Re: equation of time sundial
Ken, You wrote: I take it the motor is to keep the hole pointed toward the sun. It’s to keep the cylinder pointed in the direction that the sun would be if it kept standard time. I should have added that this device includes the longitude correction as well. I'm unclear though on what's drawn inside. Is it a single curve of EOT as a function of declination, or a family of curves? It’s a very simple straight line grid with the vertical lines showing minutes from standard time and the horizontal lines showing the sun’s declination from the equinox position. I hope that’s clearer. I’ll see if I can find any photos and contact you off list. Mike Shaw 53º 22' North 03º 02' West www.wiz.to/sundials - No virus found in this message. Checked by AVG - www.avg.com Version: 2013.0.2897 / Virus Database: 2639/6078 - Release Date: 02/03/13 --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial-heliochronometre 2
Hi, You can also see a magnificent heliochronometre made by John Carmichael at: http://www.flickr.com/photos/jlcarmichael/3576166915/in/set-72157618973847752/ best wishes, Peter On 4/02/2013 3:13 PM, Ken Baldwin wrote: Thanks, everyone. I see now how an analemmic gnomon can be used to read the EOT, if the equatorial band shows both mean and apparent time and you do the subtraction in your head. I'm most interested, though, in something which allows a direct reading of EOT... Mike, your solution sounds very interesting, but I confess I don't really understand it :-) I take it the motor is to keep the hole pointed toward the sun. Presumably this could be done manually, as on a shepard's dial? I'm unclear though on what's drawn inside. Is it a single curve of EOT as a function of declination, or a family of curves? Ken On Sun, Feb 3, 2013 at 12:02 PM, Ken Baldwin kenneth.bald...@gmail.com mailto:kenneth.bald...@gmail.com wrote: Hello, I'm a new list member, and have a beginner question: Are there examples of sundials whose sole (or primary) purpose is to compute the Equation of Time for the current date? - I know that this information is often provided as a graph in the furniture, but why should I have to know the date and perform the look-up manually? Can't I use the position of the sun to do the computation for me? - I know that the EOT correction can be incorporated into the layout of (some) hour lines, but I'm more interested in having dials which show true solar time. I'd like a separate device dedicated to computing the EOT. - I know that I can construct an analemmic noon mark to show the EOT for that day, since it's simply the east-west component of the analemma, but I'd like a design that can be read at any daylight hour. It seems to me that it should be possible to build such a dial, since the EOT is a function of date, and date lines can be read from many sundials. In principle, I can just re-label the date lines with corresponding EOT values and interpolate. I hope that makes sense. But since I haven't seen anything like that in introductory sundial books, I must be missing something... Is it that the shadow length can't be read accurately enough to get a reasonably precise EOT estimate? Or is it just too hard to make a readable layout, given that solar altitude is ambiguous between two dates, and that the component of the EOT due to the eccentricity of the earth's orbit is out of phase with the equinoxes and solstices? Thanks in advance, Ken Baldwin Corvallis, OR USA --- https://lists.uni-koeln.de/mailman/listinfo/sundial -- -- Peter Mayer Politics Department The University of Adelaide, AUSTRALIA 5005 Ph : +61 8 8313 5609 Fax : +61 8 8313 3443 e-mail: peter.ma...@adelaide.edu.au CRICOS Provider Number 00123M --- This email message is intended only for the addressee(s) and contains information that may be confidential and/or copyright. If you are not the intended recipient please notify the sender by reply email and immediately delete this email. Use, disclosure or reproduction of this email by anyone other than the intended recipient(s) is strictly prohibited. No representation is made that this email or any attachments are free of viruses. Virus scanning is recommended and is the responsibility of the recipient. attachment: tn_Mayall-Heliochronometre1.3.jpg--- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial
