
Time
Historically, the principle purpose of astronomy was record and measure time. Paraphrased from George Reed in his 1989 book, Dark Sky Legacy , "clocks and calendars are not just for crossing off the days and years; clocks and calendars are for saving the phenomena of our civilization." Although the notion of time is a fascinating and difficult one, we have measured it for centuries. A "unit" of time is the interlude between any two successive events - the fundamental one for us being the length of time between two successive risings of the Sun or one syndodic Earth rotation - 24 hours.
Primarily, our civilization has used the motion of the Sun as a measurement of time. At a very young age, children understand the difference between day-time and night-time (when we can and can't see the Sun). If we draw an imaginary line splitting the sky into eastern and western halves, called the meridian, whenever the Sun is on the eastern half of the sky, we say it is morning or A.M. (ante meridian). Whenever the Sun is on the western half of the sky, we say it is afternoon or P.M. (post meridian). Noon is, of course, when the Sun is on the meridian line.
Since antiquity,
time has been measured using the moving shadow of a stick in the ground.
This stick, called a gnomen, creates a shadow at sunrise. This shadow
rotates "clockwise" until the sun sets in the west. The shadow is shortest
when the Sun is directly in the South - thus defining local noon. The Sumerians
are credited with dividing the day and night into twelve equal parts. Twelve
was considered to be a very lucky number because of it's non-primeness
(twelve is evenly divisible by 1, 2, 3, 4, and 6). Originally, all days
and nights were 12 hours, thus in the winter daylight hours were much shorter
than summer daylight hours. Today, all hours are the same length, 1/24th
of the time needed for one Earth synodic rotation.
Sun
Most folks
realize that the Sun rises in the East every morning and moves westward
across the sky. It is also evident to observers that the Sun sets in the
West every evening and returns triumphantly to the East over the course
of the night to rise again. The activities in this section focus on systematic
observations because most of our students do not have experience as "observers."
The motion of the Sun is easy and complicated at the same time, depending
on the depth you wish to observe it. Students will quickly discover
that wristwatch time and sundial time are not the same.
Sundials
record local apparent solar time or where the Sun is is in the sky. Part
of the difference between the two "times" is the earth's revolution around
the Sun. The Earth's velocity changes throughout the year, moving faster
in the winter when close to the Sun and slower in the summer when farther
from the Sun. The result of this variation is that the time between median
crossings (local noon) varies. So, we currently measure the passage of
time by something called mean solar time. The length of the mean solar
day is the length of the average apparent solar day over one year. Sundials
and wrist watches potentially read the same only four times a year: mid
April, mid June, early September, and late December. The difference between
the two can reach more than 16 minutes and is calculated with something
known as the "equation of time" that graphically looks like a figure eight
found on most globes called the analema.
Historically, noting the Sun's daily crossing of the meridian was precise enough for earlier civilizations. However, because every location on Earth has its own "local apparent noon" accompanied with the highly mobile population in today's world, it has become necessary to create time zones. Time zones are areas in which everybody uses the same wristwatch time regardless of their "local apparent noon." They are a recent construct, internationally adopted in 1883 due to increased feasibility of train transportation. Geographically, time zones occur every 15 of longitude around the world starting in Greenwich, England. The time at that location is sometimes known as Greenwich Mean Time (GMT), Universal Time (UT) or Zulu Time. Practically, time zones are somewhat arbitrary and many municipalities and counties decide which zone they want to be a part of. Atlanta, for instance, is geographically in the Central Time Zone but community leaders believed that Atlanta should use the same time zone as New York. The issue of daylight savings time was originally suggested by Ben Franklin in 1874 but didn't really catch on until 1942 (then called War Time). However, not everywhere in the US uses daylight savings time (Arizona, Hawaii, and part of Indiana for example).
On an annual scale, the Sun's altitude (or perceived height in the sky) changes significantly over the course of the year. At the PSU/GAO, the winter-time noon Sun only reaches 29 in altitude whereas the summer-time noon Sun reaches almost 87 in altitude (the Sun is never directly overhead at the latitude of the PSU/GAO). The small altitude of the Sun results in very long winter-time shadows. The opposite is true for the large altitude for the summer-time Sun. On approximately December 21, the Winter Solstice, the Sun's noon time shadow will be at its longest noon-time length. Conversely, on approximately June 21, the Summer Solstice, the Sun's noontime shadow will be at its shortest length.
The
length of time it takes the Sun to go from its lowest altitude (winter
solstice) to its highest altitude (summer solstice) and back down again
is defined as ONE YEAR. This cycle from short noon-time shadow to long
noon-time shadow and back again takes approximately 365.24220 days (the
0.24220 fractional days or 5 hours, 48 minutes, and 46 seconds is a constant
source of headaches for calendar makers). In 45 BC, Julius Caesar implemented
a 365 day calendar with an extra day on February 29 every four years. This
however, was an imperfect solution that caused the calendar to be off one
entire day every 128 years. Catholic religious officials realized that,
if this went on long enough, Christmas would be celebrated on the same
day as Easter. In 1582, Pope Gregory XIII introduced the currently used
Gregorian
calendar that does not allow leap years in Centuries unless the
year is evenly divisible by 400. In such a system, the years 1700, 1800,
and 1900 are not leap years, but 2000 is a leap year. The systematic error
in this calendar is about 25 seconds each year or about one day in 3,300
years. Although we can measure time, it's exact nature still eludes us
(eg. Einstein, 1905).