Polaris
I am working on two modest projects, both of which require accurate coordinates (RA and declination) of Polaris several hundred years ago. I have been using a spreadsheet of my own doing to calculate precessed stellar coordinates using the formulae in Meeus, but I'm not very confident in the results as I have only one sample giving confirmed values. I am hoping someone in the sundial brain trust can direct me to an available trusted source of Polaris coordinates for years in the past. A few accurate values that I can try to replicate would also be useful.Best regards,Jim James E. Morrison janus.astrol...@verizon.net Astrolabe web site at http://astrolabes.org --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Polaris time
Hello Dialists : I've got a question that's always bugged me. As I'm more of an artist than mathematician, I doubt that my answer is the correct one and I'm sure many of you geniuses out there know the answer. I'm sure many of you have seen time lapse photography of the little circle that Polaris circumscribes around the North Celestial Pole in the northern night sky. This is because it is about 1/2 a degree away from the N.C.P. If you orient your sundial north by Polaris when it is on the meridian, then there will be no time error, because the dial will be pointed due north. Right? But if you orient it when it is due east or west of the meridian, the sundial will be turned the maximum distance from true north (1/2 degree) and the maximum time error will result. How large is this error in seconds of time? Here's how I tried to solve it: If the sun moves 15 deg./hr. then it moves 15 deg./ 60 min.= 1 deg./.25min.=1 deg/15 sec.=.5 deg./7.5 sec. Is this the answer: 7.5 seconds? I've got a feeling that I've oversimplified the problem. I bet the answer turns out to be some bellcurve with an error which changes throughout the day. It's probably something only T.J.Lauroesch and J.R.Edinger,Jr. can solve! John Carmichael
Re: Polaris
To get your mean coordinates of a particular equinox into apparent coordinates for a particular date, you will have to make a few calculations 1) apply proper motion of polaris until the new date 2) precess the result to the new date 3) convert to ecliptic cordinates using the mean obliquity of the new date 4) apply nutation to the ecliptic longitude 5) apply annual aberration to the true ecliptic coordinate 5a) apply diurnal aberration and parallax (these two are generally ignored) 6) if rqd, convert back to equatorial by using the true obliquity of the new date Also, don't forget to use the apparent lst if converting to Declination and HrAngle Finally, apply refraction to the altitude if you want it in alt-azi form Many star chart programmes miss a few of these steps, but I would check out some of them and see if they provide the numbers you are looking for. On 2014-11-28 23:52, James E. Morrison wrote: I am working on two modest projects, both of which require accurate coordinates (RA and declination) of Polaris several hundred years ago. I have been using a spreadsheet of my own doing to calculate precessed stellar coordinates using the formulae in Meeus, but I'm not very confident in the results as I have only one sample giving confirmed values. I am hoping someone in the sundial brain trust can direct me to an available trusted source of Polaris coordinates for years in the past. A few accurate values that I can try to replicate would also be useful. Best regards, Jim James E. Morrison janus.astrol...@verizon.net <mailto:janus.astrol...@verizon.net> Astrolabe web site at http://astrolabes.org --- https://lists.uni-koeln.de/mailman/listinfo/sundial -- --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Polaris time
>Hello Dialists : > >I've got a question that's always bugged me. As I'm more of an artist than >mathematician, I doubt that my answer is the correct one and I'm sure many >of you geniuses out there know the answer. > >I'm sure many of you have seen time lapse photography of the little circle >that Polaris circumscribes around the North Celestial Pole in the northern >night sky. This is because it is about 1/2 a degree away from the N.C.P. > >If you orient your sundial north by Polaris when it is on the meridian, then >there will be no time error, because the dial will be pointed due north. >Right? > >But if you orient it when it is due east or west of the meridian, the >sundial will be turned the maximum distance from true north (1/2 degree) and >the maximum time error will result. How large is this error in seconds of >time? > >Here's how I tried to solve it: > >If the sun moves 15 deg./hr. then it moves 15 deg./ 60 min.= 1 >deg./.25min.=1 deg/15 sec.