And here is a discussion of mean time as well as UTC: <http://gauss.gge.unb.ca/GMT.UT.and.the.RGO.html>. -- Richard Langley
On Sat, 11 Jun 2005, Gordon Uber wrote: > From "Explanatory Supplement to the Astronomical Almanac" by Seidelmann, >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 > > >- > =============================================================================== Richard B. Langley E-mail: [EMAIL PROTECTED] Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142 University of New Brunswick Fax: +1 506 453-4943 Fredericton, N.B., Canada E3B 5A3 Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ =============================================================================== -
