Astrocompass
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An astrocompass is a navigational tool for determining the direction of true
There are certain circumstances when magnetic
Principle of use
The Earth's axis of rotation remains effectively 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 onto 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 and 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 became useful only after the invention of the
Operation
The operation is as follows:
- Getting the local time (by means of a clock)
- Setting the latitude
- Setting (in the local hour angle(LHA) for that day obtained by means of tables or a computer program
- Pointing the compass pinnula to the star (sun or moon)
- Reading the true course on the limb
See also
- Compass direction using a watch
- Grid compass
- Solar compass
References
- ^ Barnfield, Malcolm (June 2011). "The Sundial Goes to War" (PDF). The BSS Bulletin. United Kingdom: British Sundial Society. Retrieved 28 December 2023.
- ^ "Astrocompass". How It Works, Part 6. United Kingdom: Marshall Cavendish Ltd. 1974.