MadSci Network: Astronomy |
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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