In its daily run through the sky, the Sun offers a great way to know the time. Since it always moves at the same speed it can be used as a clock. How it works?

First of all, to set everyone to agree wherever he lives on Earth, a reference time has been defined. It has been decided to use universal time or T. It corresponds to the time given in England when the Sun is above a line drawn on the ground in the town of Greenwich; It is called the Greenwich Meridian. It is the official time that is used worldwide to synchronize clocks.

When it says 12:00 am, it means it’s 12 or 12 in Greenwich, England. There is a time difference of -9 hours with California (they are behind Greenwich) and +8 hours with Sydney in Australia (they are ahead of Greenwich). However, it is also necessary to change the time in winter and summer. Now, you can fix appointments without difficulty to a person anywhere in the world using universal time without risk of deceiving you.

But here or elsewhere you can calculate yourself the exact time given by the Sun without knowing the Greenwich time or the jet lag. Simply draw a circle on the ground about 30 cm in diameter that you will divide into 24 parts separated by 15°.

A gnomon is the ancestor of the sundial, clocks and watches.

Then plant a large stick straight in the center. Looking at where the shadow of the stick is projected you will know the solar time. This very simple device is known since antiquity and is called a gnomon. There is also another version in which the sunlight passes through a small slit and in which the time is indicated by the position of the light ray.

You can also make a sundial that is based on the same principle. Takes a small wooden board of 30 cm of side and traces on it a big circle. Divide it into 24 parts by making small marks on the edge with a felt. Trace also 2 perpendicular axes passing through the center on which you indicate the 4 cardinal points, N, S, E, O.

If you live in the northern hemisphere, orient one side to the north. Place a piece of wood behind the plate to raise the south side to the latitude of the place, about 50° for Brussels, 48° for Paris, 46° for Geneva, 45° for Montreal and 32° for Marakech.

Then place it on a table that is well exposed to the sun or on the floor. If you live in the Northern Hemisphere, under each mark from the south and turning clockwise will indicate the hours between 1 and 24 (or twice between 1-12). In practice, of course, only the hours between 6am and about 9pm will see the sun. Now your sundial is ready to work. What’s going to happen?

Throughout the day you will see that in the morning the shadow of the stick projects a shadow directed towards the west and which gradually moves northwards and then towards the east. You just have to read the mark on the circle to know the solar time. If you want to know the legal time you will have to add two hours in summer or one hour in winter. So you will know the time without using your watch! For the anecdote if you realize a dial 3m radius as to the left you will be able to place yourself in the center and play the role of the stick. Your shadow will indicate the time. This is called amusing physics!

Thanks to the solar time or universal time you will have no problem with the time zones and you will be able to fix your appointments with anyone, wherever it lives on Earth, without paying attention if it is day Or night at your friend’s house. It’s a great invention!

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