The Weather Store is proud to offer a selection of fine sundials for use in your garden or yard. The sundial is the oldest known device for the measurement of time. They have been in existence for as long as the need for segmenting the day. Flat dials, Armillary Spheres, and portable compass sundials are examples of its many innumerable shapes. Yet the principle of all these time pieces remains the same. An object called a gnomon casts a shadow on a number scale to show the hour of day.