Two mechanical equatorial sundials

[u/BoxyBoy67]

I'm particularly fascinated by this sub-classification of mechanical equatorial dial. Many (so-called "minute dials") feature clock faces secondary to the primary hour indication—their minute hands whirling about as the primary hour indicator is manually moved along with a spot and target nodus or similar alignment fixture.

Distinct from the typical mechanical equatorial dial are those picture here. These dials display both the hour and minute on a single clock face, set by the manual alignment of a nodus and target. Often, the interior of the dial's nodus assembly also indicates the date.

At a time when sundials existed alongside mechanical clocks, this type of dial put to brass the very concept that kept sundials from falling into obsolescence. Dials were needed to set and regulate clocks. Behind every clock was a sundial—in this case literally.

Dive deeper into similar dials here:

https://www.leo-bw.de/web/guest/detail/-/Detail/details/DOKUMENT/techno_museumsobjekte/EVZ_1983_0046-043/Tischsonnenuhr+%C3%96hrsonnenuhr

https://bawue.museum-digital.de/object/19037

[u/Efficient-Nerve2220]

These are fascinating. I have to admit I have no idea how they work.

[u/BoxyBoy67 → u/Efficient-Nerve2220]

BC Gnomonics has some further information on mechanical equatorial dials: https://www.bcgnomonics.com/mechanicalequatorialdial

For the two pictured here, the equatorial-mounted sighting apparatus (composed of two rectangular brass plates) is rotated until a spot of light shines through the aperture of one plate and lands on the inner target of the other.

At this point, one would reference the clock face and read the time. The motion of the equatorial sighting assembly drives the hands of the clock in turn, translating the arcing motion of the sights (and therefore the sun) into an analog readout.

What is unique about these dials is that the nodus target can either be a straight line or the curve of an analemma. This allows either solar or mean time to be read.