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encyclopedia
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.
