This pair of large rectangular stone slabs is actually an Analemmatic sundial.

It's a type of altitude sundial, which uses the sun's height above the horizon to tell time. This one is quite interactive.

The user - in this case, you - functions as the sundial's gnomon, by standing on the date plate which is nearest to the actual date. The shadow you cast on the ellipse below, indicates the time!

In the Arab world, the basic principles of sundials were expanded upon and they became a key means of indicating time for prayer, in addition to more general purposes. They discovered that the gnomon needs to be parallel to the Earth's axis. Abu al-Hasan al Marrakushi, a 13th-century Moroccan astronomer, introduced the concept of equal length hours.

The Romans perfected horizontal sundials and portable varieties. While it is impossible to be certain, it is likely that they were the first to make decorative sundials and use them in gardens. Having a decorative sundial in your garden set upon a stone perch remains in style centuries later, bringing order and a focal point to the garden. While the Sundial and Rose Garden's focal point is undeniably the Tercentenary Sundial, there are six additional sundials that decorate the garden's grounds.

Early watches and clocks were not very accurate and had to be set and calibrated using a sundial to ensure the correct time was being kept. The invention of the telegraph led to the decline of sundials. With the telegraph, it became possible to transmit a time signal across a country that could be used to set clocks.

Azimuth Dials use the sun's angle on its daily arc to determine time. An azimuth is an angular measurement in a spherical coordinate system, the angle of the shadow indicates the time.

Horizontal Sundials are the most common. These are often used as decorative features in gardens. The flat horizontal plate enables these sundials to tell time as long as the sun is shining. The gnomon is the vertical part of the sundial that casts the shadow. The hour markers are used to show the time based on where the shadow is.

Vertical Sundials are often found on the walls of buildings of worship. The gnomon is positioned so that it aligns with the Earth's axis of rotation and the plane is positioned vertically. Importantly, the design of these sundials means that the shadow does not move across the plane uniformly. Vertical sundials were used as they could be read at distances, allowing the time to be known easily.

Polar Sundials have their plate parallel with the Earth's axis, in an East-West direction. The gnomon is parallel to the plate, and the hour markings are parallel to the gnomon. The plate is usually rectangular.

Equatorial Sundials are also knowns as equinoctial dials have flat or circular ring plates. The plane of the plate is parallel to the equator and perpendicular to the gnomon, which is parallel to the Earth's axial rotation. The equatorial ring sundial is a sub-type in which the hour markings are within a ring.

Altitude Dials use the sun's altitude to determine time. The sun's altitude is defined as its height above the horizon.

Analemmatic Sundials are also known as vertical center dials. The hour marks are positioned in an ellipse. An interesting feature of these sundials is the gnomon is not fixed but is required to change to determine time. The moveable gnomon is vertical, and its position depends on the date. To use the sundial, stand on the date marker that is closest to the date and read your shadow on the ellipse markers around the sundial.

Ring Sundials use light to tell time. The ring has hour marks consisting of a small hole through which light shines.

Cylindrical Sundials, also known as pole or pillar sundials, are shaped like a cylinder with a gnomon attached or positioned on a moveable top. The hour marks are inscribed by curves and as the sun reaches different heights the gnomon casts a shadow indicating the time.

Border Sundials. (n.d.). History of sundials. https://www.bordersundials.co.uk/about/history-of-sundials/

Homestratosphere.com. (2021). 8 different types of sundials. https://www.homestratosphere.com/types-of-sundials/

Internetworks Ltd. (2018). Types of sundials. https://www.sundials.co.uk/types