Ken, I’ve found a couple of small photos of my rather crude device. I’ve posted then in Dropbox at : https://www.dropbox.com/sh/tkxqw7x5mbr1axd/tYOlNAwT60 By coincidence, the 2nd photo shows the EoT is –7.5 minutes and sun’s declination is –7.5 degrees. I hope that makes things clearer. Of course, it’s just really a reverse engineered heliochronometer where you drive the alidade round using a 24 hour clock mechanism (widely available), to keep the spot of light on the analemma. However, I’ve used a graph of EoT and declination instead so that you can read off the two figures directly. Over the course of a year, the spot of light will trace out the analemma. Mike Shaw 53º 22' North 03º 02' West www.wiz.to/sundials - No virus found in this message. Checked by AVG - www.avg.com Version: 2013.0.2897 / Virus Database: 2639/6078 - Release Date: 02/03/13 --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial
Ken, Your original mail has the sentence: * It seems to me (Ken) that it should be possible to build such a dial, since the EOT is a function of date, and date lines can be read from many sundials. In principle, I can just re-label the date lines with corresponding EOT values and interpolate. * I agree with this and a sample of a dial that's suitable is in Copenhagen, Denmark. See the website of De Zonnewijzerkring. Address below. Follow: Archives 2003 - 2013 Sundial of the month 2006 06-09 Copenhagen. Lable te datelines for a half year left and the other half year right with the appropriate equation of time rounded to e.g. one minute. The dial in Copenhagen has three scales so it can be used for many hours of the day. Best wishes, Fer. Fer J. de Vries De Zonnewijzerkring http://www.de-zonnewijzerkring.nl Molens http://www.collsemolen.dse.nl Eindhoven, Netherlands lat. 51:30 N long. 5:30 E - Original Message - From: jmikes...@ntlworld.com To: Ken Baldwin ; sundial@uni-koeln.de Sent: Monday, February 04, 2013 9:59 AM Subject: Re: equation of time sundial Ken, I’ve found a couple of small photos of my rather crude device. I’ve posted then in Dropbox at : https://www.dropbox.com/sh/tkxqw7x5mbr1axd/tYOlNAwT60 By coincidence, the 2nd photo shows the EoT is –7.5 minutes and sun’s declination is –7.5 degrees. I hope that makes things clearer. Of course, it’s just really a reverse engineered heliochronometer where you drive the alidade round using a 24 hour clock mechanism (widely available), to keep the spot of light on the analemma. However, I’ve used a graph of EoT and declination instead so that you can read off the two figures directly. Over the course of a year, the spot of light will trace out the analemma. Mike Shaw 53º 22' North 03º 02' West www.wiz.to/sundials No virus found in this message. Checked by AVG - www.avg.com Version: 2013.0.2897 / Virus Database: 2639/6078 - Release Date: 02/03/13 -- --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial
A good question, however except for the solstices, there is ambiguity. Each declination has two dates. For example March and September equinoxes are 6 months apart, but same declination. Simon Simon Wheaton-Smith www.illustratingshadows.com Phoenix, Arizona, W112.1 N33.5 --- On Sun, 2/3/13, Ken Baldwin kenneth.bald...@gmail.com wrote: From: Ken Baldwin kenneth.bald...@gmail.com Subject: equation of time sundial To: sundial@uni-koeln.de Date: Sunday, February 3, 2013, 1:02 PM Hello, I'm a new list member, and have a beginner question: Are there examples of sundials whose sole (or primary) purpose is to compute the Equation of Time for the current date? - I know that this information is often provided as a graph in the furniture, but why should I have to know the date and perform the look-up manually? Can't I use the position of the sun to do the computation for me? - I know that the EOT correction can be incorporated into the layout of (some) hour lines, but I'm more interested in having dials which show true solar time. I'd like a separate device dedicated to computing the EOT. - I know that I can construct an analemmic noon mark to show the EOT for that day, since it's simply the east-west component of the analemma, but I'd like a design that can be read at any daylight hour. It seems to me that it should be possible to build such a dial, since the EOT is a function of date, and date lines can be read from many sundials. In principle, I can just re-label the date lines with corresponding EOT values and interpolate. I hope that makes sense. But since I haven't seen anything like that in introductory sundial books, I must be missing something... Is it that the shadow length can't be read accurately enough to get a reasonably precise EOT estimate? Or is it just too hard to make a readable layout, given that solar altitude is ambiguous between two dates, and that the component of the EOT due to the eccentricity of the earth's orbit is out of phase with the equinoxes and solstices? Thanks in advance, Ken Baldwin Corvallis, OR USA -Inline Attachment Follows- --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial
