Re: EOT=0
I calculate the Equation of Time as: Sun's Mean Longitude - Sun's Apparent Right Ascension. This is equivalent to: Right Ascension of the Fictitious Mean Sun - Right Ascension of the Sun (which is the definition of the Equation of Time) even though the Sun's mean longitude is measured on the ecliptic because the fictitious mean sun used to define civil time keeping is defined is such a way that its mean right ascension is always equal to the Sun's mean longitude. It has never been totally clear to me whether it is more accurate to use the Sun's mean right ascension or the apparent right ascension in calculating the equation of time. In the Electric Astrolabe, I used the Sun's apparent right ascension (calculated using VSOP87 corrected for light time and the true nutation of date and optionally corrected for dynamical time). This calculation might more correctly be called the 'apparent equation of time'. I don't think it makes much difference. I suspect the reason that the Dialist Companion gives a value for when the Equation of Time is 0 that is quite a bit different than Luke and I got is because it uses a series expansion for the calculation. Perhaps someone involved in Dialist Companion can tell us. The general series expansion gives a pretty good value, but is not accurate, in the astronomical sense, for any specific day or year (but is good enough for sundials). The US Naval Observatory used to publish the series expansion for calculating EQT for a given year, but I don't know if they still do. Best regards, Jim James E. Morrison Astrolabe web pages at: http://myhouse.com/mc/planet/astrodir/astrolab.htm - Original Message - From: Luke Coletti [EMAIL PROTECTED] To: Phil Pappas [EMAIL PROTECTED] Cc: sundial@rrz.uni-koeln.de Sent: Friday, April 16, 1999 7:18 PM Subject: Re: EOT=0 Hello JOHN, I think the difference between my reported time and that given by Jim Morrison converges very closely the same value, if as I suspect, Jim has incorporated DeltaT in his calculation i.e., the difference between Dynamical Time and Universal Time. I know that I have not, still on the punch sheet though! I don't have the exact current value handy but it is on the order of a minute of time. Therefore, I would say you have two values that are quite close to one another. The interesting thing to me is that like Jean-Paul I too use the VSOP87 periodic terms to calculate solar position however the two EoT zero times based upon it are quite different, I guess I would suspect a different computation method of the EoT itself. Regards, Luke Coletti John Carmichael wrote: Several people wrote me with their calculations of when EOT=0. I hope they don't mind if I sumerize their answers for you here. Jim Cobb:4/16/1999 3:04:36 UTC (xephem version 3.0) Luke Coletti:4/16/1999 0:40:00 UT(Solar Calculator) Jean-Paul Cornec: 4/16/1999 1:06:04 UT(VSOP87) James Morrison: 4/16/1999 0:40:57 UT(?) As you can see, all the calculations are different. I assume this is due to the different calculating methods that were used. Surely there can only be one correct answer.(Or should I use the average time of all the answers?) Thanks again to Jim, Luke, Jean-Paul, and James for taking the time to do the calculations.
Re: EOT=0
Hello JOHN, I think the difference between my reported time and that given by Jim Morrison converges very closely the same value, if as I suspect, Jim has incorporated DeltaT in his calculation i.e., the difference between Dynamical Time and Universal Time. I know that I have not, still on the punch sheet though! I don't have the exact current value handy but it is on the order of a minute of time. Therefore, I would say you have two values that are quite close to one another. The interesting thing to me is that like Jean-Paul I too use the VSOP87 periodic terms to calculate solar position however the two EoT zero times based upon it are quite different, I guess I would suspect a different computation method of the EoT itself. Regards, Luke Coletti John Carmichael wrote: Several people wrote me with their calculations of when EOT=0. I hope they don't mind if I sumerize their answers for you here. Jim Cobb:4/16/1999 3:04:36 UTC (xephem version 3.0) Luke Coletti:4/16/1999 0:40:00 UT(Solar Calculator) Jean-Paul Cornec: 4/16/1999 1:06:04 UT(VSOP87) James Morrison: 4/16/1999 0:40:57 UT(?) As you can see, all the calculations are different. I assume this is due to the different calculating methods that were used. Surely there can only be one correct answer.(Or should I use the average time of all the answers?) Thanks again to Jim, Luke, Jean-Paul, and James for taking the time to do the calculations.
