Magnetic compasses must be modified for use in high polar latitudes by reweighting the needle. As the compass gets closer to the South Pole, the south-seeking end of the needle is pulled downward toward the earth and will drag on its enclosure unless the proper nonmagnetic counterweight (copper wire) is added to the north-seeking end.
Field parties must be careful of localized magnetic variations. On Ross Island, for example, magnetic compasses are unusable because there is so much iron in the rock. Likewise, compasses are also affected by the metal in vehicles. Bearings must be taken well away from such disturbing influences. Navigation with a magnetic compass over long distances is difficult because the magnetic variation (the difference between magnetic and true north) is so high, and changes significantly over short distances. Field parties may elect to travel by using a Grid North system, versus a magnetic or true-north system. Using a compass is an accurate way to determine bearings. Using an astrocompass (in conjunction with an artificial horizon) is a good way to fix your position. This method requires an accurate chronometer and extensive knowledge on how to use navigational tables to get good results.
Polar exploration was one of the fields in which the astrocompass saw the most use, for the reasons described above. An astrocompass was the most reliable way to ascertain the direction of true north through the positions of various astronomical bodies.
Principle of use of an astrocompass
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, and a slide ruler, one of its chief disadvantages.
The bubble compass
A bubble compass was used on aircraft during the early pioneering days of flight in Antarctica to measure the altitude of a celestial body above a horizontal line of reference. (“Altitude” in this case is a special use of the word describing an angular measure, not a distance in feet above sea level.) Measurements were taken through a bubble window protruding into the slipstream from the top of the aircraft and this would enable the navigator to determine true north. When an airplane is above the clouds or flying at night, its navigator can’t see the horizon. The bubble sextant solves this problem by providing an artificial horizon using a bubble, but acceleration of the aircraft and turbulence frequently deflect the true vertical; therefore, a single reading may not be accurate so multiple readings are necessary for accuracy.
Both the astrocompass and the bubble compass have seen many years of reliable service here at Mawson guiding our field parties and aircraft to their destinations and safely home again.
Bubble sextant Mk IX – Henry Hughes & Son LTD (Left)
Garmin eTrex10 – Current AAD field issue GPS (Middle)
A.M. Astrocompass Mk II - Sperti Inc (Right)
Slide ruler – Used to calculate complex calculations (Bottom)
"Sperti Inc." Was a company that made products for the US Air force and Navy. One of the products made was the Astrocompass which went into service in 1942. The name plate containing information on what the item was, Its parts and serial number and AM indicating the instrument was made for the "Air Ministry" was stamped onto the chassis of the instrument and highlighted as white lettering.