‘This is how I find my way back to you,’ Hester’s father explained
to her once, on one of his rare visits home. ‘I just need
three things. A compass, a telescope and a knowledge of the
movements of the stars.’
‘The stars don’t move,’ said Hester.
‘Yes they do,’ said her father. ‘Except that one.’ And he
pointed out the Pole Star, twinkling in the sky. ‘That sits
right above the North Pole and never moves. The way the
rest of the stars move around it helps you work out where
you are.’
It’s an odd truth that in order to work out where we are on Earth, we have to look outwards, into space. We use celestial objects as markers and make calculations based on what we know about our relationship to them. We use special instruments to observe and measure angles and distances; reference our knowledge about their relative positions at any given time of the day or year or century; and apply maths to pinpoint locations.
Careful observation on things on Earth can tell us things about the position of celestial bodies, even without equipment. For example, lichens grow in different ways on north- or south-facing surfaces like rocks and trees, due to the varying amount of sunlight they get, and can be used to determine direction. Migratory birds use their own methods of navigation, and so we can use them to navigate too if we watch their behaviour.
In the days before GPS, sea navigators needed to have a solid grasp of astronomy. The most ancient cultures observed the night sky over millennia, mapping the constellations and the way they moved across the night sky. They knew the phases of the moon and the seasonal position of the sun. They knew that the Pole Star was a fixed point in the night sky. Over the centuries, we developed complicated equipment and methods for measuring these phenomena and using them to make more and more accurate calculations. We invented geographic coordinate systems like latitude and longitude and devices to help us measure the angle of distance of sun and stars from the horizon. We drew star charts and used dead reckoning to calculate distance using speed and time.
The greatest challenge to navigators was the accurate calculation of longitude, or east-west position from the prime meridian. While latitude could be easily determined with reference to the sun or the Pole Star, longitude is connected to the rotation of the earth. Until reliable marine chronometers – clocks that could remain accurate during conditions at sea – were invented, many ships ran aground and many sailors lost their lives due to inaccurate calculations of longitude using elaborate but much clumsier methods.
Modern navigators use the Global Positioning System (GPS) to do essentially the same thing as sea navigators did for centuries, but in this case the celestial objects are not the sun, planets, moon and stars, but satellites that we have positioned in space. GPS devices on Earth read signals from a network of satellites to determine position using a three-way calculation called trilateration. GPS is much more accurate, of course – it doesn’t get obscured by the weather, it’s not prone to human error, and it does the maths for us. In fact, high-precision versions of GPS can pinpoint location within 1 centimetre. Above all, the availability of GPS means we are all navigators, as we use apps like Google Maps to guide us around unfamiliar places, and with impressive precision too – standard phone or car GPS can achieve accuracy up to about 3 metres. We don’t need to know a thing about astronomy to use them – but perhaps this is a loss, as it is only very recently in human history that ordinary people, particularly those of us that live in cities, have lost that direction connection to the night sky and the sense it gives us of our place on Earth.
Further reading: Longitude by Dava Sobel
CATHERINE NORTON 