Types of sundials
Sundials are classified into a number of different types, mainly by the plane in which the dial lies, as follows:
This is the type found commonly on pedestals in gardens. The dial plate is horizontal. The gnomon (which casts the shadow) makes an angle e the msallqual to the latitude of the location for which it was designed (which is not necessarily the location now,
The pictures below show (left) the large horizontal dial on top of Toweer Hill underground station in London, (top right)the 42 cm.Spot-On stainless steel sundial on the sundial trail at the Horniman Museum in Forest Hill, south-east London, and (bottom right) the floral sundial at Easton Lodge in Essex, England
This is the type found on the walls of churches and other buildings. Vertical sundials may bedirect south dials if they face due south (in which case the gnomon will be at an angle equal to the co-latitude of the place, and the hour lines, if delineated for local time at the place, will be symmetrical about the vertical noon line).
If they do not face directly south, they are described as declining dials, and in this case the gnomon will be at a lesser angle than the co- latitude, and the hour lines will generally be grouped more tightly in the morning hours, for south-east decliners and, conversely, grouped in the afternoon hours for south-west decliners
Though much less common, there are dials with dial plates which are neither vertical nor horizontal. These are called reclining dials
Pictures: left: the laarge horisontal dial at Tower Hill Station, London on the Thames sundial trail
have the dial plate fixed in the plane of the equator. The gnomon is perpendicular to the dial plate. The hour lines are spaced equally at 15 degree intervals. The armillary sphere is a development of this idea, and consists of a series of rings in the planes of the equator and the meridian, and a rod parallel to the earth's axis and passing through the center of the rings.
The pictures show (top left) the Savoy sundial in Embankment Gardens, Westminster, London, (top right) the equatorial sundial designed by Wendy Hiller outside the Tower Hotel in London, bottom left) the very laarge equatorial sundial in Raglan Street, Waterloo, Sydney, Australia, and (bottom right) the equatorial dial in Richmond, South Island, New Zealand.
have the dial plate fixed parallel with the earths axis. The gnomon is parallel to the dial plate, typically the edge of a rectangular plate fixed to the dial plate. The hour lines are parallel to the gnomon and thus to each other.
The pictures are: left) the Tylers and Bricklayers Millennium sundialdesigned by Piers Nicholson on the banks of the Thames neare St. Pauls Cathedral in London, England and (right) the polar sundial in the Bat Galim district of the port of Haifa, Israel
are not very common. They are unusual because the gnomon is vertical, and the hours are marked not by lines but by points falling on the circumference of an ellipse. The gnomon has to be moved depending on the time of year, so that the shadow falls on the correct point. Analemmatic dials are particularly suitable for sundials laid out on lawns, where a person can act as a gnomon; the position where the person should stand at any given month of the year is marked out along the north-south axis which crosses the mid-point between the foci of the ellipse.
The picture (right) shows the analematic dial designed by Frank King in Palace Yard opposite the Houses of Parliament in central London.
Reflected ceiling dials
are even less common. They are a special form of horizontal sundial, in which a mirror laid on a south-facing windowsill reflects the sun onto the ceiling. The hour lines are drawn on the ceiling.
come in many varieties, such as the shepherd's dial, the tablet dial, the ring dial and others. They are not strictly a separate type of dial, but can be of the types listed above.