Re: EOT=0
Hello John and all, Well, I quite agree with everyone, I had read the mails a bit quickly (it was late here). EOT = 0 for a certain moment in UT, and that moment can correspond to different days according to the longitude and time zones. It is the same thing as the seasons occurence that was discussed here a few months ago. As far as I am concerned, when I explain what EOT is to people, I always say and write that EOT is, as John says, null at mid-april, mid-june, around september the 1st, and at Christmas, and it is quite enough for a casual reader and even for dialists. Any change in the exact date from year to year (as any change in the daily value from year to year) is included in the width of the line that will be drawn on the sundial's surface, and included in the penumbra of the style's shadow. To design sundials I take average values given, for instance, in tables of (good) gnomonics books. Sundials are not theoretical things designed for a given year but are designed to last long. Regards Jean-Paul -- De : Phil Pappas [EMAIL PROTECTED] A : sundial@rrz.uni-koeln.de Objet : 4/15:EOT=0=Taxes Date : jeudi 15 avril 1999 17:51 Hello yall: What I've been trying to do is to tell my sundial customers that there are four magical days of the year when their sundial keeps perfect Standard Mean Time. (The dials are already corrected for longitude and DLS Time). On these four days, they don't need a copy of the EOT to tell time. These are also the only days that a sundial can be set, without EOT corrections, using the time method. From what I've learned, with your help, is that these dates may change from year to year mostly due to leap year adjustments. This anual variation may be as much as 24 hrs. But the exact moment of the event, EOT=0, is absolute and occurs at the same time everywhere. The date, however, may be different due to longitudinal time zone differences. I guess what I need is the AVERAGE four dates for The United States. Do you think that it is ok to stick with: April 15, Jun. 14, Sep 1, and Dec 25 ? I can accept an error of about 1 minute (EOT=0 plus or minus 1), as my dials can only be read to about 1 min. EOT will be -1 min. tomarrow. Thank you all, John Carmichael Tucson
Re: EOT=0
[EMAIL PROTECTED] wrote: Hello John and everybody on this list, I don't want to extend this discussion endlessly , but I am surprised to read that the value of EOT depends on longitude. [...] I believe John was referring to the (local civil) date (and time) of the occurrence of the zero EOT value; the date and time do depend on time zone, which is loosely coupled with longitude. I did not read his remarks as implying anything more than that. (His goal is to write a sundial manual for his customers, who may not be astronomically versed.) Of course John can correct me if I read his remarks or intent incorrectly... I agree with the rest of your remarks. Jim --- -- | Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] | | Parametric| Salt Lake City, UT | (801)-588-4632 | | Technology Corp. | 84108-1202 | Fax (801)-588-4650 | --- -- Hateful to me as the gates of Hades is that man who hides one thing in his heart and speaks another. -- Homer
4/15:EOT=0=Taxes
Hello yall: What I've been trying to do is to tell my sundial customers that there are four magical days of the year when their sundial keeps perfect Standard Mean Time. (The dials are already corrected for longitude and DLS Time). On these four days, they don't need a copy of the EOT to tell time. These are also the only days that a sundial can be set, without EOT corrections, using the time method. From what I've learned, with your help, is that these dates may change from year to year mostly due to leap year adjustments. This anual variation may be as much as 24 hrs. But the exact moment of the event, EOT=0, is absolute and occurs at the same time everywhere. The date, however, may be different due to longitudinal time zone differences. I guess what I need is the AVERAGE four dates for The United States. Do you think that it is ok to stick with: April 15, Jun. 14, Sep 1, and Dec 25 ? I can accept an error of about 1 minute (EOT=0 plus or minus 1), as my dials can only be read to about 1 min. EOT will be -1 min. tomarrow. Thank you all, John Carmichael Tucson
RE: EOT=0 and longitude
Andrew James contributed: Please can someone point me to an explanation of exactly how the mean sun is derived? There is a clutch of very useful time-related definitions in the opening chapter of Whitaker's Almanack. Tony Moss
Re: EOT=0
