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Solving the longitude puzzle

On the open seas, sailors use the sky to pinpoint their latitude (how far north or south they are) and longitude (how far east or west they are).

Navigators in the 1700s could work out their latitude by measuring the height of a heavenly body above the horizon — the sun by day and the stars at night. Longitude was much harder to establish. Most sailors guessed their longitude using 'dead reckoning', which involved estimating how fast and how long they had travelled since their last known position. These estimates were often wrong and ships were frequently wrecked or ran out of supplies before reaching port.

A brick building seen from the front. It is symmetrical in design, with a middle front door and a window either side. The left and right sides of the building are separated by a space which is open to the outside via a raised roof centre section, front wall opening and open front door. The interior of the centre space is brightly lit. In the image foreground is a cobblestone street surface with a large white strip on the ground that runs directly at the centre of the building. The light from inside the building reflects off the cobblestones. In the left background is a two-storey domed building. Above both buildings the sky is a deep blue colour, suggesting that the image was taken either very late or very early in the day.

In 1714 the British Government offered a large cash prize for a simple and practical way of establishing a ship's exact longitude. British voyages into the Pacific during the 1700s tested many of the methods proposed — including lunar observations and the marine chronometer.

Right: British navigators in the 1700s used the Prime Meridian line at the Royal Observatory in Greenwich, England, as the zero longitude. Courtesy: National Maritime Museum, London.

What are latitude and longitude?

Any position on earth can be pinpointed using a north–south coordinate (latitude), measured in degrees, minutes and seconds from the Equator, and an east–west coordinate (longitude), measured from an agreed starting point at Greenwich, England (Prime Meridian). The National Museum of Australia's location, for example, is at 35°17'35"South and 149°07'15"East.

There is a direct connection between longitude and time. The earth rotates through 360 degrees each day, or 15 degrees an hour at the Equator. If a navigator knows the time in a fixed location, and the exact time where he or she is, the difference in time between them would equal his or her longitude in degrees.

Diagrams of the globe showing lines of latitude and longitude

This diagram of the globe shows the lines of longitude and latitude.

Navigation today

Today, there is a shift occurring in navigation and mapping as significant as that seen in Cook's time. The sextant, almanac, chronometer and calculations used by mariners for centuries are being replaced by radio navigation or global navigation satellite systems (such as the Global Positioning Systems [GPS]), which use microwave signals transmitted by satellites to establish exact locations on earth.

 A Global Positioning System satellite. It has an eight-sided centre section attached to two outer sections each made up of four large flat panels. These outer sections are taller than they are wider. The panels have many smaller sections in them and have highly reflective surfaces. The centre section surface is covered in a layer of thin gold-coloured material. There is a circular section in the bottom half of the covering. Inside this circle are twelve smaller circles. The entire satellite is located in a building interior; there is a large black area behind the satellite, which suggests a curtain or dark-coloured wall for display purposes.

A Global Positioning System (GPS) satellite on public display at the San Diego Aerospace Museum. At least 24 of these satellites orbit the earth transmitting signals to GPS receivers. Photo: Scott Ehardt.

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