Advice For Making a Sundial

Thibaud replies to the girls...

Dear Thibaud,

Our names are Cate, Stephanie, and Melissa. We are trying to find out some information about sundials. Here it goes!!

A sundial is the oldest product with a measurement of time. The sundial moves from one side to another just like the sun moves from east to west.

How can we make a sundial with 24 hours? Help us!!!!!!!!!!

Stephanie, Melissa and Cate

Hi Cate, Stephanie and Melissa,

So you want to make a sundial for Mc Murdo station. I guess the first thing you did in a project like this is to check the local library about all the literature available abou the subject. So you found that the sundial principle is indeed very old. But one thing you wrote is not completely correct: the sundial is not moving, the shadow is moving over the sundialface. Because the maker of the sundial put put several marks on the sundial face you can read the time; just like a watch! You can make a sundial for any surface that gets direct sunlight. But you never saw any surface, horizontal or vertical, that got direct sunlight during the full 24 hours of the day.

So what is the trick to make a 24 hour sundial at Mc Murdo station? Imagine the earth with its rotation axis vertical, and the sun moving around it in a horizontal plane, (Actually the earth is revolving around the sun, but that doesn't make any difference in this example) you can try this with a ball and a lightbulb. If you are on the equator of the earth you will see the sun during half of the revolving time, the other half you are in the shadow. So there you get only half the time direct sunlight. But if you are standing on the top or the bottom of the earth, i.e. you are standing on the North- or South pole, you can always see the sun, you are never in the shadow! Perhaps you understand now what a midsummernightsun is? Well, you know how many hours it takes for the sun to make a full circle around, so you might be able to 'predict' at what time the sun will be in which direction if you are standing at the bottom of the earth: the South pole. But be careful! In which direction is the sun moving? Perhaps a little model will help.

The reality is however that the earth axis is slightly tilted compared with the plane in which the sun is moving, so the reality is a bit different than the above described ideal model. Can you find out what the difference will be? Again a model might help.

By the way, how old are you three? So I can imagine a bit what you know and what might be able to be puzzled out by you.

Good luck,
Thibaud Taudin-Chabot

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