Re: Solar Noon & Equation of Time Calculator
Oops! Sorry about the banal error, and most grateful for all the comments. Fortunately nothing was carved in stone, and the sign error has now been corrected. Other points: 1. The incorrect "Equation of Time" has now been relabelled "Standard Time Correction" and it is explained that it is the sum of EOT and the longitude correction 2. I have learnt something about astronomy. However, for the current purpose we will stick with the +180 to -180 convention 3. Printing. You have to print it landscape (not portrait) and you may need to narrow the margins to get December on. We cant see any way round this one. It is explained on the final page. We have now reposted the pages of www.solar-noon.com, and I have sent a further message outlining two outstanding matters concerning the Equation of Time, on which I would welcome views. Many thanks to all Piers Nicholson
Re: Solar Noon & Equation of Time Calculator
Hi Gianni et al, Re. the Longitude convention: note that conventions are just that and, by definition, aren't right or wrong but accepted (or not). When I wrote the BSS Glossary, I consciously avoided using the IAU definition (0-360 degrees, Eastwards positive) in favour of the one almost universally used by diallists and navigators for four centuries (-180 to +180 degrees, Westward positive). The astronomers can use what they like, but they don't rule the world and the rest of us (like Steve Lelievre) use what is convenient. For the Second Edition of the Glossary (in preparation), I have stuck to my original definition but have added a note that the IAU one is different. The other area where I (and Jean Meeus) are at odds with the IAU is the sign of the EoT, but that's another can of worms. The key thing is to be consistent in a document and to make sure that the reader is given enough information to get the right answer. It's clear that we could do with a term for (EoT + longitude correction). I didn't find a common one in my trawl through the literature, though the NASS Dialist's Companion uses "Total Correction" which seems reasonable. However, there are still chinks not defined, such as: is the atmospheric refraction correction included? I quite agree that a table of the mean EoT over a long period (the lifetime of a sundial) is very useful as an addition to the exact EoT for a particular date/time. The new Glossary will include the one which you (Gianni) so kindly gave me. Best regards to all, John -- Dr J R DavisFlowton, UK52.08N, 1.043Eemail: [EMAIL PROTECTED] - Original Message - From: Gianni Ferrari To: sundial@rrz.uni-koeln.de Sent: 01 May 2001 16:13 Subject: Solar Noon & Equation of Time Calculator Hi Piers ,I have visited with a lot of interest your Solar Noon Calculator on the webat www.solar-noon.com and I have immediately made some tests to compare yourvalues with those calculated from me and published in an article in theproceedings of our "X Seminario di Gnomonica" ( X Italian Meeting onSundials - 2000)Here are some considerations of mine1)>From the comparison I have immediately seen that your results are wrongbecause of a banal error: anywhere the value of the EoT has been or takenwith opposite sign or subtracted instead that added.An example : Long. =12d E and TZ of Central Europe (central Meridian 15 dEast): longitude correction =+3d = +12mAt 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, lessapproximate, 12h 15m39s )In the Table calculated with the Solar Noon Calculator is written the value= 12h08m48s, value that is obtained adding (instead that subtracting ) thevalue of the Eot: 12h+12m+(-3m12s) = 12h08m48s2)The table of the EoT NOT gives the values of the EoT but the Totalcorrection that it is necessary to add to the Local Apparent Time to obtainthe Standard Time : it is therefore the sum of the EoT + longitudecorrection..This value is certainly very useful but, perhaps, it is necessary to givesome explanations and it is opportune not to call it EotMoreover in this way the table with the values of noon is useless becausethese values are equal to those of the EoT + 12hThe definition :"Equation of Time displays the difference between solar time and thestandard 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 (apparentsolar time) and mean solar time at the same location (NOT Standard Time).Its value varies between extremes of about +14 minutes in February and -16minutes in October.3)Checking only for the date 1/1/2001 I have found that the error between theexact value and the mean value of the EoT = 3m41s -3m12s=29 sec: almost thedouble 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 25sDoes the greater error depend on the fact that the Eot has been calculatedat 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 ofsundials with mean time, to have a table of the mean values of the Eot (asof the mean declination of the Sun), while the table that gives the meanlocalnoon (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 meantime from the apparent solar time g
Re: Solar Noon & Equation of Time Calculator
Even if, already from two centuries, in all the nations, engineers and scientists have tried of to reach an unification of the different quantities that are used in industry and in science and hundreds of International Conferences have been made for adopting the same definitions all over the world (and this for the obvious reasons of clarity in transmitting data and results), in my opinion everyone can do as he please and choose, in his calculations and jobs, the units that are more congenial to him and that he prefers for traditional or personal reasons. If then the conversion between these personal units and those recognized (and understood) from others is not made, there will be some difficulties inthe communication and in the comprehension:-) Returning to the Longitude, also I prefer the range from -180 to +180 degrees and I believe that this is not important because of the periodicity of the trigonometric functions that use these values. For the Longitude's Sign instead, if there is some difficulties in using (in our writings) the IAU convention (Long. positive if East), we could not to use the signs and speak only of East or West Longitude Everyone then, for a East/West Longitude, will put the necessary sign to get correct results with the formulas that he uses. Best wishes Gianni Ferrari
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 > > >
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
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
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
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
Solar Noon & Equation of Time Calculator at www.solar-noon.com
We have just posted on the web a new Solar Noon Calculator which prints out a table showing the time of solar noon for your longitude for every day of the year on one side of a landscape A4 sheet. It is at www.solar-noon.com. Please feel free to use it. We would welcome links to it - we have a links page if you would like a reciprocal link. You can also get a printout of the Equation of Time in the same format. We hope this will be useful. We would appreciate your comments. The page was developed as part of the Spot-On Sundial project (see www.spot-on-sundials.co.uk). This sundial can be set up precisely knowing only the time of solar noon - when the Calculator. Haopy dialling to all, Piers Nicholson
