MadSci Network: Astronomy |
Lee,
You have indeed missed some key points. Your clock measures mean solar time. A mean solar day is the time it would take a mean sun to appear to move from the meridian back to the meridain one time, 24 hours. You may say "The Sun isn't mean, it gives us light and heat." In this case we are talking about an average Sun, that appears to move along the celestial equator at a constant rate. Our Sun appears to move along the celestial equator because, as you stated, the Earth orbits the Sun every 365.2442 days. The Earth's orbit is not a perfect circle though, it is an elipse. If you've studied Keplar you know that when the Earth is closest to the Sun it moves more quickly than when it is at its furthest. Telling time with the Sun constantly appearing to move at a different rate could pose a tricky problem. To simplify things, astronomers have defined the mean solar day to correct for the different lengths of the day due to the Earth's position relative to the Sun. Thus our definition of a day is already corrected for the motion of the Earth around the Sun.
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