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RE: RE: Sidreal Time????

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Posted by P. Halsted on June 29, 2000 00:55:46 UTC

Mean solar time is the time it takes (On average) for the sun to go from transit of the meridian to the next transit of the meridian.
This is the closest thing to our well known "clock" time. In fact because the Earth is moving around the sun as it rotates on its axis, the actual time for a star at huge distance to transit the meridian twice varies slightly from mean solar time. This is the Siderial day and it is what siderial time is based upon.

In addition, the motion of the Earth around the sun effects the actual time of the solar transit of the meridian by as much as +/- 15 minutes. To adjust for this,celestial navigators and astronomers use the "Equation of Time".
It is interesting that the failure of the early clocks to accurately predict the exact time of passage of the Sun accross the meridian was considered a fault, and the equation of time was applied to the time on the clock to "correct" it for the true observed solar noon. Now we use the clock as the reference, and apply the correction to determine the "time" of the transit.
This stuff is a lot more interesting than you may believe... check out Dava Sorbel`s "Longitude" and Jo Ellen Barnett`s "Times Pendulum"
The tradition of keeping time by the stars and Sun is much more ancient and logical than our present day quartz wrist watches and atomic clocks, but they are prety good tools for telling where the stars are, and thus where we are in the universe!

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