At 06:56 PM 10/15/01 +0100, Chris Lusby Taylor wrote:Sara is right. I was mistaken. Although in some countries 'nonius' is used to refer to a vernier, it originally and correctly refers to a device consisting of 45 concentric circular scales in a quadrant, each divided into between 46 and 90 parts, so an angle can be read as, say, 57/73 of a right angle. See, for instance, < http://www.mhs.ox.ac.uk/epact/catalogue.asp?enumber=98558 >. This wouldn't seem very helpful for sundials.
By the way, John, could you please note in the BSS Glossary that the
correct term for a diagonal scale (used to interpolate the shadow edge
between successive hour lines) is a nonius, from Pedro Nuñez, the
Portuguese inventor.I do not think that this is right. I think you are referring to a
transversal rather than a nonius.Happy dialling,
Sara
A transversal is a diagonal scale. It appears to have been invented by Tycho Brahe. He used it in a quadrant to allow stellar altitudes to be read to 10 arc seconds (See <http://www.landskrona.se/kultur/tychobrahe/astro2/instrument2.html>). See also <http://www.mhs.ox.ac.uk/epact/catalogue.asp?enumber=94421> which has both a transversal scale and a grid that shows the sine and tangent of the angle. In sundials, the diagonal scales are usually graduated in minutes. I haven't been able to find a URL for one, but there's at least one pictured in Cousins' book.
By coincidence, I visited Prague last month and climbed up the astronomy
tower in the Klementinum, which is where Tycho Brahe last worked. There
are two transit telescopes with quadrants, but I didn't notice if they
had transversals. In the same room, there is an unusual meridian line.
It consists of a fine wire strung about 5cm above a slab of marble. It
is illuminated by a small hole high in the wall. At noon, I presume one
sees the sun's disc on the marble bisected by the shadow of the wire. This
was used until the 1920s to give a noon signal by gun, flag and, latterly,
radio.
On the east and west sides of the tower there are painted sundials,
except that the westerly one has been totally obliterated. The gnomon is
all that remains. There are, apparently, 17 sundials in the Klementinum,
but I only spotted 8 of them, all painted on walls.
Chris Lusby Taylor
Newbury, England
51.4N 1.3W
