Hi John, Roger & all, Thank you, John, for the link, and Roger for the extensive, thoughtful comments.
Wondering how small the sundial actually is, Mike Gaylard answered "of the order of 1 metre". In physics this would be interpreted as: between 0.3 and 3 meter. Judging from the grass around the granite slab, however, the estimate looks quite close. As Roger pointed out, the analemmatic analemma in Mike's HartRAO website is used correctly, as it has its own EoT time scale at the bottom of the date line. I know of only one other such a correct usage. Marinus Hagen, founding father of the Dutch Sundial Society, constructed the first analemmatic dial of our country in his own garden. It also had an analemma around the date line with a separate EoT time scale. The attached schematic drawing (12 kB) is from an article he published in Zenit, an amateur astronomy magazine, in 1976. Best regards, Frans ----- Original Message ----- From: "Roger Bailey" <[EMAIL PROTECTED]> To: "John Carmichael" <[EMAIL PROTECTED]> Cc: <[EMAIL PROTECTED]>; "Frans W. Maes" <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]> Sent: Sunday, September 17, 2006 5:33 AM Subject: RE: a small analemmatic > Hi John, > > Thanks for the link http://www.nfi.org.za/palaeo/millennium_sundial.htm to > the The Millennium Dial at Sterkfontein, South Africa. > > This sundial demonstrates well the shape and size of the analemma for the > southern hemisphere at a latitude close to the tropic line. But, as usual > for analemmatic dials with analemmas, this dial shows the common error of > trying to use the analemma shape as an equation of time correction. The > website tells the user to put the gnomon on date mark on the analemma to > correct for the equation of time. This obviously only works at around noon > when the correction is east or west. The north/south corrections required > for the morning and afternoon go in opposite directions. This is not > corrected with a single analemma. As Fred Sawyer describe in his article, > "Of Analemmas, Mean Time and the Analemmatic Sundial", at > http://www.longwoodgardens.org/docs/analemma.pdf , a split dial with two > analemma can be used with morning corrections on one analemma and afternoon > corrections on the other. Brian Albinson demonstrated this well with his > split analemma designs on the recent Vancouver Sundial Tour. Helmut > Sonderegger's program "ALEMMA" available at > http://web.utanet.at/sondereh/sun.htm calculates these split analemma dials > well. > > The website http://www.hartrao.ac.za/other/sundial/sundial.html is referred > to as the design basis for the Millennium dial. The designs by Mike Gaylard > on this website show how the analemma can used correctly to account for the > equation of time, but not directly. In his drawings,.the date line for the > gnomon position is shown correctly by the green line at the N/S central > axis. The red analemma shape shows the EQT correction for that date > referenced to a small scale at the base of the dial. This is a clever way to > include the EQT correction, but I do not see any evidence of such a > reference scale in the Millennium Dial picture. Perhaps the correction scale > should be added this and other dials that try to use a single analemma for > EQT correction. This renovation may also provide a good opportunity to add > Seasonal Markers. I have note yet seen these on a southern dial. > > It is interesting that many analemmatic dials in the southern hemisphere > attempt to use a single analemma for EQT correction. I have seen such dials > in Wellington NZ, Mt Annan NSW Australia and Kingston SE Australia. Click on > the links below. There are others as well . often called the "sundial of > human involvement". Perhaps the latitude close to the tropics and > compression of the hour ellipse makes the single correction appropriate. > Perhaps it is the southern analemma with the big loop near the hour ellipse. > Perhaps this was the design standard in 1990 when many of these dials were > built. I don't know. There is a rationalization for the design in the book > "Sundials Australia" by John Ward and Margaret Folkard. I read the > description of the experiments and studied the drawings but I cannot agree > that the single analemma gives an accurate EQT correction. > > Regards, > > Roger Bailey > Walking Shadow Designs > N 46.6 W 123.4 > Northern hemisphere, well north of the tropics > > > http://www.flickr.com/photos/glutnix/sets/151663/ > http://www.rbgsyd.nsw.gov.au/mount_annan_botanic_garden/garden_features/buil > dings_and_art/sundial > http://walkabout.com.au/locations/SAKingstonS.E..shtml > > For a few of my scanned pre-digital pictures of our visits to two of these > sites, to click on this Snapfish link. > http://www2.snapfish.com/share/p=481161158454699242/l=211710424/g=56484356/c > obrandOid=1000131/otsc=SYE/otsi=SALB > > -----Original Message----- > From: John Carmichael [mailto:[EMAIL PROTECTED] > Sent: September 16, 2006 7:35 AM > To: Frans W. Maes > Cc: Roger Bailey > Subject: a small analemmatic > > > Hello Frans, > > I came across this small granite analemmatic that you may want for your > photo collection of analemmatics on your website. > > see: http://www.nfi.org.za/palaeo/millennium_sundial.htm > > hope all is well, > > John >
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