Hi Ken, One of the most beautiful (IMHO) is the Schmoyer dial which incorporates the EOT in the gnomon. You can see a picture of the dial at: http://www.precisionsundials.com/schmoyer.htm best wishes, Peter On 4/02/2013 6:32 AM, Ken Baldwin wrote: Hello, I'm a new list member, and have a beginner question: Are there examples of sundials whose sole (or primary) purpose is to compute the Equation of Time for the current date? - I know that this information is often provided as a graph in the furniture, but why should I have to know the date and perform the look-up manually? Can't I use the position of the sun to do the computation for me? - I know that the EOT correction can be incorporated into the layout of (some) hour lines, but I'm more interested in having dials which show true solar time. I'd like a separate device dedicated to computing the EOT. - I know that I can construct an analemmic noon mark to show the EOT for that day, since it's simply the east-west component of the analemma, but I'd like a design that can be read at any daylight hour. It seems to me that it should be possible to build such a dial, since the EOT is a function of date, and date lines can be read from many sundials. In principle, I can just re-label the date lines with corresponding EOT values and interpolate. I hope that makes sense. But since I haven't seen anything like that in introductory sundial books, I must be missing something... Is it that the shadow length can't be read accurately enough to get a reasonably precise EOT estimate? Or is it just too hard to make a readable layout, given that solar altitude is ambiguous between two dates, and that the component of the EOT due to the eccentricity of the earth's orbit is out of phase with the equinoxes and solstices? Thanks in advance, Ken Baldwin Corvallis, OR USA --- https://lists.uni-koeln.de/mailman/listinfo/sundial -- -- Peter Mayer Politics Department The University of Adelaide, AUSTRALIA 5005 Ph : +61 8 8313 5609 Fax : +61 8 8313 3443 e-mail: peter.ma...@adelaide.edu.au CRICOS Provider Number 00123M --- This email message is intended only for the addressee(s) and contains information that may be confidential and/or copyright. If you are not the intended recipient please notify the sender by reply email and immediately delete this email. Use, disclosure or reproduction of this email by anyone other than the intended recipient(s) is strictly prohibited. No representation is made that this email or any attachments are free of viruses. Virus scanning is recommended and is the responsibility of the recipient. --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial
Hi Ken, Here's another example of a dial which may interest you. This is my adaptation of a brilliant dial coded by Steve Lelievre. To fit the size limit on this list, I've had to squeeze the image greatly, but you should be able to grasp the principle of the dial: each month is a separate concentric circle. Between month lines one must interpolate the date to read the civil time. best wishes, Peter On 4/02/2013 6:32 AM, Ken Baldwin wrote: Hello, I'm a new list member, and have a beginner question: Are there examples of sundials whose sole (or primary) purpose is to compute the Equation of Time for the current date? - I know that this information is often provided as a graph in the furniture, but why should I have to know the date and perform the look-up manually? Can't I use the position of the sun to do the computation for me? - I know that the EOT correction can be incorporated into the layout of (some) hour lines, but I'm more interested in having dials which show true solar time. I'd like a separate device dedicated to computing the EOT. - I know that I can construct an analemmic noon mark to show the EOT for that day, since it's simply the east-west component of the analemma, but I'd like a design that can be read at any daylight hour. It seems to me that it should be possible to build such a dial, since the EOT is a function of date, and date lines can be read from many sundials. In principle, I can just re-label the date lines with corresponding EOT values and interpolate. I hope that makes sense. But since I haven't seen anything like that in introductory sundial books, I must be missing something... Is it that the shadow length can't be read accurately enough to get a reasonably precise EOT estimate? Or is it just too hard to make a readable layout, given that solar altitude is ambiguous between two dates, and that the component of the EOT due to the eccentricity of the earth's orbit is out of phase with the equinoxes and solstices? Thanks in advance, Ken Baldwin Corvallis, OR USA --- https://lists.uni-koeln.de/mailman/listinfo/sundial -- -- Peter Mayer Politics Department The University of Adelaide, AUSTRALIA 5005 Ph : +61 8 8313 5609 Fax : +61 8 8313 3443 e-mail: peter.ma...