Instructions
Inspect the plate on the sundial. If it has a horizontal plate and lines marking the hours of the day, it is a horizontal sundial. The gnomon, or dial, blocks the sun on a portion of the sundial and casts a shadow onto the plate. These are the most common sundials used today and are sometimes found in people's gardens.
Observe the positioning of the dial. Some dials are located on the side of significant buildings. These sundials face south and are called "vertical sundials." In the case that these dials don't face south, they are called "declining dials." They are found on the side of buildings such as churches, but aren't common today. These dials cast a shadow on a plate, with the dial located in the middle.
Look at the surface of the plate. If it is flat, or planar, the dial is an "equatorial sundial." The plane on these dials is parallel to the equator. Each hour line is spaced at a 15-degree angle. These sundials are attached to nodes. Equatorial sundials are only found on the equator, and they don't need to be adjusted based on the Earth's axis because the sun is always in the same position in the sky. These dials also use a shadow to cast onto a number on the plate.
View the gnomon. Sundials sometimes have vertical gnomons that casts a shadow on points instead of lines that mark the time. The gnomons are movable to correspond with the time of year. These dials are called "analemmatic sundials." The gnomons are among the most accurate sundials because of their ability to change with the axis of the Earth. Many garden sundials have the ability to move the gnomons.
Notice whether the sundial is on a south-facing windowsill. Look to see if there is a mirror. Inspect the ceiling for hour lines. This is called a "reflected ceiling sundial," which was developed by Sir Isaac Newton. They are most common in significant buildings such as churches, though these dials are very uncommon. More than one series of lines along the building accommodate for the time of year, making them extremely accurate.