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TwitterThink you are flying from USA to Europe. Do you need to change the time of your clock? Yes you do. Because, time is different in various location of the world. So, like normal clock, does the time of a sundial depends upon the city where you living? Quick answer, yes. More specifically, it is important that on your location in the planet earth. Therefore, if you need tools to guide you when you deal with sundials.
First you must understand that magnetic north is not the same as True North, pointing to the globe at the North Pole. Sundials use true north as a reference point not the magnetic north. So, if the sundial is calibrated to align with magnetic North, it will not give the correct time display.
The difference between magnetic North and True North is that magnetic inclination is different in different places. Therefore, while you buy one, you need to position it with actual geographic north by compensating with the magnetic north of your location.
Also note that the position and length of the shadow cast by sundial gnomon depends not only on time of day, but also the time of the year. Different seasons mean that the position of the sun at a certain time of day is different in the sky and has significant impact on the shadow cast by gnomon.
The shadows cast by the gnomon tell the time of a day, because the shadows change direction corresponding to the times of the day. A sundial uses the position of a shadow to tell time. This is not exactly what you want on your clock that shows the standard time, compared with the solar time that is the time from the sun itself. The two measures are not exactly the same time, of course, if the trend of telling time is in the same direction: with the direction of sun.
Before the invention of the mechanical clock, for example, before 1883, people used primarily sundial. Each neighborhood has maintained their own time that they followed, depending on the sundial, which of course is based on the position of the sun in the sky of that particular neighborhood. Of course, the time may significantly vary in closer neighborhoods; say between two cities located 25 miles to the east or west from each other. But it works for that time until the arrival of the mechanical clock, which is not really important that the time was wiped out in minutes.
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