Astrocompass
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 compass
es and gyrocompass
es 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.
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.
, 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 anywhere except for areas very close to the poles where GPS coverage is not available and there are no current electronic navigation aids.
North
North is a noun, adjective, or adverb indicating direction or geography.North is one of the four cardinal directions or compass points. It is the opposite of south and is perpendicular to east and west.By convention, the top side of a map is north....
through the positions of various astronomical bodies.
There are certain circumstances when magnetic compass
Compass
A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...
es and gyrocompass
Gyrocompass
A gyrocompass is a type of non-magnetic compass which bases on a fast-spinning disc and rotation of our planet to automatically find geographical direction...
es 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
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
. 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
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...
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 timeTime
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
and geographical position in the form of latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
and longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
, 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
Sight (device)
A sight is a device used to assist aligning or aim weapons, surveying instruments, or other items by eye. Sights can be a simple set or system of markers that have to be aligned together as well as aligned with the target...
and a scale of declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
. More advanced versions may have built-in chronometers
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...
or default settings for bodies such as the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
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
Hour angle
In astronomy and celestial navigation, the hour angle is one of the coordinates used in the equatorial coordinate system to give the position of a point on the celestial sphere....
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
Nautical almanac
A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea...
or similar astronomical tables, one of its chief disadvantages.
Historical uses
Astrocompasses only became useful following the invention of the marine chronometerMarine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...
, 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
Polar exploration
Polar exploration may refer to:* Antarctic exploration* Arctic exploration-See also:* Heroic Age of Antarctic Exploration* List of Antarctic expeditions* List of Arctic expeditions* List of polar explorers* List of Russian explorers...
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
North African campaign
During the Second World War, the North African Campaign took place in North Africa from 10 June 1940 to 13 May 1943. It included campaigns fought in the Libyan and Egyptian deserts and in Morocco and Algeria and Tunisia .The campaign was fought between the Allies and Axis powers, many of whom had...
of World War 2
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
.
GPS and other similar forms of electronic navigation
Electronic navigation
Electronic navigation may refer to:*Global navigation satellite system, satellite navigation systems*Radio navigation, the application of radio frequencies to determining a position...
aids mean that the astrocompass is now functionally obsolete anywhere except for areas very close to the poles where GPS coverage is not available and there are no current electronic navigation aids.
Operation
The operation is as follows:- Getting the local time (by means of a clock)
- Setting the latitude
- Setting (in the alidade) the star's local hour angle (LHA) for that day obtained by means of tables or a computer program
- Pointing the compass pinnula to the star (sunSunThe Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
or moonMoonThe Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
) - Reading the true course on the limb