EQUATORIAL SUNDIALS

equatorial sundials

Using an Equatorial sundial at different locations   (      is the latitude angle)

The dial plate of an Equatorial sundial is either a flat plate or a circular ring whose plane lies perpendicular to the gnomon. The gnomon is usually a rod (sometimes in the form of a stylised arrow) passing through the centre of the dial plate and pointing towards the Celestial Poles, ie it is set at the latitude angle. Since the gnomon is parallel to the axis of rotation of the Earth, the dial plate of an equatorial sundial is therefore parallel to the plane of the equator (hence the name Equatorial sundial). 


The Armillary Sphere sundial is a particularly attractive form of Equatorial sundial.

 

The Earth rotates on its axis through 360 degrees once every 24 hours, which means that in a single hour the Earth rotates through 15 degrees. Each 15 degrees subtended on the equatorial scale therefore represents 1 hour’s rotation of the Earth on its axis. Since the hour divisions of an Equatorial sundial are equally spaced around the dialface regardless of the latitude of its location, the sundial can be adjusted to suit a range of different latitudes simply by altering the angle of the dialplate with respect to the horizontal plane so that the gnomon points towards the celestial poles (i.e. by tilting the gnomon to the correct latitude angle.) 

equatorial plate sundials

Shadows cast on an Equatorial Plate sundial

In our Southern hemisphere, the Sun moves South of the Celestial Equator during the summer half of the year and North of the Celestial Equator during the winter half of the year. The dial plate of an equatorial sundial lies precisely in the plane of the equator, which means that the summer shadow falls on only the uppermost face of the equatorial plate sundial while the winter shadow falls on only the underneath face.

 

Hour markings must therefore be made on both upper and lower surfaces when the dial is in the form of a solid flat plate with a rod gnomon such as that shown above. The Equatorial Plate sundial at Whyalla is of this type.

 

(Better photos to follow soon)

equatorial plate sundial summer

Upper face shows the time in summer

equatorial plate sundial winter

Lower face shows the time in winter

(no shadow in summer, when this photo was taken)

Equatorial Plate sundial in the foreshore park at Whyalla, South Australia

 An Equatorial Ring Sundial has the hour markings inscribed on the inside of a ring or partial ring rather than on the surfaces of a solid plate. At equinox when the sun travels precisely along the plane of the equator, and for a few days on either side of the equinoxes (21st March and 23rd September), the shadow cast by the top half of the equatorial ring totally obscures the sundial markings so that this type of sundial cannot tell the time near equinox.

This occurs with many armillary sphere sundials. However, simply tapering or even removing the top half of the ring solves this problem and the equatorial sundial then becomes particularly useful all through the year, and at all locations on Earth. The sundials in Botanic Gardens at Mount Tomah and Wollongong are such examples.

Mount Tomah Equatorial sundial

Equatorial sundial at the Blue Mountains Botanic Garden Mount Tomah

Wollongong equatorial sundial

Equatorial sundial at the Wollongong Botanic Garden

Please Contact us should you be interested in commissioning an Equatorial sundial