=.5 deg./7.5 sec. > >Is this the answer: 7.5 seconds? > >I've got a feeling that I've oversimplified the problem. I bet the answer >turns out to be some bellcurve with an error which changes throughout the >day. It's probably something only T.J.Lauroesch and J.R.Edinger,Jr. can >solve! > >John Carmichael John You have over simplied. The error in time read with such an error in placement depends upon the location that the dial was made for and the time of year and day. I suspect, without further detailed analysis that the error that you give is close to the upper limit. For a sundial made for a location in the tropics the sun passes directly overhead a some times during the year. On those occasions the dial will read correctly no matter what the orientation of the dial. So the error due to error in placement is somewhere between 0 and some upper limit. Dan Wenger Daniel Lee Wenger Santa Cruz, CA [EMAIL PROTECTED] http://wengersundial.com http://wengersundial.com/wengerfamily
embarassed about polaris
Thank you all for pointing out my third grade math error. I'm really not THAT stupid! Guess I was just rushing. Anyway, If I had done my math right my answer should have been that there would be a 2 minute error for a .5 degree dial plate rotation If one sets his sundial towards Polaris when it is due east or due west of the meridian. The article in vol. 3 number 4 of The Compendium deals with this problem. On page 9 on the graph it looks like the resulting time error for a one degree turn is about 5.5 minutes and indeed varies in a bellcurve throughout the day. Would it be correct to infer that a .5 degree turn would result in a 5.5/2=2.75 minute maximum error (at 12 noon)? Also, would the amount of error change at different times of the year and at different latitudes? I guess all of this is acedemic because unless you are a good astronomer how would you know when polaris is on the meridian and when it is safe to use it to set a sundial. In my Sundial Owner's Manual, I tell my customers that the best and easiest way to set their sundials is to use the "time method". This method only works whith those dials that are perfectly designed, constructed, and leveled. The dial is set by rotating the level dial until it agrees with a reliable time source (I use a cordless phone and the number for time) and correcting for the Equation of Time and longitude. I state that this method is more accurate than using Polaris. Is this statement valid?
Re: embarassed about polaris
On Sat, 9 Jan 1999, Philip P. Pappas, II wrote: > > I guess all of this is acedemic because unless you are a good astronomer > how would you know when polaris is on the meridian and when it is safe to > use it to set a sundial. > The leftmost star of the w-shaped constellation, Cassiopeia, is almost on the same hour circle as Polaris so if it is directly above or below Polaris then Polaris indicates close enough to due north. Of course, if it's below Polaris then it's the rightmost star of the m-shaped Cassiopeia. jh
1998 sun/polaris dec table
The following web page has a table for every day of 1998, giving the declination of the sun, the equation of time, and the declination of Polaris. http://www.cadastral.com/eph1998b.htm
embarassed about Re: embarassed about polaris
On Sat, 9 Jan 1999, Philip P. Pappas, II wrote: > >On Sat, 9 Jan 1999, Philip P. Pappas, II wrote: > > > > > >The leftmost star of the w-shaped constellation, Cassiopeia, is almost on > >the same hour circle as Polaris so if it is directly above or below > >Polaris then Polaris indicates close enough to due north. Of course, if > >it's below Polaris then it's the rightmost star of the m-shaped > >Cassiopeia. OOPS, Of course, the part I said of course about is wrong. I got my up and down mixed up but I had my left and right right. Cassiopeia is w-shaped when it's below Polaris. Sorry. > > > >jh > > >Dear jh: THANKS! > > jc > >
Compass Variations for True North (Polaris)
We all know that "True North" (Polaris) is not the same as "Magnetic North" when orienting a sundial. My grandfather's Boy Scout pocket "Sunwatch" is a true gem for our family -- loaded with information for any enthusiastic sundial traveler (complete with a compass, dial, and table of corrections for the major US cities). One of the corrections included was for "Degree Variance" (East or West) of magnetic north in which to point the dial to "True North" for an accurate reading. For example, Boston, MA is 14 degrees West; Seattle, WA is 23 degrees East; and ironically, Cincinnati, OH has no variance and is the same as magnetic north. Is anyone in the mailing list aware of the mathematical formula used to compute this variance for True North? Judith Romano mailto:[EMAIL PROTECTED] -
