MadSci Network: Astronomy
Query:

Re: How do we know that a year has passed.

Date: Wed Feb 14 13:50:58 2001
Posted By: Todd Whitcombe, Faculty, Chemistry, University of Northern British Columbia
Area of science: Astronomy
ID: 979308077.As
Message:

Interesting question - and I assume that you are looking for more than 
"because january 1st rolls around"!

The simple answer is that a year is the time it takes for the stars to 
return to exactly the same position in the sky - allowing for the tiny 
movements of the stars - at a specific time of day. That is, if local noon
is defined as the moment when the sun is at its zenith, then a year is the
time it takes for the Sun - at noon - to be in a precise position in the
sky with respect to all of the other stars. This is a "solar year". In the
past few years, the length of a year has become standardized based on 
Atomic clocks and the National Institute of Standards. They are now charged
with "keeping the year" and letting us know when one has passed.

A longer answer as to the question of the evolution and use of the year is
provided below. It is drawn from a talk on "Time" that I gave a few years
ago.
	The first calendar dates back to about 2800 BC, about 5000 years 
ago. I should mention that there is a cycle of intermediate length between 
the day and the year and that is the phases of the Moon. It takes 29 or 30 
days for the Moon to go through its cycle of phases, and it takes 12 or 13 
of these cycles - (or months from the word Moon) - to make up the cycle of 
the seasons.
	No one knows when people first began to attach importance to the 
months. There are indications that even prehistoric people counted them, 
but it was the people of the Tigris-Euphrates region who first systematized 
the matter. They worked out a cycle of 19 years, in which 12 years had 12 
lunar months and 7 years had 13 lunar months. Such a cycle kept the years 
even with the seasons. This lunar calendar was adopted by the Greeks and 
the Jews and is still used as the Jewish liturgical calendar.
	The Egyptians, however, did not make extensive use of the Moon. To 
them, the important feature of the year was the periodic flooding of the 
Nile. The priests in charge of irrigation carefully studied the height of 
the river from day to day and eventually discovered that, on average, the 
flood came every 365 days. That was also the time it took the Sun to make 
an apparent circuit of the sky relative to the stars - that is, against the 
background of the zodiac. 
	But the stars are not out during the day! The Egyptians realized 
that the Sun must be in the constellation opposite the one overhead at 
midnight - or where the sun would be if it was up in the sky - so they knew 
in which constellation to find the sun , although they couldn't see the 
stars. This passage of the Earth around the sun is a solar year and the 
calendar is a solar calendar.
	The Egyptians were aware that there were usually 12 new Moons to 
the year, so they had 12 months, but they made each month 30 days long, 
paying no attention to the actual phase of the Moon. That made 360 days, to 
which they added 5 more days at the end.
	This calendar was much simpler and handier than any other calendar 
invented in ancient times. The uniformity of the months provided a basis 
for regularizing the passage of time. Historians are uncertain of the date 
when it was first adopted, but priests may have been using it for their 
private computations as early as 2800 BC.
	Nothing better than the Egyptian calendar was devised for nearly 
three thousand years, and even then what was produced was a mere 
modification of it - with not all the changes being for the better. Our 
present calendar is still based on the Egyptian calendar. This makes our 
calendar, in essence, nearly five thousand years old.





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