Hello Claus, Yes, in J. L. Heilbron's "The Sun in the Church" he reports that Toscanelli designed the original meridian and solstice mark in 1510, placing a bronze plaque at the sun's noon solstice image. Around 1754 Leonardo Ximenes discovered that Toscanelli's 1510 meridian was off by almost a full degree. The I remain confused regarding the two circles (elipses), as the smaller circle is clearly north of the center of the larger circle.
Heilbron offers clues to the answer, but not a complete explanation: One is that the original gnomon hole was created around 1475, and was "reset" in 1511, the year after the the brass marker was placed. I do not know if that means the gnomon hole was moved, or by how much. Furthermore, I do not see a brass marker, so this is confusing. I think both of the circles are stone, based on the video and other photos. From 1510 to 1754 the ecliptic tilt shrank about .03 degrees, which would have moved the center of the solstice mark northward about 4.5 centimeters (my calculation). The smaller circle/ellipse is displaced northward from the lager ellipse, but to me it looks like a distance much greater than 4.5 cm. So I am stumped here. -Bill On Thu, Jun 7, 2018 at 5:18 PM, Claus Jensen <clau...@hotmail.com> wrote: > The divided line or band to the left in the picture is Leonardo Ximenes’ > so-called *new* meridian line described in his 1757 book *Del vecchio e > nuovo gnomone fiorentino* > <http://books.google.com/?id=-Go_AAAAcAAJ&printsec=frontcover>. It > deviates slightly from the *old *meridian line, dating from ca. 1475, due > to Paolo Toscanelli. The two circles are what remains of Toscanelli’s > meridian line. > > According to a sketch in Ximenes’ book, the smaller circle is called *The > solstice marble*. The greater circle contains a date, hardly visible > today, given in the old roman calendar, corresponding to 12 June 1510 in > the Julian calendar. Thus summer solstice in 1510 took place on 12 June. > > > Claus Jensen, DK >
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