Timekeeping - solar time (or tropical time ). Technically,...

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Timekeeping Historically, the regular motion of objects in the sky served as the basis for timekeeping. The diurnal motion of the sky caused by the rotation of the Earth on its axis defined the day, the year was defined by the motion of the Earth on its orbit about the Sun, and the month was defined in relation to the revolution of the Moon about the Earth. Although precise modern timekeeping is done electronically, many of the details and the terminology of timekeeping remain rooted in its astronomical heritage. Sidereal Time and Solar Time In using the sky for timekeeping, we must define a reference point to determine when a cycle of the required motion has been completed. If we choose a reference point afixed to the celestial sphere, the corresponding time is being referenced to the distant stars and is termed sidereal time . If instead we choose the Sun as the reference point, the corresponding time is called
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Unformatted text preview: solar time (or tropical time ). Technically, the sidereal time is defined as the length of time since the vernal equinox has crossed the local celestial meridian. An equivalent definition of the sidereal time is the right ascension of any star presently located on the local celestial meridian. Thus, if the star Sirius is presently on your celestial meridian, the sidereal time is 6 hours and 45 minutes because we saw earlier that Sirius is located at 6 hr 45 min right ascension on the celestial sphere. Generally our everyday (civil) time is referenced to the (average) motion of the Sun, not the vernal equinox. Thus, sidereal time generally does not coincide with the everyday (wall clock) time. To be precise, the sidereal time agrees with the solar time only at the autumnal equinox; at any other time, they differ (they are exactly 12 hours apart at the time of the vernal equinox)....
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