GB214034A - Sundial showing local or greenwich mean time - Google Patents

Abstract

214,034. Chase, L. H. Feb. 16, 1923. Sun-dials. - Greenwich mean time is read on a sun-dial by means of a director OP adjustably fixed to a rotatable disc H and having a curve, Fig. 2, marked on the vane 0, on which curve the image of the aperture Q is brought before reading time on the scale A against the pointer K. The disc H is graduated in intervals of 14‹ 22¢ and reads as a vernier against the 15‹ graduations on the fixed plate A, the reading being the nearest hour to the pointer plus or minus the minutes marked on the vernier at the coincident marks of the two scales. The displacement of the image right or left from the median line R when the pointer is at mean time is d tan #, d being the distance between Q and 0 and # being the angle in degrees corresponding to the equation of time. The value of # is obtained by multiplying the difference of time in minutes by 4. The displacement above or below the equinoctional position S due to declination is arrived at by multiplying the tangent of the angle of declination by d. The curve constructed from these displacement values is marked in months and provided with arrows showing the direction of motion. The director may be adjusted by a scale and pointer N for reading summer time. The position of the director pivot I on the disc may be varied. The equatorial plate A is adjustably connected to the fixed plate B by hinges C and telescopic arms D, graduations E being provided for different latitudes if desired. A lens may be fitted in the hole Q to give a brighter image, the focal length of the lens being the average distance Q to R. The director vane 0 may be curved and the diagram correspondingly modified.