## How to define a sundial

A sundial is an object that uses the sun to cast a shadow, which tells us what time of day it is. The earliest and simplest form of sundial is the shadow stick. The time of day is judged by the length and position of the stick’s shadow. As the sun moves through the sky from sunrise to sunset, the shadow from the stick rotates clockwise. When the shadow is shortest the time of day is always noon, midday.

## How sundials work

As the earth turns on its axis, the sun appears to move across our sky. The shadows cast by the sun move in a clockwise direction. If the sun rose and set at the same time and spot on the horizon every day, sundials would be fairly accurate clocks. However, the sun’s path through the sky changes every day because the earth’s axis is tilted. As earth orbits around the sun we are tilted one half of the year towards the sun, which is our summer months and the other half away from the sun, which of course is our winter. This is why it is hotter in the summer than the winter. This means the shadows cast by the sun change from day to day. In addition to this there is another problem, the earth’s surface is curved and the ground the sundial stick is on is not. This means that the shadow from the sun varies in length at different times of the day. If you mark the shadow at sunrise and sunset, you cannot evenly divide the space between for each hour. There are different ways to overcome this problem. One is to build a stand at an angle for the sundial stick to be placed on. This angle must be the same tilt as the earth is with the sun. This tilt is the earth’s axis. The hour marks can then be drawn out correctly giving you a simple working sundial.

## Who used sundials?

The sundials originated from a region called Mesopotamia, which is now known as Iraq. They used vertical rods on their buildings as shadow casting devices for telling the time and date, and were the first people to divide the day into 24 hours, the week into 7 days and the year into 12 months. From these beginnings, methods for designing and constructing various types of sundial have been developed by many different cultures right through to the present day. The Greeks and Romans in particular created a great variety of sundial types and used them throughout their empires. The first knowledge of sundials was back in 340BC where an astronomer-priest living in Egypt developed a sundial known as the Hemisphericum, in which a vertical post was placed centrally inside a hollowed out hemisphere. The inside surface of the hemisphere had vertical lines carved on it to divide the daylight period into 12 hours, and horizontal lines to show the seasons. The shadow cast on the inside surface of the post marked out the path of the sun as it travelled across the sky. The Greeks developed the sundial further and experimented with conical dials, cylindrical dials and flat dials set at different angles. From the Hemisphericum design another sundial was developed this was called the Hemicyclium shown below. This was used throughout the civilised world right up until the 14th century.

The Romans are not believed to have developed any new sundial types, but they certainly used all the Greek sundial developments and sundials were extremely popular throughout their empire.

## Are sundials useful?

Sundials were the only way people could tell what time of day it was. Every town and city had one. It was more important to the civilians who lived in these cities and every civilian started to own a sundial. Sundials started to appear in every conceivable shape, size and form. The most commonly used materials for these sundials were copper, brass, bronze and stone. During the 16th-19th century pocket dials were common.

## Are they still useful today?

By the 16th century sundials were very common amongst towns and villages but during this era the invention of mechanical clocks, which divided the day into 24 equal hours started to appear. The early clocks were rare and very expensive, and not very accurate. They were often wrong by several hours and were frequently calibrated using sundials. In America, sundials were an important method of keeping time until the first half of the 1800s, when the first truly affordable clocks were produced. Believe it or not the heyday of the railroad had much to do with rendering the sundial obsolete-schedules and standardised timekeeping became of primary importance for the first time. Across most of Europe in the 19th century, timekeeping, using sundials was starting to decline. The pocket watch was now affordable and accurate and as time went by the sundial started to become obsolete. In our lives today sundials are used for decorative purposes. They are found at gift shops and nurseries everywhere and they make delightful ornaments for gardens. In today’s times sundials would not be as accurate as watches and clocks and sundials are only used during sunny weather. It would be time consuming using sundials and they probably would be awkward to set up and use compared to watches.

## When to use a sundial

This is by far the most important point with sundials. For them to work you must need the SUN, without the sun no shadow will fall. Also at night the sundial will be useless. If you live in an unstable climate like ours, you cannot guarantee when the sun comes out but in hot countries this wouldn’t be a problem.