@adelaide.edu.au CRICOS Provider Number 00123M --- This email message is intended only for the addressee(s) and contains information that may be confidential and/or copyright. If you are not the intended recipient please notify the sender by reply email and immediately delete this email. Use, disclosure or reproduction of this email by anyone other than the intended recipient(s) is strictly prohibited. No representation is made that this email or any attachments are free of viruses. Virus scanning is recommended and is the responsibility of the recipient. attachment: tn_Lelievre Dial.jpg--- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial
Hi guys, Thanks for your responses. I'm not trying to read the EOT-adjusted time from the device, I want to read the actual EOT value itself for that date. For example, on Aug 9th, at any daylight hour, I'd like to be able to read, say, -5.5 min. Maybe I'm misunderstanding, but do I get that from these analemmic gnomons or hour lines? I'm suggesting something like Peter's design, but with the hour lines removed, and the date lines labelled with the EOT value (-15 min - 15 min). I understand Simon's point that the altitude of the sun is ambiguous between two dates, so perhaps it would have to be split into two plates (like half-analemmas). Of course, this requires the user to know which 6 month period of the year they are in, which partially defeats the purpose of not needing to know the date :-) Ken On Sun, Feb 3, 2013 at 2:14 PM, Peter Mayer peter.ma...@adelaide.edu.auwrote: Hi Ken, Here's another example of a dial which may interest you. This is my adaptation of a brilliant dial coded by Steve Lelievre. To fit the size limit on this list, I've had to squeeze the image greatly, but you should be able to grasp the principle of the dial: each month is a separate concentric circle. Between month lines one must interpolate the date to read the civil time. best wishes, Peter On 4/02/2013 6:32 AM, Ken Baldwin wrote: Hello, I'm a new list member, and have a beginner question: Are there examples of sundials whose sole (or primary) purpose is to compute the Equation of Time for the current date? - I know that this information is often provided as a graph in the furniture, but why should I have to know the date and perform the look-up manually? Can't I use the position of the sun to do the computation for me? - I know that the EOT correction can be incorporated into the layout of (some) hour lines, but I'm more interested in having dials which show true solar time. I'd like a separate device dedicated to computing the EOT. - I know that I can construct an analemmic noon mark to show the EOT for that day, since it's simply the east-west component of the analemma, but I'd like a design that can be read at any daylight hour. It seems to me that it should be possible to build such a dial, since the EOT is a function of date, and date lines can be read from many sundials. In principle, I can just re-label the date lines with corresponding EOT values and interpolate. I hope that makes sense. But since I haven't seen anything like that in introductory sundial books, I must be missing something... Is it that the shadow length can't be read accurately enough to get a reasonably precise EOT estimate? Or is it just too hard to make a readable layout, given that solar altitude is ambiguous between two dates, and that the component of the EOT due to the eccentricity of the earth's orbit is out of phase with the equinoxes and solstices? Thanks in advance, Ken Baldwin Corvallis, OR USA --**- https://lists.uni-koeln.de/**mailman/listinfo/sundialhttps://lists.uni-koeln.de/mailman/listinfo/sundial -- -- Peter Mayer Politics Department The University of Adelaide, AUSTRALIA 5005 Ph : +61 8 8313 5609 Fax : +61 8 8313 3443 e-mail: peter.ma...@adelaide.edu.au CRICOS Provider Number 00123M --**- This email message is intended only for the addressee(s) and contains information that may be confidential and/or copyright. If you are not the intended recipient please notify the sender by reply email and immediately delete this email. Use, disclosure or reproduction of this email by anyone other than the intended recipient(s) is strictly prohibited. No representation is made that this email or any attachments are free of viruses. Virus scanning is recommended and is the responsibility of the recipient. --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial
