#165 |ast08 – Finding your longitude without a GPS

Hi guys,

My English is still so bad, sorry if my sentences doesn’t make sense sometimes! I am so ashamed… Oh well. Anyway, today, I will talk about how to find your longitude coordinates without a GPS and only a precise chronometer (a clock).

Last time, I explained to you what is the correlation between the timezones and the longitude degrees. For the reminder, there are 24 hours timezones, 12h West from the time reference GMT (Greenwich Mean Time meridian) defined by a “minus”, and 12h East defined by a “plus”. The Earth is defined by 360° so when you divide it by 24, you have 15° for each timezone. Let me give you an example, when you are in Las Vegas, you are at -7h (7 hours West from GMT). If you convert it in degrees, you have 7*15°=105°W.

But how to calculate your timezone or precise longitude coordinate?

Well, you need two clocks: the first one will show the GMT. This one will give you the exact hour in Greenwich. The second one, will give you your local time hour, it is the unknown of the equation.

First of all, you need to know the local noon compared to Greenwich noon. Why? If you are in Vegas the sun position will be different when it will be noon in Greenwich. Timezone will differ, and if you want to know the exact coordinates, you need to know this difference by taking the time difference compared to your local noon.

How? The method is simple but can take some time. You need a stick and something to mark the stick shadow depending on the sun position through the sky. When you take the position of the the shadows, don’t forget to check and write the GMT time at noon and on each marks. When you have done, check the shortest shadow on your paper. The shortest shadow of the day gives you your local noon, and if you check your GMT clock, it will give you the difference between your local time (12h) and the GMT when you are at noon in your position (19h). There is a difference of 7h but with a precise clock, you will be able to have the minutes and seconds. You will be able to convert the HMS to degrees coordinates with simple mathematics.

However, this method isn’t used actually because the Earth rotation is slowing down each centuries, and sun position isn’t the same depending on the day, month and year. That’s why the navigators and explorers use tables and schema to correct the coordinates depending on the Day, Month and Year. These data are known as the Nautica Almanach and the Analemma, they are used for latitude and longitude correction.

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