Solar Noon Equation of Time Calculator
Hi Piers , I have visited with a lot of interest your Solar Noon Calculator on the web at www.solar-noon.com and I have immediately made some tests to compare your values with those calculated from me and published in an article in the proceedings of our X Seminario di Gnomonica ( X Italian Meeting on Sundials - 2000) Here are some considerations of mine 1) From the comparison I have immediately seen that your results are wrong because of a banal error: anywhere the value of the EoT has been or taken with opposite sign or subtracted instead that added. An example : Long. =12d E and TZ of Central Europe (central Meridian 15 d East): longitude correction =+3d = +12m At 1/1/2001 the exact value of the Eot = - 3m41s an so : - Local Apparent Time (apparent solar time) = 12h - Local mean time (local mean solar time) = 12h 3m 41s - Standard Time = 12 15m 41s (NASS Dialist Companion gives the value, less approximate, 12h 15m39s ) In the Table calculated with the Solar Noon Calculator is written the value = 12h08m48s, value that is obtained adding (instead that subtracting ) the value of the Eot: 12h+12m+(-3m12s) = 12h08m48s 2) The table of the EoT NOT gives the values of the EoT but the Total correction that it is necessary to add to the Local Apparent Time to obtain the Standard Time : it is therefore the sum of the EoT + longitude correction.. This value is certainly very useful but, perhaps, it is necessary to give some explanations and it is opportune not to call it Eot Moreover in this way the table with the values of noon is useless because these values are equal to those of the EoT + 12h The definition : Equation of Time displays the difference between solar time and the standard times where you are (note at the foot of the page) it is not correct. Davis' Sundials Glossary gives the following: Equation of Time: the time difference between Local Apparent Time (apparent solar time) and mean solar time at the same location (NOT Standard Time). Its value varies between extremes of about +14 minutes in February and -16 minutes in October. 3) Checking only for the date 1/1/2001 I have found that the error between the exact value and the mean value of the EoT = 3m41s -3m12s=29 sec: almost the double of the maximum error (in the Note). With NASS Dialist Companion we obtain the less approximate value Eot = 3m37s: also with this value the error would be of 25s Does the greater error depend on the fact that the Eot has been calculated at 0h (UT) instead that at 12h (UT) In fact the Eot can also change till 20s a day. 4) In my opinion it is very useful, for instance in the construction of sundials with mean time, to have a table of the mean values of the Eot (as of the mean declination of the Sun), while the table that gives the mean local noon (mean on 4 years) can be used in a wrong way. In fact these tables, that should be used only for the search of the mean time from the apparent solar time given by a sundial, could be considered right also in the search of the declination of the walls. In this case it is better to use the true value of the EqT calculated with programs as NASS Diallist Companion Perhaps a note could clarify the thing. 5) As in almost all Web sites, also you take as positive the Longitudes for places West of Greenwich. Despite the opinion of the known astronomer J. Meeus, with which also Davis agrees in his Sundial Glossary, even if a secular tradition justifies this definition, it is NOT correct. The Explanatory Supplement to the Astronomic Almanac (USNO 1992) at page 203 affirms: The geocentric longitude is defined by the angle between the reference (or zero) meridian and the meridian of point p, measured EASTWARD around the Earth from 0 to 360 deg (IAU, 1983, p.47) Perhaps it would be opportune, at least in new programs and in new Web sites , to use this correct definition Best wishes Gianni Ferrari
Re: Solar Noon Equation of Time Calculator
Gianni wrote: As in almost all Web sites, also you take as positive the Longitudes for places West of Greenwich. Despite the opinion of the known astronomer J. Meeus, with which also Davis agrees in his Sundial Glossary, even if a secular tradition justifies this definition, it is NOT correct. The Explanatory Supplement to the Astronomic Almanac (USNO 1992) at page 203 affirms: The geocentric longitude is defined by the angle between the reference (or zero) meridian and the meridian of point p, measured EASTWARD around the Earth from 0 to 360 deg (IAU, 1983, p.47) Perhaps it would be opportune, at least in new programs and in new Web sites , to use this correct definition Just because there is a definition which professional astronomers have adopted for their purposes, I don't see any reason to assume automatically that it is the correct one for practical sundialling. It seems to me that for our purposes a range -180 to +180 is more convenient than 0 to 360, especially when trying to visualise what's going on. For instance, in the morning the sun has a negative Hour Angle which links conveniently to the idea that the East has less longitude than Here. Another example is that for calculating my standard time, I can just add my longitude number and my time zone meridian to get the adjustment required. That