## Finding Your Exact Longitude

As already described, it is easy to find the the North-South
direction and the Local Noon at your location. Once you have done
this, you may now calculate your accurate geographical longitude. The
crucial point is to measure WHEN the Sun crosses YOUR local meridian,
that is when reaches its highest position and is seen due South.

These data must now be compared with the official data from
Greenwich (further details below).

We will demonstrate how to do this with an **Example:**

Suppose that with a gnomon (or other simple equipment), you find
that the Sun reaches its highest point in your place at, say, 1144
CET, that is, at 1044 GMT (remember that GMT = CES - 1 hour).

On that very same day, the Sun reaches its highest point above
Greenwich at, say, 1150 GMT. Local Noon at your place is therefore
exactly 1 hour 6 minutes (or 1.1 hour) earlier than in Greenwich.

This time difference corresponds to a longitude difference of 1.1
hour x 15°/hour = 16.5°.

In your place, the Sun reached its maximum height before it did in
Greenwich, that is, you must be East of Greenwich. Since the longitude
of Greenwich is 0°, this shows that you are at Eastern
Longitude = 16.5° East.

Thus the trick is simply to know accurately, when the Sun - on the
same day - reaches its maximum elevation above the horizon at your
place, and also in Greenwich. You need these two times to determine
your geographical longitude.

The first value must be measured by yourself. The second value may
be found with a certain accuracy for instance on the web site of the
Nautical
Almanac. Here you have to enter the coordinates of Greenwich (that
is, merely longitude = 0°), and subsequently ask for (rise,
set and) **transit times** (this means when the Sun passes
South).

Another method is by clicking **here**,
whereby you will obtain a full 1-year almanac, as given in the
author's high-school book on Sundial Math and Navigation (see the
front page for this section on *Astronomical Navigation*).

#### Exercise:

On March 21, 1998, a school class somewhere in Europe finds that
Local Noon in their town occurs at 12 hours 13 minutes CET. What is
the longitude of their town? Find some towns on a map of Europe which
are located near this longitude (and which may therefore be where that
school is located).