7 Annotations

TerryF  •  Link

Double horizontal dial

"A double horizontal dial is a horizontal dial with two scales for reading the hours. The first is a standard scale, which is used with the polar edge of the gnomon. The second is formed by the vertical edge of the gnomon (set at the centre of the dial) and the lines of projection of the celestial sphere onto the plane of the horizon (the horizontal projection). This projection shows the lines of solar declination, the ecliptic and the right ascension of the sun.... The double horizontal dial was designed by the 17th-century English mathematician William Oughtred. Elias Allen was a friend of Oughtred and he produced several double horizontal dials. They were useful not only for telling the time but also for demonstrating the motion of the sun through the day and also through the year. They could be used for calculating the altitude of the sun at its azimuth, as well as showing the position of the sun on the ecliptic and the time of day...." Scroll down for a link to an image of the Dial. http://www.nmm.ac.uk/collections/explore/index.cf…

Bill  •  Link

Double horizontal dial, one with a double gnomon, one of which points out the hour on the outward circle, and the other shews the hour upon the stereographic projection drawn upon it. This dial not only finds the meridian, hours, &c., but shews the sun's place, rising and setting, declination, amplitude, altitude, and azimuth, with many other useful propositions.
---A New and Complete Dictionary of Arts and Sciences. 1763,

Terry Foreman  •  Link

Image of a double horizontal dial, Royal Museums Greenwich

Double horizontal pedestal dial for latitude 51° 30' North. This dial is made up of an octagonal base-plate with concentric circles to show (from the rim inwards) a minute circle, an hour circle, a compass circle and finally a degree circle. Within the degree circle is the horizontal projection and the equinoxes are marked at 10th March and 13th September. A very faint solar altitude scale runs from the North-East point of the degree circle to the centre. The dial has a simple brass gnomon with a central knife-edge and a conventional polar gnomon for use with the main dial. 'Elias Allen fecit' is engraved in the hour-circle. http://collections.rmg.co.uk/collections/objects/…

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