Roman Constellation Engine

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Vitruvius (CE 70-25 BCE) devotes almost an entire chapter of his Ten

Books of Architecture to the design and construction of a mechanical

zodiac. Vitruvius demonstrates his knowledge of astronomy in the

passage, which is incorrect at some points, but as the Books are

intended as a dissertation (not a shop manual) the exercise proves

the need for a qualified project-astrologer.

From Book IX;

8. Fiunt etiam alio genere horologia hiberna...

Other kinds of winter-dials are made, which are called Anaporica.

They are constructed as follows. With the aid of the analemma the

hours are marked by brazen rods on their face, beginning from the

centre, whereon circles are drawn, shewing the limits of the months.

Behind these rods a wheel is placed, on which are measured and

painted the heavens and the zodiac with the figures of the twelve

celestial signs, by drawing lines from the center, which mark the

greater and smaller spaces of each sign. On the back part of the

middle of the wheel is fixed a revolving axis, around which a pliable

brass chain is coiled, at one of whose ends a phellos or drum hangs,

which is raised by the water, and at the other end a counterpoise of

sand equal to the weight of the phellos.

9. Ita quantum ab aqua phellos sublevatur...

Thus as the phellos ascends by the action of the water, the

counterpoise of sand descends and turns the axis, as does that the

wheel, whose rotation causes at times the greater part of the circle

of the zodiac to be in motion, and at other times the smaller; thus

adjusting the hours to the seasons. Moreover in the sign of each

month are as many holes as there are days in it, and the index which

in dials is generally a representation of the sun, shews the spaces

of the hours; and whilst passing from one hole to another, it

completes the period of the month.

 

(BH)