Astronomical Images : Astronomical ring dial: hinged articulation for the meridian and equator rings

Various

Astronomical Images

<p style='text-align: justify;'>The astronomical ring dial is a time-keeping device adaptable to a large range of latitudes. These instruments were very popular in the early modern period, with several different variants described and a number surviving to the present day. The number of rings is variable, but typically includes a ring representing the meridian, mounted perpendicular to the ring of the equator, with a third ring corresponding to a great circle of constant right ascension. A sliding device on this latter ring enabled the user to adjust the sights to the seasonal declination of the Sun. Some rings also included a suspension device that allowed the instrument to be adjusted for a given latitude. This image comes from a work of Johannes Dryander, published in an edition of collected works on the <i>annulus</i>. It shows the perspective and side-view of the hinge joint between the meridian and equator rings. This design allowed the rings to lie flat when closed, yet to open out and be locked at ninety degrees, thus representing the correct configuration of a celestial meridian to the celestial equator.</p>