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Timekeeping
has been the pervue of astronomers for most of history. Agraian
socieities needed to know when the rains would begin or when religious
ceremonies needed to be held. It is natural to use the Sun as
a timekeeping device since it defines when we are awake and when we
are asleep.
The sundial was the first attempt to keep track of apparent solar time. An apparent solar day is the time it takes the Sun to go from one high noon to the next. For more precise measurements astronomers use the meridan, which is a circle on the celestial sphere that passes through the zenith and both celestial poles. It is noon at your location when the sun crosses the meridian above the horizon and midnight when the Sun crosses the meridian below the horizon. The crossing of the meridian by any object is called a meridian object. If the crossing occurs while the Sun is above the horizon (noon) it is called an upper meridian tranist. An apparent solar day is defined as the interval between two successive upper meridian transits of the Sun as observed from any fixed spot on the Earth. |
