Equatorial sundial with simple time and date interpretation

Abstract

This invention represents an improvement to an equatorial sundial comprising an independent simultaneous single scale indication of both time and date as represented from a primary and secondary gnomon. This allows one unfamiliar with the physics of an equatorial sundial to with simple observation quickly assess both time and date as a singular measurement. The sundial displays conventional time measurement on a single scale transcribed on the equatorial ring from a primary gnomon aligned with the polar axis. The improvement includes a date scale transcribed on the primary gnomon that is cast a shadow from the secondary gnomon comprised of the upper equatorial ring. Since the sun maintains an essentially equivalent angle of declination throughout the day, the shadow cast by the upper equatorial ring (secondary gnomon) on the primary gnomon (containing the date scale) remains consistent in position and date can be observed at any time of the solar day.

Description

FIELD OF THE INVENTION This invention pertains to equatorial sundials BACKGROUND OF THE INVENTION The basic equatorial sundial is comprised of a gnomon that is aligned with the star Polaris and essentially represents the axis of the earth. The time scale is composed of a semicircle that is aligned with the equatorial plane of the earth. On the time scale, hours of the day are separated by 15 degree increments that correspond with a one hour rotational angle of the earth. Refer to U.S. Pat. No. 2,754,593 (Sunblad), and U.S. Pat. No. 2,192,750 (Mead) which describe an adjustable time scale equatorial sundial. Equatorial sundials have been described which can measure the date as well as time based on the declination of the sun: U.S. Pat. No. 89,585 (Johnson) describes a blackened glass sphere with a hole to transmit a ray of sunlight against a two axis grid etched on the opposing surface of the glass. By interpreting alignment of the illuminated dot on one axis a time is determined...

AI Technical Summary

Benefits of technology

The improvement in using two separate scales (time on the equatorial grid and date on the primary gnomon grid) is that a user at a glance can see either the time or the approximate date independently and with simple interpretation. Two critical components are necessary for proper date representation on a single axis scale throughout the day; a constant secondary gnomon position with regard to the declination of the sun regardless of time of day, and an equidistant relationship between said gnomon and the date scale so the projected shadow always falls on the same point. The equatorial ring serves well for the purpose of the secondary gnomon since it is at a constant position with regard to the declination of the sun throughout the day and the primary (polar) gnomon serves best for the scale since it maintains an equidistant relationship with the equatorial ring regardless of solar time.

Problems solved by technology

Unfortunately because of the geometry of a sphere, resolution is only good during midday hours (high solar altitudes), and the observer must follow the grid pattern to extrapolate both date and time.

Unfortunately, this only occurs when the sun reaches maximum altitude (solar noon), and the intersecting shadows pass through the scale (a 30 minute period or less), so the date function has a finite time span of utility.

Like patent U.S. Pat. No. 89,585, this device losses resolution during low solar altitudes (beyond the midday hours) and requires the user to follow a grid pattern to extrapolate both time and date.

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