Ken, I think I made a device some time ago to do what you want . A cylinder was tilted to the appropriate latitude angle and direction. There was a small hole on one side of the cylinder which gave a projected a spot of light from the sun on the inner opposite surface where there was a graph which showed: left to right – the equation of time up and down – the sun’s declination. The cylinder was driven round using a 24 hour clock motor, so the spot of light remained apparently stationary except for changes in the EoT and sun’s declination that were read directly from the graph. Sadly, I don’t think I have it any more. Mike Shaw 53º 22' North 03º 02' West www.wiz.to/sundials - No virus found in this message. Checked by AVG - www.avg.com Version: 2013.0.2897 / Virus Database: 2639/6076 - Release Date: 02/02/13 --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial
Hello Ken, The examples using an analemma shaped gnomon that project the shadow the equatorial line do what you want. An improvement could be a wire stretched along the centerline of the gnomon in Willy's Spanish example. This would provide solar time. The two values of the EQT at that declination would be the shadow line of the aperture. Thus the two possible values of EQT could be read directly without knowing the the date. Regards, Roger Bailey Walking Shadow designs. From: Ken Baldwin Sent: Sunday, February 03, 2013 2:19 PM To: peter.ma...@adelaide.edu.au Cc: sundial Subject: Re: equation of time sundial Hi guys, Thanks for your responses. I'm not trying to read the EOT-adjusted time from the device, I want to read the actual EOT value itself for that date. For example, on Aug 9th, at any daylight hour, I'd like to be able to read, say, -5.5 min. Maybe I'm misunderstanding, but do I get that from these analemmic gnomons or hour lines? I'm suggesting something like Peter's design, but with the hour lines removed, and the date lines labelled with the EOT value (-15 min - 15 min). I understand Simon's point that the altitude of the sun is ambiguous between two dates, so perhaps it would have to be split into two plates (like half-analemmas). Of course, this requires the user to know which 6 month period of the year they are in, which partially defeats the purpose of not needing to know the date :-) Ken On Sun, Feb 3, 2013 at 2:14 PM, Peter Mayer peter.ma...@adelaide.edu.au wrote: Hi Ken, Here's another example of a dial which may interest you. This is my adaptation of a brilliant dial coded by Steve Lelievre. To fit the size limit on this list, I've had to squeeze the image greatly, but you should be able to grasp the principle of the dial: each month is a separate concentric circle. Between month lines one must interpolate the date to read the civil time. best wishes, Peter On 4/02/2013 6:32 AM, Ken Baldwin wrote: Hello, I'm a new list member, and have a beginner question: Are there examples of sundials whose sole (or primary) purpose is to compute the Equation of Time for the current date? - I know that this information is often provided as a graph in the furniture, but why should I have to know the date and perform the look-up manually? Can't I use the position of the sun to do the computation for me? - I know that the EOT correction can be incorporated into the layout of (some) hour lines, but I'm more interested in having dials which show true solar time. I'd like a separate device dedicated to computing the EOT. - I know that I can construct an analemmic noon mark to show the EOT for that day, since it's simply the east-west component of the analemma, but I'd like a design that can be read at any daylight hour. It seems to me that it should be possible to build such a dial, since the EOT is a function of date, and date lines can be read from many sundials. In principle, I can just re-label the date lines with corresponding EOT values and interpolate. I hope that makes sense. But since I haven't seen anything like that in introductory sundial books, I must be missing something... Is it that the shadow length can't be read accurately enough to get a reasonably precise EOT estimate? Or is it just too hard to make a readable layout, given that solar altitude is ambiguous between two dates, and that the component of the EOT due to the eccentricity of the earth's orbit is out of phase with the equinoxes and solstices? Thanks in advance, Ken Baldwin Corvallis, OR USA --- https://lists.uni-koeln.de/mailman/listinfo/sundial -- -- Peter Mayer Politics Department The University of Adelaide, AUSTRALIA 5005 Ph : +61 8 8313 5609 Fax : +61 8 8313 3443 e-mail: peter.ma...