The traditional nativity story tells of three wise men making their way from the East to Bethlehem guided by an unusually bright star – later postulated to have been Halley’s comet.

Member Stories

Before being replaced by technology, navigating by the stars was commonly used by seafarers to work out their position and which direction they should be heading – and was successful enough to cross oceans.

The simplest use of the stars is to find your orientation. For example, travellers in the northern hemisphere might use the star Polaris in the constellation Ursa Minor – also known as the North Star as it’s close to the celestial north pole, so if you’re facing Polaris, you’re looking approximately north. Other compass directions can then be worked out accordingly.

But it’s no good knowing which direction you’re going if you don’t know where you are. Sailors can also work out their longitude from the positions of celestial bodies.

One of the simplest is to observe the time at which the Sun reaches the highest point in the sky. On the prime meridian, the Sun is at its zenith at 12pm GMT. Because we know that the Sun covers 360 degrees of sky in about 24 hours, the position of an observer can be easily calculated. If the Sun is directly overhead at 1pm GMT then you must be 15 degrees east of the prime meridian.

Because the Earth’s orbit around the Sun is elliptical, rather than circular, corrections must be made to these calculations, and a table of daily variations is published in each year’s Nautical Almanac.

The stars can be used to determine longitude in a similar way, when this method is used in conjunction with an almanac of the coordinates of celestial bodies. HM Nautical Almanac Office in the UK and the US Naval Observatory jointly publish a list of 57 stars that can be used for astronavigation.

But knowing that you’re 15 degrees east of the prime meridian could put you in either Naples or Kinshasa, so latitude must also be calculated separately.

The distance of the Sun or a star above the horizon changes as you move north or south – for example at the north pole Polaris is directly overhead whereas on the equator it’s near the horizon. Measuring the angle between a star and the horizon using an instrument such as a sextant, the seafarer’s latitude can be worked out using a little trigonometry.

Because the calculations are dependant on knowing the time, this has to measured extremely accurately – small inaccuracies in time can be magnified into major errors in position. Due to the motion of a ship and slight local variations in gravity, pendulums were useless for making accurate time measurements at sea.

The clockmaker John Harrison spent more than 30 years developing and perfecting the marine chronometer for precisely this reason, and received a prize of £20 000 from the British Parliament – equivalent to almost £3 m in today’s money. Marine chronometers are accurate to within 0.1 seconds per day, allowing sailors to calculate their position accurate to within a kilometre and a half after a month at sea.

For a time, chronometers were replaced by radio time signals, and now the ancient art of astronavigation has been superceded by the Global Positioning System. But when technology goes wrong and makes you go 100 miles out of your way, or gets you stranded in a desert, you can still follow the example set by those three wise men two thousand years ago.

Other IOP websites

Your guide to physics on the web

Cookie Settings