The Shadow of Time
J. Rienhardt, 2006
In its simplest of forms a sun clock displays the shadow caused by the sun as it apparently moves across the sky. This motion repeats itself everyday, but not exactly. Upon close observation it can be seen that the shadow does not reach the same length every day and moves more quickly in the winter than in the summer.
All of this is due to the motion of the Earth as it spins and travels around the Sun. This was not understood in very ancient times, but tracing the shadows made by the Sun gave early humans some clues about the workings of the Sun/Earth system.
Time, as we know it, was invented by people and is based on the motion of the Earth. If we had been inhabitants of another planet, our time would have been different. Just one planet away, on Mars, the day is just a little longer (about 40 minutes) but the time Mars takes to orbit the Sun once is 320 days and 18 hours longer than Earth. A year on Mars is almost twice as long as on Earth.
It seems important for us to keep track of time. In modern days our lives depend on it. In much earlier times the events of morning, noon and afternoon were sufficient. Before tracking daily time the event and passing of seasons was most important and calendars to keep track of seasons were created. The most famous of these being Stonehenge. Certainly smaller versions of calendar time tracking were also invented.
About 5000 to 6000 years ago civilizations in the Middle East and North Africa found it necessary to begin keeping more finely divided time than days. Days were divided into parts – morning and afternoon and of course, night. A very simple arrangement of a shadow casting device easily divides the day. Eventually marks were placed in the path of the shadow to further divide the day into hours.
The first evidence of 12 hour days comes from Egypt around 3500 BC. The Egyptian sundial divided the day into 10 segments of sunlight and two segments of twilight, morning and evening. At first “hours” were not equal lengths of time at every time of year. The hours were shorter in the winter than in the summer.
The Sundial as we know it today was probably invented around 300 BC. Since then many styles and variations have been developed.
Sun time is not local civil time. If you keep track of the time on your sundial you will find that most often the sundial is “wrong”. Actually your clock is wrong. The sundial tells the local time your clock tells a more rigid civil time. The closer you are to the center of your time zone, the more often the sundial will agree with civil time. Why is it different?
As travel and communication became faster it was much more noticeable that the time in one place was different than the time in another place. In 1884, 27 nations met in Washington, DC and agreed on a world wide time zone system. Most of the world’s nations adhere to this time system today.
But, we are interested in sun time. To keep track of sun time we use the sundial. To set your sundial correctly you need to know where north is. You may be surprised to find that north is not the north you are probably thinking of. True North is different than Magnetic North. A compass points to magnetic north. True north is the point of rotation of the Earth at the North Pole.
The Earth spins on an axis as if a rod was running through the planet as an axel. The point where this axel would exit the earth, both north and south, is the true north and south of the Earth. Magnetic north is the magnetic pole which is in a different place. You can imagine the Earth having a giant bar magnet inside that is not aligned with the rotation axis. The magnetic north pole is in northern Canada and is constantly moving. It is currently predicted that the north magnetic pole will be in Siberia in about 50 years!
There are three ways to orient your sundial to true north. Using the compass you can offset north by the current offset angle. Or you can find true north directly using the sun’s shadow. The third way involves the darkness of a clear night and aligning with the North Star.
Start with a simple sundial. Once you understand that, design some elaborate way to track the shadow of time. As many of us would like to do, sundials only count the sunny days and ignore the cloudy ones.
Recommended books on sundials:
Sundials – Their Theory and Construction, Albert E. Waugh, Dover publications
The Great Sundial Cutout Book, Robert Adzema and Mablen Jones, Hawthorn (out of print, but worth finding)
SUNCLOCKS - Paper Sundials to Make and Use, JVT Publications
Additional link: Ancient Calendars from Online University http://www.onlineuniversity.net/resources/ancient-calendars/
(Thanks to Jennifer Jenkins for providing this link)
Connect to Amazon Books
 The word “hour” means moment
 Magnetic Declination information and computation http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp
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