Since time immemorial, sundials have been used to give an indication of time, allowing people to function to a timetable. The ancient Babylonians, Egyptians, Greeks and Mayans were just some of the great civilizations who understood that the position of the sun in the sky, and the shadows it casts, could be used to make an estimate of the time of day.
|Ancient sundial from Marcianopolis, Museum of Mosaicas, Devnya, Bulgaria. (Creative Commons)|
Of course, many of these, such as the Greeks and the Romans, believed that the earth was fixed in the center of the heavens and the sun orbited around it, making it move across the sky. We now know that the movement of the sun across the sky is caused by the rotation of the earth, although this makes little significant difference when it comes to sundials.
In the morning, a shadow points to the west as the sun rises in the east, gradually getting shorter and moving towards the north. After noon, the shadow points eastwards, steadily lengthening as the sun moves towards sunset in the west. A sundial takes advantage of this by using a device called a gnomon to cast a shadow, with a dial and markings allowing you to gauge the time.
A sundial also depends upon your position on the earth and it can only be used at a certain longitude and latitude. You cannot move away from your latitude and, although you can move along it, you will need to add or subtract four minutes for each degree moved, depending upon the direction.
This is the most basic sundial, and depends upon the ancient principle of placing a stick in the ground and using the shadow to tell the time.
|Sundial in thyme garden at Minnesota Landscape Arboretum. (Creative Commons)|
If you monitor your sundial over the course of the year, you will notice that it does not exactly match the time. Why?
Many parts of the world use daylight saving time during the summer, so you will have to recalibrate your sundial twice per year.
For every degree you live away from your local meridian, the time will vary. For every degree east, add four minutes; for every degree west, subtract four minutes. A degree in latitude is about 69 miles or 100 kilometers, which will help you to take this variance into account.
For example, Penzance is approximately 5.5 degrees west of Greenwich. If it is noon at Greenwich, what time is it in Penzance?
To work this out:
If it is 6pm in Los Angeles (118.25 Degrees west), what time is it in New York (74 Degrees west).
|Moot Hall Sundial (Public Domain)|
If you want to move your sundial, you have to be extremely careful when repositioning and ensure that it points north (or south in the southern hemisphere). You cannot use a compass because this points to the magnetic pole, which is in a very different location from true north.
One way is to use the star Polaris to set your sundial at night, which will ensure that it is accurate during the day. Alternatively, buy a map with true north gridlines, allowing you to use landmarks to position your sundial.
Some people may want to make a sundial with a circular dial, where the markers showing the shadows fall are all of the same length. This is very easy but requires a little planning, because you have to set the gnomon at an angle. For this, you could make a sturdy triangle from a drinking straw or use wood – there are many ways to achieve this goal.
Hopefully, this has helped you to build your own sundial, or at least given you a few ideas about using the sun to tell the time. There is, of course, much more to learn, such as the Equation of Time and, because these devices have been around for thousands of years, there is a huge array of types. Some of the links below will help you to explore further:
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