A sundial uses the sun’s position to measure time, by casting a shadow on a flat surface that is usually marked with lines that indicate the hours of the day. The shadow is cast by the gnomon, also called the sundial’s style, which is either a thin rod, or a sharp, straight edge.

As the sun moves across the sky, the shadow of the gnomon’s edge aligns progressively with the different hour lines that are etched on the sundial’s plate or on the ground. You can see this is more common in sundials such as the Horizontal sundial, where the style’s edge is aligned to the Earth's axis of rotation. To tell the correct time, the style of the gnomon should point towards true north.

There are several kinds of sundials; these are classified based on the plane in which the dial lies, such as horizontal dials, vertical dials, polar dials, equatorial dials, portable dials, and reflected ceiling dials.

Horizontal Sundials

The typical horizontal sundial is commonly found on garden pedestals. This kind of dial has a horizontal dial plate on which lines are marked to indicate hours of the day. The shadow is cast on the ground by the gnomon, which creates an angle that matches the angle made by that location’s latitude. Horizontal dials are designed specifically for specific locations.

Vertical Sundials

You’ll find vertical sundials on church walls and the walls of other significant buildings. Vertical sundials that face due south form a shadow at an angle is equal to the location’s co-latitude. The hour lines will be symmetrical around the vertical noon line, if they have been delineated for the local time at the location. Such vertical dials are also known as south dials.

If the dial does not face direct south, you can call these vertical dials as declining dials. In this case, the gnomon will form a lesser angle than the angle of the location’s co-latitude. Also, the hour lines will be grouped in a tighter fashion during the morning hours for southeast facing decliners. This grouping changes in the afternoon hours for south-west decliner dials.

Equatorial Sundials

The equatorial dial, also called the equinoctial dial, has a planar surface that is exactly perpendicular to the style of the gnomon. The shadow falls on this planar surface. Since the plane on which the shadow falls is parallel to the Earth’s equator, this kind of dial is called equatorial. The hour lines are spaced apart at a uniform 15 degrees, which makes the equatorial sundial easy to construct. To tell the time of the year, equatorial sundials are attached to a nodus, which might even be the ends of the gnomon, whose shadow moves on a circle on the equatorial plane. This shadow indicates the time of the year based on the radius of the circle that measures the sun’s declination.

Analemmatic Sundials

These sundials have vertical gnomons, where points rather than lines mark the hours that fall on an elliptical circumference. One has to move the gnomon depending on the time of year, to allow the shadow to fall on the correct point. Most lawn sundials are of the Analemmatic type, with a human being acting as the gnomon. The position where one stands is marked out along the north-south axis. This axis crosses the mid-point between the foci of the ellipse, thereby allowing us to know the time of the year.

Reflected Ceiling Sundials

Reflected ceiling sundials are a special, rather uncommon form of horizontal sundials. In this type of sundial, a mirror is laid on a south-facing windowsill to reflect the on the ceiling, where hour lines are drawn. Sir Isaac Newton developed this type of sundial. The mirror acts as a nodus and casts a spot of light on the ceiling, which follows a conic section depending on the latitude and time of year. This type of sundial has been known to be very accurate and was quite popular in its day.