Sea and Space / Navigation / Astronomical

## Rotation of the Earth

There is a simple mathematical relation between Local Noon and your longitude. We will work with this in some detail now. First of all, here is a picture of our planet Earth, as seen from a point high above the North Pole.

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As you know, the Earth performs one full rotation (360°) every 24 hours. Thus, it rotates 15° every hour.

In other words, Local Noon at two geographical locations which are separated by a longitudinal difference of 15° occurs with a time difference of 1 hour.

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On this globe, we have marked the longitudes at intervals of 30°.

All points along the green "longitude parallel" will have Local Noon at the same time. On the figure, we have marked a situation where the Sun is at the meridian in Greenwich. In this case, the time at Greenwich will be approximately 1200 hours - or "1200 GMT" that is, 12 hours Greenwich Mean Time.

However, at the longitude parallel which is situted 30° to the East, Local Noon will have appeared 2 hours before, that is at 1000 GMT. And, similarly, at the longitude parallel that is 30° to the West of Greenwich, Local Noon will happen 2 hours later, that is, at 1400 GMT.

#### Exercise:

Imagine that you are the captain of a small sailing vessel. Most unfortunately, your ship has stranded on a reef in the Caribbean Sea, due to the low tide. You know that your position is approximately longitude 82° West, latitude 23° North.

The native sailors tell you that high tide will return during Local Noon. On this very day, the Local Noon in Greenwich (at longitude 0°) takes place at 1145 GMT. At what time (GMT) may you expect to escape from the reef?

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