Extensive Definition
An astrocompass is a navigational tool for
determining the direction of north through the positions of
various astronomical bodies.
There are certain circumstances when magnetic
compasses and gyrocompasses are
unreliable. The most obvious is in polar
regions, where the force exerted on the needle of a magnetic
compass is nearly vertical and gyrocompasses become unstable due to
the rotation of the Earth. Magnetic
compasses are also particularly susceptible to magnetic fields,
such as those produced by the hulls of some metal vehicles or
craft. Before the advent of electronic navigational aids such as
GPS
the most reliable way to ascertain north in such circumstances was
through the use of an astrocompass.
Principle of use
The earth's axis of rotation remains, for all
intents and purposes, stationary throughout the year. Thus, with
knowledge of the current time and geographical position in
the form of latitude
and longitude, which
are set on the instrument using dials, an astrocompass can be
sighted on to any astronomical object with a known position to give
an extremely accurate reading.
In its most basic form, the astrocompass consists
of a base plate marked with the points of the compass, with a
mechanism known as an equatorial drum mounted on it. On this drum
is a set of adjustable sights and
a scale of declination. More advanced
versions may have built-in chronometers
or default settings for bodies such as the Sun.
To use the compass, the base plate is first
levelled with the horizon then pointed roughly to what the user
believes to be north. The equatorial drum is then tilted in
relation to this base according to the local latitude. The sights
are then set using the local hour angle and
the declination of whatever astronomical body is being used. Once
all these settings have been made, the astrocompass is simply
turned until the astronomical body is visible in the sights: it
will then be precisely aligned to the points of the compass.
Because of this procedure, an astrocompass requires its user to be
in possession of a nautical
almanac or similar astronomical tables, one of its chief
disadvantages.
Historical uses
Astrocompasses only became useful following the
invention of the marine
chronometer, without which it is almost useless for navigation.
Even then, they saw only limited use, with first magnetic compasses
and then gyrocompasses being preferred in almost all cases.
Polar
exploration was one of the fields in which the astrocompass saw
the most use, for the reasons described above. They have also been
used throughout history in other climes to check the accuracy of
other forms of compasses: they saw use, for example, in the
North African Campaign of World War
2.
GPS and other similar
forms of electronic
navigation aids mean that the astrocompass is now functionally
obsolete.
See also
References
- 'How it Works, Part 6' Marshall Cavendish Limited 1974