## 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.

[GIF,
6k]

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.

[GIF,
8k]

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 **G**reenwich **M**ean
**T**ime.

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?