3-12
plane. These gyros are used to establish vertical and horizontal planes to be used where stabilized
reference planes are needed.
In the second group, the gyro's spin axis is either parallel to the surface of the Earth or at some angle
other than perpendicular. The spin axis of the gyro in the gyrocompass, for example, is maintained in a
plane parallel to the surface of the Earth It is aligned in a plane of the north-south meridian. Once set, it
will continue to point north as long as no disturbing force causes it to precess out of the plane of the
meridian.
Effect of Rotation of the Earth
As you have learned, a free gyro maintains its spin axis fixed in space, and not fixed relative to the
Earth's surface. To understand this, imagine yourself in a space ship somewhere out in space and looking
at the South Pole of the Earth. You see a sphere rotating clockwise, with the South Pole in the center.
Maneuver your ship until it is on a direct line with the South Pole and then cut in the automatic controls to
keep it in this position. You will now see the Earth make a complete rotation every 24 hours.
You could keep track of that rotation by driving a big post into the Equator as shown in view A of
figure 3-11. If this post were upright at 1200, the Earth's rotation would carry it around so it would be
pointing to your right at 1800. Likewise, the Earth's rotation would carry the post around so that at 2400 it
would be upside down. Then, at 0600 the next day, the post would be pointing to your left. Finally, at
1200 the next day the post would be back in its original position, having been carried, with the Earth,
through its complete rotation. Notice that the post has many positions as you observe it—because it is
attached to the Earth's surface and does not have rigidity in space.
Figure 3-11A.—Fixed direction in space. Post on the equator viewed from space.
If you put a gyroscope in place of the stake, you will see a different action. Imagine a gyroscope
mounted at the Equator with its spin axis aligned with the E/W axis of the Earth. The gyro is spinning and
has rigidity in space. Now look at view B. At 1200 the spinning axis is horizontal with respect to the
Earth's surface. At 1800 the spinning axis is vertical with respect to the Earth's surface; but the gyro is
still spinning in the same plane as before, and the black end is pointing away from the Earth's surface. At
2400, the spinning axis is again horizontal. At 0600 the spinning axis is again vertical, and the black end
points toward the Earth. Finally, at 1200 the next day, the gyro is in the same position as when it started.
The plane of spin of the gyro wheel did not change direction in space while the gyro rotated with the
Earth. This is because the gyro is rigid in space.
