A sundial brings an element of cultured antiquity to your garden, and contradictorily enough, it's something of a timeless addition. You probably don't actually need it to tell you the time, but even so, it's a more interesting showpiece if it's capable of doing that. Your sundial must be properly oriented to tell time; its operation depends on the incident angle of the sunlight, which changes not only according to time of day but also according to the time of year and latitude.
Finding the Best Spot
It's axiomatic that your sundial must be in a location that receives full sun all day, which means locating it out the shadow of overhead branches and away from shrubs and flowers that cast early-morning or late-afternoon shadows. You can install a horizontal sundial, incorporating it into the paving stones of your patio, or you can mount an armillary sundial on a pedestal. You can also mount a vertical sundial on a tree or post. Whichever you choose, beware of fast-growing grasses that may obstruct the dial face at the height of summer.
Orienting the Sundial
The working apparatus on a sundial -- if it can be called that -- is called the gnomon. It's typically a triangular protrusion extending up from the face of the dial or -- in the case of armillary sundials -- a rod positioned over the face of the dial. Either way, the shadow it casts on the dial face is what tells you the time of day.
The gnomon must be calibrated for the latitude at which the sundial is to be used; a sundial purchased in Rome won't work properly in Wisconsin unless the angle of the gnomon is readjusted. Consequently, it's best to buy your sundial from a local source.
Find True North
Once you have a sundial designed for your latitude, you must orient it so the gnomon points toward true north. You can't find true north with a compass because the compass needle points to magnetic north, which is actually located somewhere in northern Canada -- not at the North Pole. The easiest way to find true north is to wait until noon, then orient the sundial so that the shadow cast by the gnomon falls on the mark representing noon or, if you're on daylight savings time, 1 p.m. You can also wait until night and point the gnomon toward Polaris. A third method doesn't rely on the sundial at all:
Stick a pencil in the ground at about 9 a.m. and note the shadow it casts. Put another pencil in the ground at the end of the shadow.
Measure the distance between the pencils with a tape measure or ruler and draw a circle around the first one, using the distance you measured as the radius.
Wait until the shadow of the first pencil crosses the circle again., This should happen at around 4 p.m. Put a third pencil in the ground to mark this intersection point.
Draw a line between the second and third pencils, and then draw a second line from the original pencil that intersects the first line at an angle of 90 degrees. The second line points toward true north.
Adjust the Dial With the Seasons
Your sundial will gradually fall out of synchronization with clock time because the movement of the sun through the sky is irregular; it varies with the season. In the Northern Hemisphere, it appears to move faster from April through June and to slow down after December. If it's important for you that the sundial keeps accurate time, you should reorient it four times a year on specific dates. The dates are April 15, June 15, Sept. 1 and Dec. 24.