I classify sundials into three principal categories-those that find time from the sun's hour angle; those that use the sun's azimuth; and those that use the sun's altitude-plus various combinations of two out of the three. In the first category, we generally subdivide dials according to the plane on which the shadow is projected-i.e., horizontal, vertical, equatorial, polar, etc.
Sundials that project the sun's hour angle onto a plane parallel to the equator are equatorial dials. These include many forms such as the universal equatorial dial, the Augsburg-type dial, those with an armillary or crossed-Cs form, mechanical equatorial dials, and more. The convention in the US is to call all of these equatorial dials. In the United Kingdom, it is a convention to call them equinoctial dials. This is perhaps because the celestial equator is often called the equinoctial circle in the UK and the equinoctial circle/celestial equator are in the same plane as the terrestrial equator I prefer the label "equatorial" because it is a typology constructed in parallel to the others (horizontal, vertical, polar) which refer to an orientation on the earth and don't mix in celestial designations. Cheers, Sara Sara J. Schechner, Ph.D. David P. Wheatland Curator of the Collection of Historical Scientific Instruments Department of the History of Science, Harvard University Science Center 251c, 1 Oxford Street, Cambridge, MA 02138 Tel: 617-496-9542 | Fax: 617-496-5932 | sche...@fas.harvard.edu<mailto:sche...@fas.harvard.edu> http://www.fas.harvard.edu/~hsdept/chsi.html
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