It begins when the Sun is invisible by being 12 hours from its zenith. Astronomers use the so-called Julian Day, which runs from noon to noon; the concept was invented by the astronomer Joseph Scaliger, who named it after his father, Julius. To avoid the problems caused by leap-year days and so forth, Scaliger picked a conveniently remote date in the past (4713 B.C.) and suggested just counting days without regard to weeks, months, and years. The reason for having the Julian Day run from noon to noon is the practical one that astronomical observations usually extend across the midnight hour, which would require a change in date if the astronomical day, like the civil day, ran from midnight to midnight.
Mean solar time, rather than apparent solar time, is the basis for local civil and standard time. The mean solar time is based on the position of a fictitious “mean sun.” The reason why this fictitious sun has to be introduced is the following: Earth turns on its axis regularly; it needs the same number of seconds regardless of the season. But the movement of Earth around the Sun is not regular because Earth’s orbit is an ellipse. This has the result (as explained in the section that Earth moves faster in January and slower in July. Though it is Earth that changes velocity, it looks to us as if the Sun does. In January, when Earth moves faster, the apparent movement of the Sun looks faster. The mean sun of time measurements, then, is a sun that moves regularly all year round; the real Sun will be either ahead of or behind the mean sun. The difference between the real Sun and the fictitious mean sun is called the equation of time.
Time zones. But if all clocks were actually set by mean solar time we would be plagued by a welter of time differences that would be “correct” but a major nuisance. A clock on Long Island, correctly showing mean solar time for its location (this would be local civil time), would be slightly ahead of a clock in Newark, N.J. The Newark clock would be slightly ahead of a clock in Trenton, N.J., which, in turn, would be ahead of a clock in Philadelphia. This condition prevailed until 1884, when a system of standard time was adopted by the International Meridian Conference. Earth’s surface was divided into 24 zones. The standard time of each zone is the mean astronomical time of one of 24 meridians, 15 degrees apart, beginning at the Greenwich, England, meridian and extending east and west around the globe to the International Date Line. For practical purposes, this convention is sometimes altered. For example, Alaska, for a time, consisted of four of the eight U.S. time zones: the Pacific standard time zone (east of Juneau) and the 6th (Juneau), 7th (Anchorage), and 8th (Nome) zones, encompassing the 135°, 150°, and 165° meridians, respectively. In 1983, by act of Congress, the entire state (except the westernmost Aleutians) was united into the 6th zone, Alaska standard time.
The eight U.S. standard time zones are: Atlantic (includes Puerto Rico and the Virgin Islands), eastern, central, mountain, Pacific, Alaska, Hawaii-Aleutian (includes all of Hawaii and those Aleutians west of the Fox Islands), and Samoa standard time.
The Date Line. While the time zones are based on the natural event of the Sun crossing a meridian, the date must be an arbitrary decision. The meridians are traditionally counted from the meridian of the observatory of Greenwich, in England, which is called the zero meridian. The logical place for changing the date is 12 hours, or 180°, from Greenwich. Fortunately, the 180th meridian runs mostly through the open Pacific. The Date Line makes a zigzag in the north to incorporate the eastern tip of Siberia into the Siberian time system and then another one to incorporate a number of islands into the Hawaii-Aleutian time zone. In the south there is a similar zigzag for the purpose of tying a number of British-owned islands to the New Zealand time system. Otherwise, the Date Line is the same as 180° from Greenwich. At points to the east of the Date Line the calendar is one day earlier than at points to the west of it. A traveler going eastward across the Date Line from one island to another would not have to reset his watch because he would stay inside the time zone, but it would be the same time of the previous day.
History of the Calendar
The purpose of the calendar is to reckon past or future time, to show how many days until a certain event takes place-the harvest or a religious festival-or how long since something important happened. The earliest calendars must have been strongly influenced by the geographical location of the people who made them. In colder countries, the concept of the year was determined by the seasons, specifically by the end of winter. But in warmer countries, where the seasons are less pronounced, the Moon became the basic unit for time reckoning; an old Jewish book says that “the Moon was created for the counting of the days.”
