Galileo
Lines connecting the Geographic North and South Poles are called Meridians, and the one passing through Greenwich (just southeast of London) is known as the Greenwich or Prime meridian. Angular measurement East of West in degrees is called Longitude. There is an observatory located at Greenwich with a powerful optical telescope. This forms the basis for time measurement. All time is measured from the time at Greenwich called the Greenwich Mean Time (G.M.T.). Places to the west of Greenwich are said to be behind and those east are ahead. There are a total of 360 degrees of Longitude. At the point diametrically opposite Greenwich 180 degrees East coincides with 180 degrees west. This line is called the International Date Line. Here the date will change. For more information and answers to questions. click below.
We divide the day into 24 hours, an hour into sixty minutes and each minute into sixty seconds. In 1964, the twelfth General conference on Weights and measures, meeting in Paris, adopted an atomic definition of the second as the international unit of time. The cesium 133 atom provides a highly accurate and stable reference frequency of 6,192,631,770 cycles per seconds.
The earliest measurement of time was by means of a Gnomon which was simply a vertical stick in the ground, the shadow indicating the time of the day. The vertical pointer of the Sundial is the Gnomon. The Sundial can only be used when the Sun is shining and always gives the Local time.
The water clock in which water falls from one vessel to another with
the size of the opening fixed for a specific rate was invented in the third
century B.C. A similar principle was used for sand, or burning candles.
In the sixteenth century, Galileo noticed a large hanging lamp swinging
back and forth at regular intervals of time as measured by his pulse rate.
This gave him an idea for the construction of a pendulum clock, and paved
the way for the building of the first pendulum clock by Christian Huygens.
Galileo
For more information about Galileo, click below
Today we use electric clocks controlled by the frequency of alternating current, and wrist watches controlled by quartz crystal.
The 24 hour day starts and ends at midnight. For convenience,
it is divided into two equal parts of 12 hours each. The time from
midnight to noon is called Ante Meridiem (A.M.) and that from noon
to midnight called Post Meridiem (P.M.).
The local time is noon when the sun is at its highest position in the
sky. Local Time was suitable before mass communications. To
know local time for specific cities around the World click below.
In order to facilitate communications, time was standardized into zones having the same time. In the conterminous United States there are four zones: Eastern, Central, Mountain, and Pacific. They are 5, 6, 7, and 8 hours behind Greenwich Mean Time respectively. There are other zones for Alaska and for Hawaii. The Earth rotates 360 degrees in 24 hours. So fifteen degrees constitute an hour. Thus the time zones are approximately 15 degrees apart. If the longitude of a place is known, determine the nearest multiple of 15. Then divide this number by 15 to find the number of hours the place is ahead or behind G.M.T., or find the difference in time between two places. One degree is further equal to four minutes. For more information on Standard time click below.
During World War I, daylight saving Time was introduced in summer to
give more daylight hours thus conserving fuel used to generate electricity
for lighting. This practice has now become standard for most of the
United States and many other countries around the globe. Humans are
creatures of habit. During the summer months, in locations far from
the Equator, the Sun rises around 3 to 4 A.M. and sets around 8 to 9 P.M.
(Local Time). People prefer to rise at 6 A.M. and go to sleep at
10 A.M. So they need an hour or two of electricity at night and waste
2 to 3 hours of glorious sunshine in the early morning. If people
could be persuaded to rise at 5 A.M. and go to bed at 9 P.M. they could
save an extra hour of electricity. Due to their reluctance to do
so, they are fooled by having the clock set ahead. So when the clock
shows 10 P.M. (time for bed) by the Sun it is 9 P.M. while when the clock
reads 6 A.M. (time to rise) by the Sun it is just 5 A.M. In the winter
since it gets dark at 4 P.M. and the Sun does not rise until 8 A.M.
we would just exchange an hour of electricity in the evening by an
hour in the morning and not achieve any real savings. Daylight Saving
time (D.S.T.) starts at 2 A.M. on the first Sunday of April and ends at
2 A.M. on the last Sunday of October in the United States. In Europe
it starts on the last Sunday of March. During the war, double daylight
time was in effect in Europe and Asia, since there blackouts and curfews
were often enforced.
For problems in time, click below
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