Welcome to You Ask Andy

Philip Poulin, age 14, of West Warwick, R.I., for his question:

How did our days, months and years originate?

Originally these passing periods of time were donated by the sun and the stars, the moon and the whirling world. For our time system is based, more or less, on the motions of the heavenly bodies. In a general way, this was established long ages ago. Later, astronomers gradually made the details more precise.

Most likely our days, months and years originated with the Magi of ancient Babylon more than 5,000 years ago. These early astronomers studied the motions of the sun, the stars and the moon  and bisected the circle to cope with the problem. The planet earth, of course, is surrounded on all sides by star studded space. As it rotates on its axis and revolves around the sun, we are turned to face changing views of outer space.

The sky overhead is 'like a huge dome, a celestial hemisphere reaching from horizon to horizon. As the earth rotates toward the east, the heavenly bodies rise in the east, arch up overhead and set in the west. Then we turn around to face the opposite direction and behold the opposite celestial hemisphere.

As the earth completes each rotation, the two hemispheres give us a complete picture of the celestial sphere. our calendar day is based on the earth's rotation  and the celestial sphere provides us with a great clock in the sky. This problem was solved, more or less, by early astronomers many thousands of years ago.

The year is based on the time it takes the tilted earth to orbit once around the sun. As this happens, our view of the celestial sphere gradually changes with the seasons. For example, in summer the stars of our winter night pass overhead during the daytime when they are masked by the dazzling sun. The heavens that provide a celestial clock also provide a celestial calendar.

Early astronomers noted that twelve major constellations overhead with the changing seasons Possibly the Babylonians first divided these star groups into the twelve constellations of the zodiac. This was one way to measure the months.

Other early astronomers noted the changing phases of the moon and tried to fit the lunar months into the solar year. But after many tries, this idea was discarded as downright impossible.

The day is based on the earth's rotation and the year on its revolution around the sun. These celestial patterns originated when the earth was born. Thousands of years ago, several groups of early astronomers began using the heavenly parade to measure the passage of time.

 

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