Taking it to the edge: did you know? Navigating on sea and land

Know your latitude and longitude and you know where you are in the world.  But how do you know where you are when you are exploring a previously unknown place on the globe?

A compass, because of the earth's magnetism, will point to the north - not true north however.  The magnetic poles move around and so magnetic variation needs to be taken into account: it is however still true that the compass will indicate north. In the northern hemisphere Polaris, the north pole star, could also be used (at night) to find north. In the southern hemisphere, the Southern Cross, aids in identifying the south, but it is not accurate, because it is not as near to its pole as is Polaris.

Speed will also need to be calculated - on a ship it could be calculated using a logline - thrown overboard at the bow, it was timed how long it took to reach the stern, and knowing the length of the vessel, its speed could be calculated. Later the line was marked at set positions with a series of knots, and the number of knots that had run out was counted off - hence the term 'knots' for speed at sea. On land, explorers could either measure distance travelled very accurately using a surveyor's or Gunter chain (both Charles Sturt and Thomas Mitchell used this method), by using an odometer or 'measuring wheels' where each revolution of the wheel would be counted by a device on the handle, or more simply by experienced estimates of how much ground was covered by each step taken, either by man, or beast.

Latitude, or distance north or south of the Equator, is measured in degrees and each degree is made up of 60 minutes.  On land latitude was determined using a sextant.  An artificial horizon was also frequently used to assist the process - the sun's reflection from a pool of mercury in a dish was observed using a sextant. A theodolite could also be used, but because it is a heavy instrument was not generally used by explorers on land.

Longitude up until the late 18^th century was difficult to determine but once the marine chronometer had been invented by John Harrison, became increasingly easy to calculate - provided the chronometer (or watch) was accurate. A small error in timekeeping could have dire results for later explorers. The prime meridian (O degrees ) today runs through the Greenwich Observatory in England. This was agreed to in 1884: prior to that the prime meridian could be run through Paris, Madrid, Tenerife in the Canary Islands, or some other point depending on the nationality of the explorer. Longitude and time are closely related. Because the earth rotates once every 24 hours, it turns 15 degrees every hour. To calculate longitude you need to know how the time where you are, differs from the time at Greenwich. (Longitude can also be calculated using the method of lunar distances, with accurate tables setting out the Moon's movements compared to the Sun, or some of the very bright stars. These tables are published in the Nautical Almanac.

Further reading: 

Badger, Geoffrey The explorers of Australia East Roseville, N.S.W.: Kangaroo Press, 2001.  Chapter 2, Navigation at sea and on land.

Macinnis, P. Australia's pioneers, heroes & fools: the trials, tribulations and tricks of the trade of Australia's colonial explorers Millers Point, N.S.W.: Pier 9, 2007 Chapter 6: essential instruments and unwanted items

Williams, JED. From sails to satellites Oxford; New York: OUP, 1994.

Sobel, Dava. Longitude New York: Walker, 1995.

Bennett JA. The Divided Circle Oxford: Phaidon-Christies, 1987.