Time and Calendar
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. Unfortunately Earth needs 365 days plus
about six hours to go around the Sun once, so that the year does not consist
of so and so many 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. Astronomers use a point which they call the "vernal equinox"
for the actual determination. Such a sidereal day is somewhat shorter than
the "solar day," namely by about three minutes and 56 seconds of so-called
mean solar 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.E.) 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 (or in the Julian Day number) if the
astronomical day, like the civil day, ran from midnight to midnight.

Mean solar time, rather than apparent solar time, is what is actually used
most of the 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 The Seasons) 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.

When the real Sun is west of the mean sun we have the "sun fast" condition,
with the real Sun crossing the meridian ahead of the mean sun. The opposite
is the "sun slow" situation, when the real Sun crosses the meridian after
the mean sun. Of course, what is observed is the real Sun. The equation of
time is needed to establish mean solar time, kept by the reference clocks.

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. (This system was actually put into use a year
earlier by the railroad companies of the U.S. and Canada who, until then,
had to contend with some 100 conflicting local sun times observed in
terminals across the land.)

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 (provided he does so where the
Date Line does not coincide with the 180° meridian), but it would be the
same time of the previous day.



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Sent: Sunday, December 26, 1999 1:22 AM
Subject: A New Funpage: Snowman on Vacation


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