Equatorial Dial

A equatorial dial , sometimes called equinoctial dial, is a Sundial time whose table is located in a plan parallel with that of the terrestrial equator, from where its name. Its style, perpendicular to the table of the dial and placed in the plan of the Meridian local, is parallel to the axis of the Earth (pole Southern North-pole) and is thus tilted compared to the horizontal plane of an angle equal to the Latitude of the place.

It is the type of the dial simplest to build and include/understand. It functions like a terrestrial sphere in miniature, making a turn on itself of 24 hours, with the variation due except for the terrestrial ray, which is negligible had regard to the distance Ground-Sun. It follows that the time lines are equidistant of 15°, the line of midday being that of greater slope, in the plan of the meridian line. Its main feature is that the reading of the hour is done on a face of the table between the equinox of spring and that of autumn and on the other face other half of the year. A disadvantage is that the rays of the sun arrive exactly in the plan of the table the days of the equinoxes. One can facilitate the reading by replacing the table by a half-cylinder perpendicular to this one, which transforms it into dial armillaire.

It can function under all the latitudes, on the condition of modifying its slope which must be adjustable. Placed at the equator, it becomes a vertical Cadran whose face functions six months and the other face the six other months. Placed at the poles, it becomes a horizontal Cadran which gives the hour only during the six months of the polar day.

The equatorial dial allows a very easy correction of the time variations due to the equation of time, a difference of Longitude or to the passages hour of winter/summer-time: it is enough to turn the table of the dial around the style of the wanted angle, for example 15° for a one hour shift.

See too

Sundial

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