Figure 4-6.--Free gyroscope at the equator viewed from space.

Figure 4-6.--Free gyroscope at the equator viewed from

Figure 4-7.--Free gyroscope at the equator viewed from the

space.

earth's surface.

to as the horizontal earth rate effect. If the gyro

and partly about the vertical axis. The horizontal earth

were started with its axle vertical at one of the

rate causes the gyro to tilt, whereas the vertical earth rate

earth's poles, it would remain in that position and

causes it to turn in azimuth with respect to the earth. The

produce no apparent rotation around its horizontal

magnitude of rotation depends on the latitude of the

axis. Figure 4-7 illustrates the effect of apparent

gyro.

rotation at the equator, as seen over a 24-hour

Apparent rotation is illustrated by placing a

period.

spinning gyroscope with its axle on the meridian

Now assume that the spinning gyroscope, with

its spinning axis horizontal, is moved to the North

Pole (fig. 4-8). To an observer on the earth's surface,

the gyroscope appears to rotate about its vertical

axis. To an observer in space, the gyroscope axle

appears to remain fixed, and the earth appears to

rotate under it. This apparent rotation about the

vertical axis is referred to as vertical earth rate

effect. It is maximum at the poles and zero at the

equtator.

When the gyroscope axle is placed parallel to

the earth's axis at any location on the earth's surface,

the apparent rotation is about the axle of the

gyroscope and cannot be observed. At any point

between the equator and either pole, a gyroscope

whose spinning axis is not parallel to the earth's

spinning axis has an apparent rotation that is a

combination of horizontal earth rate and vertical earth

rate.

The combined earth rate effects at this point make

Figure 4-8.--Apparent rotation of a gyoscope at the North

the gyro appear to rotate partly about the horizontal axis

Pole.

4-5