Скачать или смотреть Teach Astronomy - Precession

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A spinning top or gyroscope that has not pointed straight up will wobble. That is, its axis of rotation traces out a circle. This is called precession. The spinning Earth does exactly the same thing. It's tilted by twenty-three and a half degrees on its axis, and the North Celestial pole traces out a large circle on the sky with a 26,000 year motion. This causes in a very subtle way the position of the North Celestial pole to change among the fixed stars. As subtle as this motion was, it is detectable by observations over many centuries and was known to ancient cultures thousands of years ago. This means that the North Celestial pole has not always pointed at the bright star Polaris. In the time of the ancient Egyptians it pointed at a different star called Thuban, and in the future the North Celestial pole will point at a different star.