Garden Sundials

Backyard garden sundials are are not only unique conversation peices, they're also functional. Provided a sundial is installed properly (and this is the key factor), it can keep quite accurate time.

Types of garden sundials

At its simplest a sundial consists of a dial plate -- marked off in hours -- and a gnomon -- a blade or post in the center of the dial plate that casts a shadow on the dial plate. Within this form there are several variations.

Horizontal
This is the type most people think of when they hear "sundial". The dial plate is horizontal and the gnomon projects upright from the dial plate.

Vertical
The vertical types, hung on walls, are commonly "direct south dials", meaning they face dew south. In a direct south dial the gnomon will be at an angle equal to the co-latitude of the location, and the hour lines, if delineated for local time on location, will be symmetrical near the vertical noon line.

Equatorial sundials
This version has its the dial plate fixed in the plane of the equator, with the gnomon perpendicular to the dial plate.

Polar discs
These 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.

Analemmatic dials
This unusual type consists of a verticle gnomon and hours 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. The correct position of the gnomen for any given month of the year is marked out along the north-south axis which crosses the center-point between the foci of the ellipse.

Reflected ceiling dials
A 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.

Garden sundial designs

Some dials are designed to sit on pedestal, while others can be placed directly on a table. The gnomon is often part of a small statue such as the wings dragonfly pictured on the dial. In addition to hour lines, the dial plate of may contain a design such as a frog or leaves, and many with carry an inscription around the dial plate. Brass is one of the most popular materials for sundials, but other materials are also used.

Setting up a sundial

In order to tell the correct local time the gnomon must be parallel with the earths axis, or, in other words, that it should point towards the celestial pole. In the northern hemisphere, this means that the gnomon should point at the Pole Star. If sundial is wasn't made for the place at which it is to be set up it will have to be corrected. For more on calibrating a sundial: http://adelie.harvard.edu/ed/Activities/Sundial.html