The relationship between distance and time

From early human navigation by stars to modern exploration of space, our ability to accurately know where we are has always been inherently linked to knowing what time it is.

Navigating by stars

Before humans built satellites, navigation was done by looking at fixed stars in the night sky. Maritime navigators used nautical charts to determine their position in the sea. A tool called a sextant was the most essential instrument for celestial navigation.

You can still use tools like this to determine your position anywhere on the Earth’s surface to within a hundred metres or so, as long as you have an accurate clock.

This navigational need for an accurate clock (with the stars alone you can measure latitude but not longitude), drove the development of accurate mechanical clocks in the mid-18th century. At that time most clocks were based on a swinging pendulum or a hairspring, which were not always reliable at sea.

John Harrison is credited with the creation of the first highly accurate marine chronometer, based on a pair of counter-oscillating weighted beams connected by springs.

Satellite navigation and atomic clocks

Satellite navigation systems such as GPS require unprecedented accurate knowledge of the time. GPS receivers work by measuring the relative time delays of signals from at least four satellites. Electric and quartz clocks had followed on from he marine chronometer with increasing accuracy,  but neither of these was well-suited to the extremities of space or the particular accuracies required to measure this time delay.

Atomic clocks are the most accurate time and frequency standards known. Based on atomic physics, atomic clocks measure the electromagnetic signal that electrons in atoms emit when they change energy levels. First suggested by Lord Kelvin in 1879, the magnetic resonance necessary for this wasn’t developed until the 1930s by Isidor Rabi.

GPS satellites have at least two onboard caesium and as many as two rubidium atomic clocks. The relative times of these clocks are combined into one absolute time coordinate. The European Galileo Global Navigation Satellite System operates in a similar way, with a control centre in Fucino, Italy, generating Galileo System Time.

Once you know the absolute time coordinates for a satellite, you can determine the time delay it took a signal to reach you from it, and from this you can deduce how far away it is.