Most of the oldest calendars were lunar calendars, based on the time interval from one new moon to the next-a so-called lunation. But even in a warm climate there are annual events that pay no attention to the phases of the Moon. In some areas it was a rainy season; in Egypt it was the annual flooding of the Nile River. The calendar had to account for these yearly events as well.
The Egyptian Calendar
The ancient Egyptians used a calendar with 12 months of 30 days each, for a total of 360 days per year. About 4000 B.C. they added five extra days at the end of every year to bring it more into line with the solar year. These five days became a festival because it was thought to be unlucky to work during that time.
The Egyptians had calculated that the solar year was actually closer to 3651/4 days, but instead of having a single leap day every four years to account for the fractional day (the way we do now), they let the one-quarter day accumulate. After 1,460 years, or four periods of 365 years, they added an entire leap year of 365 days. This means that as the years passed, the Egyptian months fell out of sync with the seasons, so that the summer months eventually fell during winter. Only once every 1,460 years did their calendar year coincide precisely with the solar year.
In addition to the civic calendar, the Egyptians also had a religious calendar that was based on the 291/2-day lunar cycle and was more closely linked with agricultural cycles and the movements of the stars.
During antiquity the lunar calendar that best approximated a solar-year calendar was based on a 19-year period, with 7 of these 19 years having 13 months. In all, the period contained 235 months. Still using the lunation value of 291/2 days, this made a total of 6,9321/2 days, while 19 solar years added up to 6,939.7 days, a difference of just one week per period and about five weeks per century.
Even the 19-year period required adjustment, but it became the basis of the calendars of the ancient Chinese, Babylonians, Greeks, and Jews. This same calendar was also used by the Arabs, but Muhammad later forbade shifting from 12 months to 13 months, so that the Islamic calendar, even today, has a lunar year of 354 days. As a result, the months of the Islamic calendar, as well as the Islamic religious festivals, migrate through all the seasons of the year.
The Roman Calendar
When Rome emerged as a world power, the difficulties of making a calendar were well known, but the Romans complicated their lives because of their superstition that even numbers were unlucky. Hence their months were 29 or 31 days long, with the exception of February, which had 28 days. However, four months of 31 days, seven months of 29 days, and one month of 28 days added up to only 355 days. Therefore the Romans invented an extra month called Mercedonius of 22 or 23 days. It was added every second year.
Even with Mercedonius, the Roman calendar eventually became so far off that Julius Caesar, advised by the astronomer Sosigenes, ordered a sweeping reform in 45 B.C. One year, made 445 days long by imperial decree, brought the calendar back in step with the seasons. Then the solar year (with the value of 365 days and 6 hours) was made the basis of the calendar. The months were 30 or 31 days in length, and to take care of the 6 hours, every fourth year was made a 366-day year. Moreover, Caesar decreed the year began with the first of January, not with the vernal equinox in late March.
This calendar was named the Julian calendar, after Julius Caesar, and it continues to be the calendar of the Eastern Orthodox churches to this day. However, despite the correction, the Julian calendar is still 111/2 minutes longer than the actual solar year, and after a number of centuries, even 111/2 minutes adds up.
The Gregorian Reform
By the 15th century the Julian calendar had drifted behind the solar calendar by about a week, so that the vernal equinox was falling around March 12 instead of around March 20. Pope Sixtus IV (who reigned from 1471 to 1484) decided that another reform was needed and called the German astronomer Regiomontanus to Rome to advise him. Regiomontanus arrived in 1475, but unfortunately he died shortly afterward, and the pope’s plans for reform died with him.
