Re: Equinox and Equatorial Rings
Dear Frank, You point out here that the declinations lines normally used (which I assume are conic sections) are not really correct, and it made me wonder - what is the maximum deviation from the conic section line, eg on a dial the size of your Pembroke dial? - if it is significant, are there any programs that will give the truer line? - have any dials been made which take this into account? Regards, John When I was setting up the Pembroke Sundial in 1997, I spent a lot of time tracing the path of the shadow of the nodus across the dial and comparing the observed results with my calculations. For each day's observations I assumed the declination at noon held good all day. Observed results and calculated results matched pretty well during June and July and I became complacent. At the autumnal solstice, observed results and calculated results didn't agree so well. I was puzzled by this at first. Then I appreciated the obvious. The declination is changing so rapidly at the equinoxes (about 1 minute an hour) that you can no longer assume the declination at noon holds good all day. Contrary to what simple sundial guides tell you, the shadow of a point cast onto a plane DOESN'T follow a straight line on the day of an equinox. It actually follows an S shape, albeit a very narrow S. --- https://lists.uni-koeln.de/mailman/listinfo/sundial
RE: Equinox and Equatorial Rings
John Foad asked: are there any programs that will give the truer line? One problem presumably is that the lines are different according to whether the solar declination is increasing or decreasing, so there are two for every value of declination (except - more or less - at the solstices), making a rather confused appearance - I guess there would probably in effect be a thickening of the double ends of the lines compared with the middle? Regards, Andrew James PRI Limited, PRI House, Moorside Road Winchester, Hampshire SO23 7RX United Kingdom Tel: +44 (0) 1962 840048 Fax: +44 (0) 1962 841046 www.pri.co.uk PRI Limited is a company registered in England and Wales with company number 2199653 Measure - Inform - Empower This correspondence is confidential and is solely for the intended recipient(s).If you are not the intended recipient, you must not use, disclose, copy, distribute or retain this message or any part of it. If you are not the intended recipient please delete this correspondence from your system and notify the sender immediately. This message has been scanned for viruses by MailControl. --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Equinox and Equatorial Rings
Frank, Declination lines normaly used on a dial are correct but you have to see them as lines for constant declination. In a day the declination changes so the lines aren't correct as lines for the date. Also the average declination changes in a period of 4 years. So in fact datelines never are correct. It is for this reason I prefer a zodiac calendar on a dial in stead of a our month/day calendar. 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: John Foad john.f...@keme.co.uk To: Frank King frank.k...@cl.cam.ac.uk Cc: sundial@uni-koeln.de Sent: Thursday, September 24, 2009 11:20 AM Subject: Re: Equinox and Equatorial Rings Dear Frank, You point out here that the declinations lines normally used (which I assume are conic sections) are not really correct, and it made me wonder - what is the maximum deviation from the conic section line, eg on a dial the size of your Pembroke dial? - if it is significant, are there any programs that will give the truer line? - have any dials been made which take this into account? Regards, John When I was setting up the Pembroke Sundial in 1997, I spent a lot of time tracing the path of the shadow of the nodus across the dial and comparing the observed results with my calculations. For each day's observations I assumed the declination at noon held good all day. Observed results and calculated results matched pretty well during June and July and I became complacent. At the autumnal solstice, observed results and calculated results didn't agree so well. I was puzzled by this at first. Then I appreciated the obvious. The declination is changing so rapidly at the equinoxes (about 1 minute an hour) that you can no longer assume the declination at noon holds good all day. Contrary to what simple sundial guides tell you, the shadow of a point cast onto a plane DOESN'T follow a straight line on the day of an equinox. It actually follows an S shape, albeit a very narrow S. --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Equinox and Equatorial Rings
Dear John, This is a good riposte... You point out here that the declinations lines normally used (which I assume are conic sections) are not really correct... They are indeed conic sections but they ARE correct provided you call them Constant-Declination Lines or something equivalent. If you could somehow freeze the declination of the sun at spot-on zero for 12 hours then the shadow of the nodus would run along the equinoctial line seen on many dials. This is, of course, a straight line (on a plane dial) and is, strictly, the line associated with declination zero rather than the day of an equinox. ... have any dials been made which take this into account? Well, there isn't a strong case for drawing narrow S shapes because it would be different for each equinox. The S would cross the straight equinoctial line at the instant of the equinox and the hour-angle of that instant at a given place is different for each equinox. That said, you could argue that the cronometri that Bianchini incorporated into the meridiana in S. Maria degli Angeli in Rome are there to measure the point where the (undrawn) S shape crosses the (also undrawn) equinoctial line. In short, these instruments enable you to estimate the instant of the equinox by the offset of the point where the centre of the solar image crosses the noon line. To check their performance, Mario Catamo has recorded the crossing on the day of every equinox since 2003. Best wishes Frank --- https://lists.uni-koeln.de/mailman/listinfo/sundial
