John My recent postings relate to this question. The leap year is not relavent in the use of an analemma for reading standard time. The leap year is an adjustment to keep the number of rotations of the earth in synch with the revolution about the sun. The reading of standard time using an analemma should make use of the declination of the sun and that is completely independant of issues relating to the rotation of the earth. The difference in right ascension of the sun and of the mean sun (the EoT) is independant of the rotation of the earth on its axis and is only dependant upon the revolution of the earth about the sun. The EoT has meaning if there were no rotation of the earth or an arbitary rate of rotation for the earth. The mean sun is a construct that can be defined completely independantly of the rotation of the earth. Once the mean sun has been defined it may be used to measure the rate of rotation of the earth and of the position of Greenich meridian as a function of the mean sun time.
I would suggest that a spherical dial is the most accurate as the reading of the time is as accurate at noon as at any other hour of the day. If the Singleton dial uses an analemma based upon a mean EoT then it is date related and not declination related. Per my arguments this analemma is not correctly designed to be accurate and invarient over a period of years. If the mean EoT is the same as the declination related analemma then the word mean can be removed and it will be accurate over a period of years. Dan Wenger >Hi all, > >I have a question/challenge to all you sundial designers: what is the most >accurate design for a Standard Time dial? > >The reason behind the question is to find a way to stop members of the >public looking at a public dial, inspecting their watches, and concluding >that dials never tell the right time! > >The criteria for the dial are, in my opinion: > >a) it should tell Standard Time, (or possibly Daylight Saving Time - BST in >the UK) >b) it should be in a fixed location >c) it must have no moving parts (which rules out adjustable equatorials and >changeable gnomons etc) and > should be as physically robust as possible. >d) it must not require reference to a separate table or computer program eg >to get an exact declination for the sun. > All data must be built into the dial plate. >e) the "accuracy" should be interpreted as the mean error for the hour >lines 3 hours either side of noon (or 12:00) for the years 2000 to 2050. > >As a starter, the "Singleton" dial recently discussed here would seem to be >a reasonable candidate. It's main limitation, common to all dials which >incorporate an EoT correction, is that it is drawn for a some MEAN EoT >curve, and no allowance is made for the leap year cycle and the other minor >variations. Is there some geometry of dial plate and style which minimises >the time error caused by small year-to-year variations in the mean daily >declination? If this is achieved, then the small change in the EoT over a >single day may be allowed for. > >There is no prize for the competition, but I promise I will build a physical >example of the best suggestion, and share it with the list! > >Happy designing, > >John >---------------------------------------------------------------------------- >---------------------------------- > > >Dr J R Davis >Flowton, UK >52.08N, 1.043E >email: [EMAIL PROTECTED] Daniel Lee Wenger Santa Cruz, CA [EMAIL PROTECTED] http://wengersundial.com http://wengersundial.com/wengerfamily
