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