RE: Equinox and Equatorial Rings
Hi All, I would like to recommend a very interesting and useful astronomical tool / program. It's not cheap at about $280 but definitely worth it, especially if you have a telescope to guide. You do need a medium grade computer; I use a 4 yr old Dell laptop. The program is from Software Bisque and is called The Sky 6, Professional. Example: I was able to look at this equinox down to very fine detail...though unproven down to the second of time or angle but far greater than what sundialists would require. I could adjust the time by one second and watch the change in the sun's altitude for sunset / azimuth / RA / sun's apparent dia. etc or find the second where the declination was closest to zero (I believe the closest was something within 5 arc seconds to zero). I saw that the Sun's declination was closest to zero at about 2:19 PM, my Pacific Daylight Savings Time, I don't remember the exact second because I don't have it all here now. I do remember that the sun's declination the day before at the same time was +22'22 and the day after it was -24'22...so it was changing only about .3 degree per 24 hr. day! Not that much when the sun's apparent dia. is something like 1/2 degree (31'?). The program is really fun in other ways also. You can set it up for anywhere on the earth and at any reasonable time...say within + - 5 thousand years, to the second! You can look at just about any object in the heavens, get its coordinates and so much data about it, including pictures of many! You can watch the changes of star patterns thru the millenniums, watch the sun drift from Aries towards Aquarius at the 1st pt of Aries (Spring) and so much more, the possibilities are unlimited! I can check and see when one of Jupiter's moons is going behind it then have my Wife watch thru my telescope's eyepiece and I will tell her when it will happen by timing with my watch, and...usually within seconds! I will leave it to you to imagine what can be done in designing / setting up / checking sundials etc! Have fun all, Fritz Stumpges -Original Message- From: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de]on Behalf Of Frank King Sent: Wednesday, September 23, 2009 1:39 PM To: Roger Bailey Cc: sundial@uni-koeln.de Subject: Re: Equinox and Equatorial Rings Dear Roger, It is good of you to remind us of the bronze ring set in the Square Hall of Alexandria. You add... You can repeat this experiment on any correctly aligned equatorial sundial or armillary sphere. You can in fact undertake the essentials of the experiment using ANY sundial equipped with a nodus and an equinoctial line. When I was setting up the Pembroke Sundial in 1997, I spent a lot of time tracing the path of the shadow of the nodus across the dial and comparing the observed results with my calculations. For each day's observations I assumed the declination at noon held good all day. Observed results and calculated results matched pretty well during June and July and I became complacent. At the autumnal solstice, observed results and calculated results didn't agree so well. I was puzzled by this at first. Then I appreciated the obvious. The declination is changing so rapidly at the equinoxes (about 1 minute an hour) that you can no longer assume the declination at noon holds good all day. Contrary to what simple sundial guides tell you, the shadow of a point cast onto a plane DOESN'T follow a straight line on the day of an equinox. It actually follows an S shape, albeit a very narrow S. Have a good winter. Frank --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
RE: Equinox and Equatorial Rings
I agree. I have a polar dial with an equinox line and I see this every equinox. It's not that subtle. You can see it easily over just a few hours. -Original Message- From: sundial-boun...@uni-koeln.de [mailto:sundial-boun...@uni-koeln.de] On Behalf Of Frank King Sent: Wednesday, September 23, 2009 1:39 PM To: Roger Bailey Cc: sundial@uni-koeln.de Subject: Re: Equinox and Equatorial Rings Dear Roger, It is good of you to remind us of the bronze ring set in the Square Hall of Alexandria. You add... You can repeat this experiment on any correctly aligned equatorial sundial or armillary sphere. You can in fact undertake the essentials of the experiment using ANY sundial equipped with a nodus and an equinoctial line. When I was setting up the Pembroke Sundial in 1997, I spent a lot of time tracing the path of the shadow of the nodus across the dial and comparing the observed results with my calculations. For each day's observations I assumed the declination at noon held good all day. Observed results and calculated results matched pretty well during June and July and I became complacent. At the autumnal solstice, observed results and calculated results didn't agree so well. I was puzzled by this at first. Then I appreciated the obvious. The declination is changing so rapidly at the equinoxes (about 1 minute an hour) that you can no longer assume the declination at noon holds good all day. Contrary to what simple sundial guides tell you, the shadow of a point cast onto a plane DOESN'T follow a straight line on the day of an equinox. It actually follows an S shape, albeit a very narrow S. Have a good winter. Frank --- https://lists.uni-koeln.de/mailman/listinfo/sundial --- https://lists.uni-koeln.de/mailman/listinfo/sundial