@adelaide.edu.au CRICOS Provider Number 00123M --- This email message is intended only for the addressee(s) and contains information that may be confidential and/or copyright. If you are not the intended recipient please notify the sender by reply email and immediately delete this email. Use, disclosure or reproduction of this email by anyone other than the intended recipient(s) is strictly prohibited. No representation is made that this email or any attachments are free of viruses. Virus scanning is recommended and is the responsibility of the recipient. --- https://lists.uni-koeln.de/mailman/listinfo/sundial No virus found in this message. Checked by AVG
Re: equation of time sundial
Hello All, I think Mike nailed it, as to what Ken is asking. I know of no sundial, other than Mike's here, that directly measures the Eot, rather than somehow incorporate the EoT calculated elsewhere. I did not think this was possible until I saw Mike's solution just now, because a sundial has no way of measuring Mean Solar Time without a previously calculated EoT chart. In Tom Hank's movie Cast Away, he records an analemma on a cave wall, but this is movie fiction. His watch was broken and he had no way to measure mean time, only local solar time. -Bill On Sun, Feb 3, 2013 at 5:51 PM, jmikes...@ntlworld.com wrote: Ken, I think I made a device some time ago to do what you want . A cylinder was tilted to the appropriate latitude angle and direction. There was a small hole on one side of the cylinder which gave a projected a spot of light from the sun on the inner opposite surface where there was a graph which showed: left to right – the equation of time up and down – the sun’s declination. The cylinder was driven round using a 24 hour clock motor, so the spot of light remained apparently stationary except for changes in the EoT and sun’s declination that were read directly from the graph. Sadly, I don’t think I have it any more. Mike Shaw 53º 22' North 03º 02' West www.wiz.to/sundials No virus found in this message. Checked by AVG - www.avg.com Version: 2013.0.2897 / Virus Database: 2639/6076 - Release Date: 02/02/13 --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial
Thanks, everyone. I see now how an analemmic gnomon can be used to read the EOT, if the equatorial band shows both mean and apparent time and you do the subtraction in your head. I'm most interested, though, in something which allows a direct reading of EOT... Mike, your solution sounds very interesting, but I confess I don't really understand it :-) I take it the motor is to keep the hole pointed toward the sun. Presumably this could be done manually, as on a shepard's dial? I'm unclear though on what's drawn inside. Is it a single curve of EOT as a function of declination, or a family of curves? Ken On Sun, Feb 3, 2013 at 12:02 PM, Ken Baldwin kenneth.bald...@gmail.comwrote: Hello, I'm a new list member, and have a beginner question: Are there examples of sundials whose sole (or primary) purpose is to compute the Equation of Time for the current date? - I know that this information is often provided as a graph in the furniture, but why should I have to know the date and perform the look-up manually? Can't I use the position of the sun to do the computation for me? - I know that the EOT correction can be incorporated into the layout of (some) hour lines, but I'm more interested in having dials which show true solar time. I'd like a separate device dedicated to computing the EOT. - I know that I can construct an analemmic noon mark to show the EOT for that day, since it's simply the east-west component of the analemma, but I'd like a design that can be read at any daylight hour. It seems to me that it should be possible to build such a dial, since the EOT is a function of date, and date lines can be read from many sundials. In principle, I can just re-label