The Equation of Time:

If we use only a Sundial to measure time, the Sun keeps perfect time, wherever we are. But the moment we use any other sort of clock, we find there are usually differences between clock time and sundial time. Why? The most obvious difference between sundial time and the time shown by a modern clock is in the length of the hours due to the seasonal variation in the length of the day.

When you are setting up a sundial, you must also align it correctly with the Earth's axis for it to tell accurate time more frequently. Since the Sundial Bridge at Turtle Bay is a bridge that was made to span a narrow portion of the Sacramento River, it could not be aligned at the correct degrees.
Because of this, and because the angle of the style must be exactly equal to the latitude for which the dial is designed, there is only one day a year when the Sundial Bridge at Turtle Bay accuratley tells time--June 21, Summer Solstice!

The essential parts of a sundial:

In its simplest form a sundial really only consists of two parts. One part casts a shadow, and the other part is a surface upon which the shadow falls. To gain precision and accuracy, the two essential parts of the sundial have been refined over the centuries. The part of the horizontal dial that casts the shadow is called the gnomon, while the part that receives the shadow is called the dial plate or face.

The Gnomon:

The gnomon is the part of the dial that extends into a vertical plane above the horizontal face of the dial. Gnomons for horizontal dials can be of many different shapes, from a simple rod to an intricately scrolled work of art. The most common mental image of a gnomon is that of a triangular piece that extends above the dial plate. The inclined or sloping edge of the gnomon is called the style. The style is the edge that cast the shadow onto the dial plate and is used to indicate the time. The angle the style makes with the face of the dial must be exactly equal to the latitude for which the dial is designed.

The Dial Plate:

The dial plate is the flat, horizontal surface that receives the shadow of the style. The plate is marked with lines that the shadows move across to indicate time of day.

Friends stroll on a path at the north side of the bridge. The tile-covered garden border in the right-hand side of the photo acts as a dial plate for the Sundial Bridge in Redding. (for the photo with people walking)

The dial plate at Sundial Bridge features hour markers from 10:00 a.m. to 3:00 p.m. with plaques like the one pictured here. The shadow of the Sundial Bridge gnomon moves across its dial plate at approximately 1 foot per minute! You can literally watch the shadow move across the dial plate.