circumference QPnQPs represents a meridian.

You measure latitude from a standard great

QQ´ represents the equator, whose plane passes

circle (the equator). You also use a standard great

through the axis of rotation. Let M be some

circle when measuring longitude. This great

position north of the equator on a meridian. The

circle is the meridian. The standard meridian is

number of degrees in arc QM is the measure of

the *prime meridian. *By international agreement

angle QOM. If arc QM is 30°, then angle QOM

in 1884, the meridian adopted as the prime merid-

is 30°. Thus, you measure a central angle by

ian was the one on which Greenwich Observatory

measuring its subtended arc.

(near London, England) was located. This is was

Let MM´ be the plane of a small circle parallel

the 0° longitude.

to QQ´, the equator. Then arc QM measures the

Figure 7-6 represents the earth. QQ´ is the

distance of any point on MM´ from the equator.

equator, with its plane passing through the center,

You can describe the whole parallel MM´ by

O. G is the position of Greenwich, and M is a

saying that it is 30° north of the equator.

position in north latitude west of Greenwich.

Similarly, you can say any point on NN´ is 45° south

PnM´ and PnG´ are portions of the meridians

of the equator. The angular distance of a

through M and G intersecting the equator at M´

position north or south of the equator is the

and G´. The longitude of M includes

position's latitude. You measure latitude north-

ward or southward through 90° and label it N or

1. the arc of the equator G´M´,

S to show the direction of measurement. You

2. the angle GPnM formed by the meridional

express latitude in terms of the angle at the center

planes through G and M, and

(see angle QOM in figure 7-5). Latitude, then, is

3. the angle G´OM´ formed at the center

the north-south geographical coordinate.

between G´ and M´.

The east-west geographical coordinate is

longitude. You can define longitude in three

You should recognize that each of these expres-

ways:

sions measures the same coordinate--longitude.

The longitude of a position is also described

1. as an arc of the equator or a parallel,

as being east or west of Greenwich. In figure 7-6,

2. as the angle at the pole or the angle at the

the longitude of M is west; the longitude of N is

center between the planes of the prime meridian,

east. You can see that longitude east or west

and

cannot exceed 180°.

3. as the meridian of a point on earth. You

You can subdivide the degree into smaller

measure this point eastward or westward from the

units, as in the decimal system. However, the

prime meridian through 180°. Label it E or W

more common method of subdivision is to divide

to show the direction of measurement.

each degree into 60 minutes (´) of 60 seconds (´´)

each. Another method is to divide the degree into

60 minutes and tenths of minutes.