Oops, I neglected to post this to the list. Sorry, John. t -- Tom Semadeni O [EMAIL PROTECTED] o aka I (Ned) Ames . Britthome Bounty ><<((((*> Box 176 Britt ON P0G 1A0 'Phone 705 383 0195 fax 2920 45.768* North 80.600* West
X-Mozilla-Status2: 00000000 Message-ID: <[EMAIL PROTECTED]> Date: Sat, 20 Feb 1999 12:24:11 -0500 From: Tom Semadeni <[EMAIL PROTECTED]> Reply-To: [EMAIL PROTECTED] Organization: Britthome Bounty X-Mailer: Mozilla 4.5 (Macintosh; I; PPC) X-Accept-Language: en MIME-Version: 1.0 To: John Shepherd <[EMAIL PROTECTED]> Subject: Re: Equation of Time References: <[EMAIL PROTECTED]> <[EMAIL PROTECTED]> Content-Type: text/plain; charset=us-ascii; x-mac-type="54455854"; x-mac-creator="4D4F5353" Content-Transfer-Encoding: 7bit Thanks, John for telling of the situation which led to tha Aha! of the pointed double gnomon. The eye's extrapolation of the two shadows removes so much of the uncertainty caused by the geometrical and physical optics of the formation of the shadow. John Shepherd wrote: > With a vertical sundial at our latitude (45 degrees N) the shadow length > varies considerably and therefore so does the distance moved by the shadow > on the wall in a minute. So unless we were to vary the width of the bar > making the annalemmas (very costly) we had to chose a compromise for this > width and that of the pipes for the . The main factor turned out to be > visibility of the shadow and the annalemmas. We hung bar stock on the wall > and just looked at it from different distances:-) 1.25 by 0.75 inch turned > out to be best. Similarly we tried various pipes for the gnomon with > various terminations. When we viewed at the extreme hours we found the > discs and cross pieces on the end were too blured to see, hence the double > gnomon was born:-) snip > Actually as a Physicist part Astronomer the calculations were done first > using the Ephemeris Tables to calculate the Right Ascension/Declination of > the Sun at each hour for each day from this the direction of the Sun was > calculated for any latitude longitude and hence the intersection of the > shadow with the plane was found. Later I used "Compact data for navigation > & astronomy for the years 1991-1995" from the Royal Greenwich Observatory, > a set of tables that contain the Altitude azimuth data for the sun as a > function of time in polynomial form which is even easier to use for > computation. > Hence I could calculate the direction of the sun for any time > on any date. I have since purchased the tables for 1996 - 2000. In plotting > the annalemas for different years I realized that there shape was year > independent which from our preceding discussion is not at first obvious:-) Yes, except for the slowly changing change in magnitudes of obliquity and eccentricity that Luke refers to. It is so much easier for me, now, to fit the calendar to the analemma rather that the other way around, in a way similar to your use of the weeks on the analemma themselves. The ONLY value to this, methinks, is to show the ability of a sundial to act as a calendar. (I guess that it will also demonstrate the need for Pope Gregory's modification to the Julian Calendar!) <<The next task was to decide the scale which is determined by the size of <<the gnomon. This is where the advantages of direct plotter output were > realized as I could draw many designs and pick the one I thought was best > for the size and shape of the wall. I also discovered that the previous > program based on the Ephemeris was in error having corrected for Sidereal > to solar time twice! This led to an error of 4 minutes at the end of the > day:-( In numerical work such as this its nice to have at least two > different ways of calculating something as an error check:-) Oops! (Yes) > Once the size was decided I generated a file of x-y coordinates in inches > from the base of the Gnomon for every minute to generate a smooth curve. You selected the "hour" first then did the minutes around the "hour" for each day of the year to generate a figure-of-eight for that "hour"? Then repeated the process for all of the other "hours"? > Gene Olson then took this into his CAD program an generated the full size > drawings which he used as templates to bend the bar stock to. You can see > a picture of Gene placing them on the wall at: > http://www.concentric.net/~mettlewk/sundial.htm > > Thats probably more detail than most of you wanted. At the risk of being selfish on this list, this is a great "how to" explanation, which I for one, appreciate very much, John. Thank you Tom