the date lines with corresponding EOT values and interpolate. I hope that makes sense. But since I haven't seen anything like that in introductory sundial books, I must be missing something... Is it that the shadow length can't be read accurately enough to get a reasonably precise EOT estimate? Or is it just too hard to make a readable layout, given that solar altitude is ambiguous between two dates, and that the component of the EOT due to the eccentricity of the earth's orbit is out of phase with the equinoxes and solstices? Thanks in advance, Ken Baldwin Corvallis, OR USA --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: equation of time sundial-heliochronometre
Hi Ken, I think it would be straightforward to modify a heliochronometre to achieve the same result as Mike's solution. In addition to the usual figure-of-eight analemma, you'd add a centre line for local solar time. After adjusting the dial for local solar time and 'zeroing' the (new) minutes scale, you would then rotate it slightly to the EOT curve and directly read out the difference between the two in minutes. Here are two scanned images from the Mayall's _Sundials: Their Construction and Use_ pp. 193ff which illustrate how one is constructed. I'll attach a third image in a separate message. On 4/02/2013 3:13 PM, Ken Baldwin wrote: Thanks, everyone. I see now how an analemmic gnomon can be used to read the EOT, if the equatorial band shows both mean and apparent time and you do the subtraction in your head. I'm most interested, though, in something which allows a direct reading of EOT... Mike, your solution sounds very interesting, but I confess I don't really understand it :-) I take it the motor is to keep the hole pointed toward the sun. Presumably this could be done manually, as on a shepard's dial? I'm unclear though on what's drawn inside. Is it a single curve of EOT as a function of declination, or a family of curves? Ken On Sun, Feb 3, 2013 at 12:02 PM, Ken Baldwin kenneth.bald...@gmail.com mailto:kenneth.bald...@gmail.com wrote: Hello, I'm a new list member, and have a beginner question: Are there examples of sundials whose sole (or primary) purpose is to compute the Equation of Time for the current date? - I know that this information is often provided as a graph in the furniture, but why should I have to know the date and perform the look-up manually? Can't I use the position of the sun to do the computation for me? - I know that the EOT correction can be incorporated into the layout of (some) hour lines, but I'm more interested in having dials which show true solar time. I'd like a separate device dedicated to computing the EOT. - I know that I can construct an analemmic noon mark to show the EOT for that day, since it's simply the east-west component of the analemma, but I'd like a design that can be read at any daylight hour. It seems to me that it should be possible to build such a dial, since the EOT is a function of date, and date lines can be read from many sundials. In principle, I can just re-label the date lines with corresponding EOT values and interpolate. I hope that makes sense. But since I haven't seen anything like that in introductory sundial books, I must be missing something... Is it that the shadow length can't be read accurately enough to get a reasonably precise EOT estimate? Or is it just too hard to make a readable layout, given that solar altitude is ambiguous between two dates, and that the component of the EOT due to the eccentricity of the earth's orbit is out of phase with the equinoxes and solstices? Thanks in advance, Ken Baldwin Corvallis, OR USA --- https://lists.uni-koeln.de/mailman/listinfo/sundial -- -- Peter Mayer Politics Department The University of Adelaide, AUSTRALIA 5005 Ph : +61 8 8313 5609 Fax : +61 8 8313 3443 e-mail: peter.ma...@adelaide.edu.au CRICOS Provider Number 00123M --- This email message is intended only for the addressee(s) and contains information that may be confidential and/or copyright. If you are not the intended recipient please notify the sender by reply email and immediately delete this email. Use, disclosure or reproduction of this email by anyone other than the intended recipient(s) is strictly prohibited. No representation is made that this email or any attachments are free of viruses. Virus scanning is recommended and is the responsibility of the recipient. attachment: tn_Mayall-Heliochronometre1.1.jpgattachment: tn_Mayall-Heliochronometre1.2.jpg--- https://lists.uni-koeln.de/mailman/listinfo/sundial
