The best treatment was found in 1951, by P. Terpstra. He treated the analemmatic sundial as a projection of the equator circle, the time being indicated by the intersection of a gnomon shadow with this projection. After further examination, building on Terpstra's proof, it appeared to me that all known kinds of equator projection sundial can be derived from one principle of construction. This possibility showed up an endless number of new varieties. View from above of the prototype of an equator projection sundial.
G is a vertical gnomon at a point on the line of dates.
This sundial was constructed in 1975 on the occasion of the third centenary of the Royal Greenwich Observatory. This circle makes an angle of about 40 degrees with the horizontal, and the movable gnomon is vertical. Because the hour points are placed at equal distances around the circle, corrections for longitude, the equation of time and summer time can be carried out simply by turning the graduated scale so that this sundial can show standard time. The equiangular sundial at the Royal Greenwich Observatory, Herstmonceux Castle. The sundial now is at Cambridge.