For centuries, the stars in the night sky were a vital part of sailors' navigation at sea, and a reliable guide for finding their way, provided you knew how to read them.
Many fine works capture the spirit of sea voyages in the days of sail, such as Sea Fever, in which the British poet laureate John Masefield (1878–1967) wrote the line: "And all I ask is a tall ship and a star to steer her by.”
More astronomy history
But for that image of a full-rigged ship cutting a foamy ocean swathe to become reality, the crew needed the knowledge and skill to determine the ship’s position at sea and the direction of the next port.
Early sea exploration
There are many accounts from around the globe of humankind’s early sea excursions.
In the Mediterranean, the Minoans, Phoenicians and ancient Greeks are documented as making numerous ocean voyages using simple coastal navigation – keeping the shoreline in sight, for fear of becoming lost.
However, even then – going back to around 2000 BC – basic navigational sea charts were in use, some of which incorporated positional references to the Sun and Moon.
On the other side of the world, in the Pacific Ocean, Polynesians have been navigating by the stars for thousands of years, their fixed positions giving a constant that the Polynesian seafarers memorised.
European explorers encountering this long tradition called it Polynesian wayfinding, as it also used natural phenomena such as seasonal bird movements.
As a more technical approach to seafaring was undertaken in Europe, detailed but still relatively basic sea charts began to emerge, mainly due to mariners of the 13th century keeping records of their endeavours.
While providing no longitude or latitude points, the charts highlighted well-travelled routes between major ports.
As the evolution of the sea chart continued, the number of people with the ability to comprehend them narrowed, making the task of the navigator rather specialised and, given the increase in sea traffic, one that carried much responsibility for crew and cargo alike.
After the advent of a reliable compass in the 13th century, the 15th century saw the era of the great explorers and the introduction of the mariner’s astrolabe, a seaworthy version of the instrument that had been used in the Near East for at least a millennium.
This device helped determine a ship’s latitude from the Pole Star or the Sun.
The 18th century saw further advancement, with the introduction of the sextant and the sea-going chronometer in the late 1700s, which helped determine a ship’s longitude with high accuracy.
The sea chart and the star chart were then combined in the navigational star chart.
Together with the US Naval Observatory, the UK Hydrographic Office today collates astronomical data into the annual Nautical Almanac.
Perhaps not everybody’s idea of bedtime reading, this navigational bible contains tabulations of navigational planets and 57 nominated stars that can determine the position of a ship at sea with a sextant.
Today, the advent of GPS has afforded the ship’s navigator pinpoint positioning.
But even without it, at Earth’s ‘middle of nowhere’ location – ‘Point Nemo’ in the South Pacific, 2,688km (1,670 miles) from land in every direction – the heavens still provide seafarers with a guide to leaving nowhere and going somewhere.
