For millennia, cultures around the world based their time on the rising and setting sun, and their calendar on the moon's movement around the Earth, based on the visible monthly cycles its appearance goes through.
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Objects like sundials could be used to measure a time based on the movement of the sun across the sky. They needed to be orientated precisely to north and south and account for the latitude - as the motion of the sun is different across the sky depending on where you are.
However, while these tools worked well, they were not as accurate as needed as technology, and in particular navigation, advanced. The invention of the pendulum clock improved the accuracy and precision.
In 1944, Mt Stromlo Observatory, then the Commonwealth Observatory, became the official time keeping service of Australia.
Instead of just using the sun, stars could be used to precisely calibrate time - offering the advantage of more objects to calibrate to, allowing the precision of milliseconds.
As stars moved across the sky, stars that moved across the zenith were recorded. Zenith is the imaginary line that goes straight up from a location - the point directly above you.
As the Earth rotated, certain stars would cross this line, and the moment they did, a series of quartz crystal clocks were set.
The observatory needed enough stars so that during the night, multiple stars could be used to calibrate it, and account for variations in the stars themselves. Enough stars were needed as well, so that as the Earth moved around the sun, the visibility of stars would change throughout the year.
At first, a small 7.5cm telescope was used for this purpose. Eventually a purpose-built facility, the Photographic Zenith Tube, was built at Mt Stromlo for the time keeping service.
The design of the instrument was similar to those in the UK at the Greenwich Observatory as well as the US Naval Observatory.
Once a star's position was recorded, a series of four quartz oscillator crystals were used to calibrate this time. These instruments were much more precise than a pendulum clock, and accurate to the scale of about a millisecond.
The clocks had their own power supply and battery back-up so that if power was lost or was interrupted, they would be able to keep going. These systems were also used to calibrate the traditional pendulum clocks as well, as well as clocks held at the US Naval Observatory.
A large number of people - from the astronomers to measure the positions of the stars, engineers to manage and operate the quartz clocks, and dedicated staff just for the time keeping service, were one of the many jobs up at Mt Stromlo for nearly 25 years
The service was eventually transferred back to the Commonwealth, now contained in the National Measurement Institute - which is responsible for not only the measurement of time, but other fundamental parameters - needed for the modern-day world.
Now, we use atomic time. Atomic time measures how often a certain atom vibrates to define a second, then we add those seconds up to an hour, and eventually a day.
So when you clock off for the day, think about all the people working throughout the years to make sure the time was known so you could.
- Brad Tucker is an astrophysicist and cosmologist at Mt Stromlo Observatory and the National Centre for the Public Awareness of Science at the ANU.