Hello John and everybody on this list, I don't want to extend this discussion endlessly , but I am surprised to read that the value of EOT depends on longitude. Perhaps I am totally wrong, but for me EOT is absolute. It is linked to the motion of Earth about the Sun and has nothing to do with longitude. Taking again Jim Cobb's formula, for any place on Earth : UT = Local Solar Time - EOT + 12 + Longitude(west is positive). The time when EOT = 0 is : UT = Solar Time + 12 + Longitude; or UT = Local Sidereal Time - AD + 12 + Longitude ;and : UT = Greenwich Sidereal Time - AD + 12 Longitude of the place has gone away. Am I wrong ? Best regards Jean-Paul Cornec . Jean-Paul I was involved some weeks ago (relative to the location of the analemma) with this issue and now view the issue as follows. When the EOT is zero the geographical position of the sun and the mean sun (longitude only) are the same. This happens at a particular moment, ie. a certain time. The time is the same for everyone, subject to time zone changes, and the event certainly occurs associated with a definite longitude. This longitude changes somewhat from year to year due to the fact that the earth's rotation is not coupled exactly to its location in its orbit about the sun. Hope I am clear and correct for everyone here. Dan Wenger Daniel Lee Wenger Santa Cruz, CA [EMAIL PROTECTED] http://wengersundial.com http://wengersundial.com/wengerfamily
Re: EOT=0
Hello John and everybody on this list, I don't want to extend this discussion endlessly , but I am surprised to read that the value of EOT depends on longitude. Perhaps I am totally wrong, but for me EOT is absolute. It is linked to the motion of Earth about the Sun and has nothing to do with longitude. Taking again Jim Cobb's formula, for any place on Earth : UT = Local Solar Time - EOT + 12 + Longitude(west is positive). The time when EOT = 0 is : UT = Solar Time + 12 + Longitude; or UT = Local Sidereal Time - AD + 12 + Longitude ;and : UT = Greenwich Sidereal Time - AD + 12 Longitude of the place has gone away. Am I wrong ? Best regards Jean-Paul Cornec . -- De : Phil Pappas [EMAIL PROTECTED] A : sundial@rrz.uni-koeln.de Objet : EOT=0 Date : mercredi 14 avril 1999 15:47
EOT=0
John, Here's another for you, though I am unsure of its accuracy! I was playing just now with the NASS Dialist's Companion and changing the date and time to find when their calculation of EoT turns to zero. For the longitude of Greenwich (and, as it happens, 52 Lat and with other corrections turned off in case they had some effect) it gave 17.53.30 on 15th April 1999. Hmmm I would expect there to be a maximum excursion of at least a day because of the leap year problem but we shouldn't forget that the leap year correction is not just one day every four years there is a gradual build up of drift in other years as well. We are seeing that at the end of 1999 when 2000 is (unusually for a century) a leap year. Therefore the maximum drift may well be more than a day. Patrick
Re: WHEN DOES EOT=0
My own computations (derived from VSOP87) give 16th April at 01h 06m 04s UT. Jean-Paul Cornec -- De : Phil Pappas [EMAIL PROTECTED] A : sundial@rrz.uni-koeln.de Objet : WHEN DOES EOT=0 Date : mardi 13 avril 1999 17:20 Hello all: Does anybody know the exact time (UT) when the Equation of Time equals zero this April 15th (or is it the 16th)? Thanks John Carmichael Tucson
Fw: WHEN DOES EOT=0
I get Julian Date 2451284.52918 which is UT 0:40:57.6 on April 16, 1999. I think this corresponds to 17:40:57.6 MST on April 15, 1999, in Tucson. It will be interesting to see what other people come up with. Best regards, Jim James E. Morrison Astrolabe web pages at: http://myhouse.com/mc/planet/astrodir/astrolab.htm - Original Message - From: Phil Pappas [EMAIL PROTECTED] To: sundial@rrz.uni-koeln.de Sent: Tuesday, April 13, 1999 11:20 AM Subject: WHEN DOES EOT=0 Hello all: Does anybody know the exact time (UT) when the Equation of Time equals zero this April 15th (or is it the 16th)? Thanks John Carmichael Tucson
Re: WHEN DOES EOT=0
Hello all: Does anybody know the exact time (UT) when the Equation of Time equals zero this April 15th (or is it the 16th)? Thanks John Carmichael Tucson I used the solver in xephem version 3.0 to find the zero of the equation Sun.HA+12-UT (that's hour angle of the sun + 12 - universal time). and got the result 4/16/1999 3:04:36 UTC Jim --- -- | Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] | | Parametric| Salt Lake City, UT | (801)-588-4632 | | Technology Corp. | 84108-1202 | Fax (801)-588-4650 | --- -- To stumble twice against the same stone is a proverbial disgrace. -- Cicero
Re: WHEN DOES EOT=0
Hello Phil, Per my Solar Calculator, I get a zero for the EoT between 00:39UT and 00:40UT on April 16. The solar declination values are 09d53m43s and 09d53m44s respectively. Regards, Luke Coletti Phil Pappas wrote: Hello all: Does anybody know the exact time (UT) when the Equation of Time equals zero this April 15th (or is it the 16th)? Thanks John Carmichael Tucson