Re: Compass Variations for True North (Polaris)
compute this variance for True North? The anomaly is truly anomalous: it varies with coordinates relative to the Magnetic Pole, but also with local geodetic anomalies, such as large concentrations of iron; and over time as the M.P. floats around. I doubt there is a formula, although empirical tables must exist. -- Bill Thayer 41N53 87W38 col cuore a 42N59.5 12E42.4 alt.313m http://www.ukans.edu/history/index/europe/ancient_rome/I/home.html -
Re: Compass Variations for True North (Polaris)
Hello Judith: The Earth's magnetic field is in a constant state of change so there is no simple answer to your question. Every 5 years or so the World Magnetic Model is updated. This model will tell you what the magnetic field is for a given location and date anywhere on the Earth's surface (but not near anomalies like Iron mountains). For more see my sensors web page at: <http://www.pacificsites.com/~brooke/Sensors.shtml#Earth's Magnetic> Brooke "Romano, Judith" wrote: > We all know that "True North" (Polaris) is not the same as "Magnetic North" > when orienting a sundial. My grandfather's Boy Scout pocket "Sunwatch" is a > true gem for our family -- loaded with information for any enthusiastic > sundial traveler (complete with a compass, dial, and table of corrections > for the major US cities). One of the corrections included was for "Degree > Variance" (East or West) of magnetic north in which to point the dial to > "True North" for an accurate reading. For example, Boston, MA is 14 degrees > West; Seattle, WA is 23 degrees East; and ironically, Cincinnati, OH has no > variance and is the same as magnetic north. > > Is anyone in the mailing list aware of the mathematical formula used to > compute this variance for True North? > > Judith Romano > mailto:[EMAIL PROTECTED] > > - -
Re: Compass Variations for True North (Polaris)
Judith, Go to everyone's favorite search engine, Google.com, and enter "magnetic declination" (or "magnetic variation") and you'll find more information, both text and graphic, than you can possibly browse through before the end of this decade! Cheers, Mac Oglesby We all know that "True North" (Polaris) is not the same as "Magnetic North" when orienting a sundial. My grandfather's Boy Scout pocket "Sunwatch" is a true gem for our family -- loaded with information for any enthusiastic sundial traveler (complete with a compass, dial, and table of corrections for the major US cities). One of the corrections included was for "Degree Variance" (East or West) of magnetic north in which to point the dial to "True North" for an accurate reading. For example, Boston, MA is 14 degrees West; Seattle, WA is 23 degrees East; and ironically, Cincinnati, OH has no variance and is the same as magnetic north. Is anyone in the mailing list aware of the mathematical formula used to compute this variance for True North? Judith Romano mailto:[EMAIL PROTECTED] - -
Re: Compass Variations for True North (Polaris)
"Romano, Judith" wrote: > > We all know that "True North" (Polaris) is not the same as "Magnetic North" > when orienting a sundial. My grandfather's Boy Scout pocket "Sunwatch" is a > true gem for our family -- loaded with information for any enthusiastic > sundial traveler (complete with a compass, dial, and table of corrections > for the major US cities). One of the corrections included was for "Degree > Variance" (East or West) of magnetic north in which to point the dial to > "True North" for an accurate reading. For example, Boston, MA is 14 degrees > West; Seattle, WA is 23 degrees East; and ironically, Cincinnati, OH has no > variance and is the same as magnetic north. > > Is anyone in the mailing list aware of the mathematical formula used to > compute this variance for True North? > > Judith Romano > mailto:[EMAIL PROTECTED] Judith, Yes, there are geomagnetic models for computing magnetic declination, a good resource is the National Geophysical Data Center: http://www.ngdc.noaa.gov/seg/potfld/magmodel.shtml There is online software available to compute local magnetic declination. Also, You can get the software, GEOMAG, for free (source code included. I actually debugged some of the code when the IGRF model data for 2000 came out. The format of the data set had changed and an array was undersized resulting in some bogus values. ftp://www.ngdc.noaa.gov/Solid_Earth/Mainfld_Mag/Models/unixsoftware/ Lastly, I use this software (GEOMAG) in my online Solar Calculator to report the local magnetic declination, a useful point of info for solar/sundial data, feel free to use it. http://www.gcstudio.com/suncalc.html Regards, Luke Coletti -
