GB537924A - Improvements in and relating to sun compasses - Google Patents

Abstract

537,924. Sundials. FORSTER, C. M. Jan. 11, 1940, No. 661. [Class 139] [Also in Group XX] In a sun compass for use on transport units the apparent time for a given latitude is indicated on a time scale by the intersection of an ellipse and a confocal hyperbola, means being provided for correlating the indications with a date indicator. The ellipse and hyperbola are determined respectively by the formulae where # is the latitude, a is a constant and h is the time in hours from apparent noon. As shown, a protractor A is mounted on gimbals, with its 0‹-180 degrees diameter parallel to the axis of progression of the transport unit. A vertical gnomon projects from the fixed pivot pin C of the protractor. A time scale E, a shadow pointer K and an elongated date scale D are pivotally mounted about the pin C, and the time scale may also be slid along the date scale to set the date against an index mark E<SP>1</SP>. The time scale bears a series of ellipses E<SP>2</SP> referring to various latitudes, and the ellipses are intersected by confocal hyperbola to graduate the ellipses in hours. The date scale D is engraved with a North-South line D'. After setting the time scale E and date scale D at the required date the two scales may be locked to the protractor A at the required azimuth by means of a clamping nut beneath the dials. The shadow pointer K is adjusted by hand at frequent intervals to indicate the correct apparent time and facilitates the maintenance of the shadow of the gnomon thereat under working conditions.