I try to answer to some of the last messages on the subject
John Carmichael has written: > And I give you think that we should stop using the term " monofilar " to >describe Singleton dials? I don't know the Singleton Sundial and so I cannot answer -------------------------- Sara Schechner has written: >I am very much against using monofilar in this way. It is confusing and >unhelpful. It seems to me that monofilar should refer to a special case of >string-gnomon. I think that the definition Monofilar causes the same confusion of Bifilar,definition used and accepted by all, from almost 80 years. I think that, in general, there is some confusion also on the Bifilar sundials -------------------------- To John Davis, Steve Lelievre, Warren Thom .Sara Schechner and others I have called (in an article on "The Compendium" September 1998) Monofilar "those sundials in which the element that casts the shadow is a rectilinear element and the set of the date lines have a shape chosen by the designer (as he wants)" As Fer de Vries has already written the element DOESN'T HAVE TO BE necessarily a thread, a string or a cable but it can be the edge of a plate or of a balcony or the ledge of a building, a pole, etc. Moreover the date lines must not have necessarily the shape of concentric circles (even if these are simple to be traced) Therefore the common sundials with a polar style are included, as very particular cases, in this group (category) of sundials The Monofilar and Bifilar sundials can be built with any kind of Time: Middle Time (Standard), Local Apparent Time, with Italic, Babylonian, Temporary hours, etc. On the name of this category of sundials I accept whatever suggestion. I had also thought to other name as " Mono-style ", "Mono-line", "With linear shadow", etc.: every name can give confusion and be erroneously interpreted. As Fer has already written also in the Bifilar Sundials the elements that cast the shadows don't have to be necessarily threads, strings or cables, despite the fact that word "Bifilar" literally means "two threads" . Moreover they don't even have to be straight lines . A fine example of a great Bifilar Sundial is in Barcelona - Spain near the beach. It can be seen in the beautiful book of Rafael Sun Gayá "Relojes de Sol". In it the two " threads" are : 1) an vertical element (of Plexiglas or of glass), in the plane E-W, with the edge with the shape of a parabola and 2) the horizontal edge of a plate (of Plexiglas or of glass) in the vertical plane North-South . The fact that almost always the Bifilar Sundials are built in a horizontal plane with two horizontal perpendicular " threads" has, in my opinion, historical and practical reasons : the sundial designed by Michnik in 1922 had these characteristics and the calculation with this configuration is more simple. To overcome the obstacle we can now use programs to calculate Bifilar Sundials on a whatever plane and with " threads " placed in a whatever way. -------------- Altitude Sundials As I have written in a precedent message that, in my opinion, the azimuthal sundials must have a vertical style. I try now to explain my thought on Altitude sundials The position of the Sun in sky is determined when we know his 2 coordinates in a given system of coordinates of reference. The main systems of coordinates that are used and have been used are : 1) the equatorial coordinates: hour angle and declination 2) the azimuthal coordinates (or local): azimuth and altitude (height) 3) the Ptolemaic coordinates: meridian angle and hectemoros angle 4) the Ptolemaic coordinates: vertical angle and horarius angle In each of these systems the first coordinate (the first angle) is a dihedral angle between the planes that belong to a pencil of planes all passing for the same straight line (axis or fundamental direction of the system of coordinates) The axis are: 1) the polar axis, 2) the vertical of the place (Zenith - Nadir), 3) the direction East-West, 4) the direction North - South The plane perpendicular to the axis is said "equatorial plane of the system." They are : 1) the plane of the Celestial Equator, 2) the horizontal plane, 3) the vertical plane North-South (meridian p.), 4) the vertical plane East-West (First Vertical p.) The second coordinate (the second angle) is measured from the axis of the system to the direction of the Sun Or, and it is the same thing, this angle is measured on the plane passing through the axis of the system and the Sun between the direction of the Sun and the "equatorial plane of the system" As I have already written, if we want that the shadow in a sundial can be calculated knowing only the first coordinate (angle) the style has to be a straight line parallel to the axis of the system. So in 1) if we want that the position of the shadow is known when we know only the hour angle, .the style has to be parallel to the polar axis; in 2) if we want to use only the azimuth (azimuthal s.) the axis has to be vertical; etc. On the contrary, if we want that the position of the shadow can be calculated when we know only the second coordinate (for ex. the height or Altitude of the Sun) , we need that the sundial can rotate around the axis of the system and that the geometric element that casts the shadow is a point (except exceptions) We can also make fixed sundials if we take many points (in theory an infinite number) : one for every direction. All the Altitude dials therefore must have the possibility to be rotated around the vertical axis (except exceptions). The common shepherd sundial has this property. An example of "exception" is the cylindrical hat-sundial in which a circular disk (hat) casts its shadow on the surface of a vertical cylinder. In it we have an infinite number of points (on the edge of the disk) that cast shadow, one point for every direction. In the same way if we want to make a sundial that uses only the Hectemoros angle, we need that it can rotate around the direction East-West (see the article of F.Sawyer in the issue of September 1998 of "The Compendium") --------------------- I apologize for the length of the message Best Gianni Ferrari --------------------------------------------------------------------------- Ing.Gianni Ferrari Via Valdrighi, 135 41100 - MODENA (ITALY) EMail : [EMAIL PROTECTED] ---------------------------------------------------------------------------
