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FIGURE 16.25 The international date line follows the 180° meridian but is arranged in such a way that land areas and island chains
have the same date.

A sidereal year is defined as the interval of time required of July was named in honor of Julius Caesar, and the following
for Earth to move around its orbit so the Sun is again in the same month was later named after his successor, Augustus.
position relative to the stars. The sidereal year is slightly longer than There was a slight problem with the Julian calendar be-
the tropical year because Earth rotates more than 365.25 times dur- cause it was longer than the tropical year by 365.25 minus
ing one revolution. Thus, the sidereal year is 365.25636 mean solar 365.24220, or 0.0078 day per year. This small interval (which
days, which is about 20 minutes longer than the tropical year. is 11 minutes, 14 seconds) does not seem significant when com-
The tropical and sidereal years would be the same inter- pared to the time in a whole year. But over the years, the error of
val of time if Earth’s axis pointed in a consistent direction. The minutes and seconds grew to an error of days. By 1582, when Pope
precession of the axis, however, results in the axis pointing in Gregory XIII revised the calendar, the error had grown to 13 days
a slightly different direction with time. This shift of direction but was corrected for 10 days of error. This revision resulted in
over the course of a year moves the position of the spring equi- the Gregorian calendar, which is the system used today. Since the
nox westward, and the equinox is observed 20 minutes before accumulated error of 0.0078 day per year is almost 0.75 day per
the orbit has been completely circled. The position of the spring century, it follows that four centuries will have 0.75 times 4, or
equinox against the background of the stars thus moves west- 3 days of error. The Gregorian system corrects for the accumulated
ward by some 50 seconds of arc per year. error by dropping the additional leap year day three centuries out
It is the tropical year that is used as a standard time inter- of every four. Thus, the century year of 2000 was a leap year with
val to determine the calendar year. Earth does not complete 366 days, but the century years of 2100, 2200, and 2300 will not
an exact number of turns on its axis while completing one trip be leap years. You will note that this approximation still leaves an
around the Sun, so it becomes necessary to periodically adjust error of 0.0003 day per century, so another calendar revision will
the calendar so it stays in step with the seasons. The calendar be necessary in a few thousand years to keep the calendar in step
system that was first designed to stay in step with the seasons with the seasons.
was devised by the ancient Romans. Julius Caesar reformed the
calendar, beginning in 46 b.c., to have a 365 day year with a
366 day year (leap year) every fourth year. Since the tropical Monthly Time
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year of 365.24220 mean solar days is very close to 365 ⁄4 days, In ancient times, people often used the Moon to measure time
the system, called a Julian calendar, accounted for the ⁄4 day by intervals that were longer than a day but shorter than a year.
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adding a full day to the calendar every fourth year. The Julian The word month, in fact, has its origins in the word moon and
calendar was very similar to the one now used, except the year its period of revolution. The Moon revolves around Earth in an
began in March, the month of the spring equinox. The month orbit that is inclined to the plane of Earth’s orbit, the plane of the

418 CHAPTER 16 Earth in Space 16-14