is, I'm at 64.50°W in the Atlantic time zone which is -4. So I divide +64.5 degrees by 15 to get 4.3 hours and add -4, and I know I'm 0.3 hours from my TZ meridian. I'm to the West, which is consistent with positive sense of the number I calculated. If I treat my longitude as 295.5 measured going East, it is a harder sum. I have to multiply my TZ meridian by 15 and subtract that from 360 to get 300, then subtract my longitude of 295.5 to get 4.5 and divide that by 15 to get the 0.3 final result. It is positive which fits with positive now being to the East, so it is still consistent and the final result is the same, but it more work mentally and thus I suspect more error prone for those of us who are still learning or who don't work routinely with these concepts. I'd like to know what other people think about the right convention to use. Steve Want to know who's going to win in your constituency? Try my UK Tactical Voting Wizard at http://users.eastlink.ca/~srgl/election2001.htm
Re: Solar Noon Equation of Time Calculator
On Tue, 1 May 2001, Steve Lelievre wrote: Gianni wrote: As in almost all Web sites, also you take as positive the Longitudes for places West of Greenwich. The Explanatory Supplement to the Astronomic Almanac (USNO 1992) at page 203 affirms: The geocentric longitude is defined by the angle between the reference (or zero) meridian and the meridian of point p, measured EASTWARD around the Earth from 0 to 360 deg (IAU, 1983, p.47) for our purposes a range -180 to +180 is more convenient than 0 to 360, especially when trying to visualise what's going on. For instance, in the Steve Steve, I agree with your point that -180 to +180 is more convenient, and is also more common usage, I believe. However, if I understood the above correctly, the more significant difference between your convention and that espoused by Gianni, is that you are have positive longitude West of the Prime Meridian, rather than East... Dave 37.3 N 121.9 W
Re: Solar Noon Equation of Time Calculator
Message text written by Steve Lelievre I'd like to know what other people think about the right convention to use. I don't think that it matters ONE JOT what convention is actually used so long as the end result is correct, explained and understandable to others of a different persuasion. The difference in the preferred usage of the sign of the Equation of Time has been known for so long that we need to be able to accommodate it, not try to change it. It is no different to the use of different weights or temperature scales. There will always be those strange folk who somehow find an equally strange need to seek a common standard be it of units or conventions and apply it to everyone, but there are those (whom frankly I applaud) who seek to solve the problem and not argue about the means by which the answer has been obtained - even if it means using a 'less approved' system of units or a 'different' convention. The logical end argument for all this is that there should be a common language amongst peoples of the world. What an absurd nonesense. However I do agree that there should be defined a 'preferred system of units' or a 'preferred convention' and perhaps a 'preferred language' for some purposes, and those who choose to use a different one should accept that they should always explain their calculations, views and conventions for the benefit of others. Perhaps as an Englishman I am an 'Imperial Measures' man after all? Yes, for areas where it is sensible - like for much of life for most people - even youngsters - in the UK, I suppose I am. Patrick
Re: Solar Noon Equation of Time Calculator
Amen! John S - Original Message - From: Patrick Powers [EMAIL PROTECTED] Message text written by Steve Lelievre I'd like to know what other people think about the right convention to use. I don't think that it matters ONE JOT what convention is actually used so long as the end result is correct, explained and understandable to others of a different persuasion. The difference in the preferred usage of the sign of the Equation of Time has been known for so long that we need to be able to accommodate it, not try to change it. It is no different to the use of different weights or temperature scales. There will always be those strange folk who somehow find an equally strange need to seek a common standard be it of units or conventions and apply it to everyone, but there are those (whom frankly I applaud) who seek to solve the problem and not argue about the means by which the answer has been obtained - even if it means using a 'less approved' system of units or a 'different' convention. The logical end argument for all this is that there should be a common language amongst peoples of the world. What an absurd nonesense. However I do agree that there should be defined a 'preferred system of units' or a 'preferred convention' and perhaps a 'preferred language' for some purposes, and those who choose to use a different one should accept that they should always explain their calculations, views and conventions for the benefit of others. Perhaps as an Englishman I am an 'Imperial Measures' man after all? Yes, for areas where it is sensible - like for much of life for most people - even youngsters - in the UK, I suppose I am. Patrick