Then in 1545, the Council of Trent authorized Pope Paul III to reform the calendar once more. Most of the mathematical and astronomical work was done by Father Christopher Clavius, S.J. The immediate correction, advised by Father Clavius and ordered by Pope Gregory XIII, was that Thursday, Oct. 4, 1582, was to be the last day of the Julian calendar. The next day would be Friday, Oct. 15. For long-range accuracy, a formula suggested by the Vatican librarian Aloysius Giglio was adopted: every fourth year is a leap year unless it is a century year like 1700 or 1800. Century years can be leap years only when they are divisible by 400 (e.g., 1600 and 2000). This rule eliminates three leap years in four centuries, making the calendar sufficiently accurate.
For in spite of the revised leap year rule, an average calendar year is still about 26 seconds longer than the Earth’s orbital period. But this discrepancy will need 3,323 years to build up to a single day.
Reform Adopted Gradually
The Gregorian reform was not adopted throughout the West immediately. Most Catholic countries quickly changed to the Pope’s new calendar in 1582. But Europe’s Protestant princes chose to ignore the papal bull and continued with the Julian calendar. It was not until 1700 that the Protestant rulers of Germany and the Netherlands changed to the new calendar. In Great Britain (and its colonies) the shift did not take place until 1752, and in Russia a revolution was needed to introduce the Gregorian calendar in 1918. In Turkey, the Islamic calendar was used until 1926.
Despite its widespread use, the Gregorian calendar has a number of weaknesses. It cannot be divided into equal halves or quarters; the number of days per month is haphazard; and months and years may begin on any day of the week. Holidays pegged to specific dates may also fall on any day of the week, and few Americans can predict when Thanksgiving will occur next year. Since Gregory XIII, many other proposals for calendar reform have been made, but none has been permanently adopted. In the meantime, the Gregorian calendar keeps the calendar dates in reasonable unison with astronomical events.
The Hindu (Indian National) Calendar
The Indian National Calendar, often called the “Hindu Calendar,” is based on both lunar and solar years. This calendar was introduced in 1957 in a government push for all of India to use the same calendar, but various traditional calendars are also used. The start of the Indian National Calendar year coincides with March 22, except in a leap year, when it coincides with March 21. The year is counted from the first year of the Saka era, in A.D. 78. The year 2005 translates to Saka era 1926-1927. (Iranian Saka emigrated and settled in India during the Parthian and Sassanian times and established their kingdoms)
The Chinese Calendar
The Chinese lunar year is divided into 12 months of 29 or 30 days. The calendar is adjusted to the length of the solar year by the addition of extra months at regular intervals. The years are arranged in major cycles of 60 years. Each successive year is named after one of 12 animals. These 12-year cycles are continuously repeated. The Chinese New Year is celebrated at the second new moon after the winter solstice and falls between January 21 and February 19 on the Gregorian calendar. The year 2005 translates to the Chinese year 4702-4703.
The Jewish Calendar
The Jewish calendar is based on both solar and lunar years. The average lunar year of 354 days is adjusted to the solar year by the addition of a leap year and an intercalary month. Nisan is considered the first month, although the new year begins with Rosh Hashanah, on the first of Tishri, which is in fact the seventh month-the calendar has different starting points for different purposes. The year 2005 translates to the Jewish year 5765-5766.
The Islamic (Hijri) Calendar
The Islamic calendar is based on the lunar year of 354 days. The number of days each month is adjusted according to the lunar cycle, beginning about two days after the new moon. The months drift backward over the seasons, beginning again on the same day every 321/2 years. The Islamic year begins on the first day of Muharram, and is counted from the year of the Hegira (anno Hegirae)-the year in which Muhammad emigrated from Mecca to Medina (A.D. 622). The year 2005 translates to A.H. 1425-1426.
Note by AAJ: Islam, like Christianity, is a product of Judaism. In act, it is closer to Judaism than Christianity. However, the Quran prohibits the addition of the intercalary month (Quran: Surah 9:36-37) and therefore, the Islamic calendar is based purely on lunar months. Islam does not follow any solar or intercalary lunisolar year.