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Date Posted: Tue, Nov 11 2003, 4:51:37
Author: no name
Subject: Learn
In reply to: Tripathy 's message, "I need nothing from trolls!" on Tue, Nov 11 2003, 4:20:52

What's a calendar?

A calendar is a system for measuring time, from hours and minutes, to months and
days, and finally to years and centuries. The terms of hour, day, month, year and
century are all units of time measurements of a calender system.

How does one measure time?

Distance can be measured with a stick. Time is measured by observing the movements
of the sun, moon and stars. People from all cultures have discovered this from
pre-history to the present time.

What's a 'Day?'

Every one knows about the rotation of the Earth about its axis, which causes
(apparent) movement of the Sun from East to West across the sky. So we define one
cycle of movement of the Sun as one 'Day.'
The Chinese word is very straight forward and calls one day as one 'Sun.'
More...

What's a 'Month?'

For this we look at the Moon! Each night, the apperance of the Moon changes. From
'new moon' to 'full moon' and back. So we define a 'Month' as the time it takes for the
Moon to go through one cycle of motion. As it happens, this takes about 29.5 days. So
we round the month to be either 29 days or 30 days.
Again, in the Chinese language a month is simply a 'Moon.'
More...

What about a 'Year?'

The next larger unit of measurement of time is the 'Year.' For this we go back to the
Sun again. Careful observations reveal that the over a period of many months (12),
the position of the Sun shifts from very high overhead to a much lower point at Noon
time. The length of daylight time also changes from longer to shorter. Even more
importantly, the weather changes from hot to cold, giving rise to the four seasons of
Winter, Spring, Summer and Fall.
It is logical, therefore, to define the length of this time period as a 'Year.'
The Chinese word for 'Year' is 'Nian', written as . Since the word 'Sun' has already
assigned the meaning of a 'day', a new word has to be used to denote the 'year.'

How long is a year?

A little more than 365 days. In 104 B.C. the length of a year was determined to an
accuracy of 365.2502 days. By 480 A.D., Ju Chongzhi refined it to 365.2428 days, or 52
seconds more than
the modern value of 365.2422 days.
To put it another way, in 2,000 years the total discrepency is less than one day!
Together with the voluminous anals of Chinese historians, the Chinese has provided
the most accurate and uninterrupted time-line records.

Although 0.24 day does not look like much, over many years it becomes significant.
How do we round it to a whole number of days? There are several different schemes to
do this. The Chinese scheme is called the 'lunar calendar', and the nearly standard
calendar is called the 'solar' calendar system.

Chinese Lunar Calendar

A normal year has 12 lunar months, with the length of lunar month defined above.
In order to make up to 365.24 days, an extra month is added during the Leap Year.

Solar or Western Calendar

In the Solar Calendar system, a normal year has 365 years. Every 4-th year, an extra
day is added in February to make up to 365.25 days. This is called the Leap Year.
Each year still has 12 months, but the number of days in each month vary illogically,
so the start of each month does not conicide with the phase of the New Moon at all.

Decades, Century, Millennium vs Great Year, Cycle and Epoch

For historical discussions of long periods, longer units of time are handy.

In the Western calendar terminology:
Decade = 10 years
Century = 100 years
Millennium = 1,000 years

In Chinese calendar terminology:
Great Year = 12 years
Cycle = 5 Great Years = 60 years
Epoch = 60 Cycles = 60 x 60 years = 3,600 years

We are now in the Third Millennium, or Second Epoch.

How is a day divided?

Since the Han Dynasty, a day is divided into 12 'double-hours' of equal length.

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