pp. 74-6:
"However, the sun's motion in right ascension [along the celestial equator]
is not uniform because the Sun moves in the ecliptic rather than the
equator, and the Sun's motion in the ecliptic is not uniform [due to
elliptical orbit].
Mean solar time was defined by an abstract fiducial point ... fictitious
mean sun ... to move uniformly in the equatorial plane at a rate that is
virtually equal to the mean rate of the true Sun's motion in the ecliptic.
The difference between mean solar time and apparent solar time is called
the equation of time."
At least formerly, solar time was computed from observed sidereal [star]
time. What is today called Greenwich Mean Time (civil time) in the UK is
actually Coordinated Universal Time for the Greenwich time zone. It is kept
by atomic clocks and modified by leap seconds as required by variations in
the Earth's rotation.
I am guessing that the observed noon (sun due south or at its highest
altitude) defined 12:00 noon for apparent solar time at Greenwich.
For those interested in this topic, this book has a sixty-page chapter on
time, including its history.
From "Astronomical Algorithms" by Jean Meeus, p. 379, "The mean solar time
is the hour angle of the mean sun , and this is measured from mean noon.
The civil time is the mean solar time increased by 12 hours, and thus is
measured from mean midnight."
Gordon
At 11:42 6/10/05, Noam Kaplan wrote:
Does anyone know how mean time is set up?
To clarify, I know that local mean time has 24 hours every day set for a
mean daily rotation for any particular location. I understand that
timezones are set up by the local mean time for the center of every 15
degrees of longitude, i.e. 0, 15, 30, 45 etc.
What I don't understand is how was it chosen when 12:00 AM/PM would be.
I used to think that mean time was set by the equinox that midday was
12:00 for the whole year. This is obviously incorrect as the equation of
time which shows how much local mean time is off from apparent solar time
is not 0 at 12 on the equinox.
If anybody can explain to me what and how mean time was set up, I would
appreciate it.
Thanks,
Noam Kaplan
N 40.5833 W 74.1166
Staten Island
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