Remark :
To my knowledge, only this one Foster-Lambert sundial was constructed with a horizontal clockface. In 1680 the Englishman Samuel Foster discovered that the ellipse of an analemmatic sundial could be changed into a circular form, providing there was a specific direction of projection. In 1777 the Alsation astronomer Johann H. Lambert succeeded in doing the same. Hence, today, these types of clocks have come to be known as Foster - Lambert dials. The circular, horizontal clockface possesses a homogenous division of hours that enables a precise reading down to the exact minute. The circle ring can then be adjusted according to date, depending on the time-equation. Thus, both local and mean time can be read.
Princip of Construction :
There are two ways to set the direction projection of an equator-parallel clock so that the ellipse of a horizontal analemmatic sundial changes into circular form. In this case, the gained analemmatic clock is homogenous, i.e., the angle-distances between the hour lines are consistantly 15°. The illustration below depicts the equatorila clock using P_{A} - P_{B} (magenta) and the polar axis using A_{W} - A_{V} (green). The two straights of projection for the construction of the horizontal analemmatic sundials (red) are represented by (90°-j)/2 and (90°-j)/2, tending horizontally. By these means one receives two circular analemmatic clocks with homogenous clockfaces. The date ribbons, however, are only then of the same length when the clock is located at the equator.
Last update : 7. October 2002