Star witness: how the Royal Observatory transformed our understanding of astronomy and time

The big bang moment

1675: Charles II sparks a star-gazing revolution

In the 17th century, the great maritime nations were vying to solve a puzzle that had confounded philosophers for many centuries: how to determine exact positions on the Earth’s surface. And in Britain, Charles II took a major step to ensure his realm’s position at the head of the pack.

On 4 March 1675, the king signed a royal warrant appointing John Flamsteed as the first “astronomical observator”, tasked with “perfecting the art of navigation”.

Flamsteed’s specific objective was to devise a method by which sailors could establish their longitude (east–west position) at sea – which involved measuring the positions of celestial bodies – and of knowing the time at a ship’s local position. And Flamsteed would get to carry out this vital research from the surroundings of the country’s first state-funded, purpose-built scientific institution: the Royal Observatory.

It was urgent work, because England’s great rival, France, had stolen a march: Louis XIV had established an observatory in Paris nearly a decade earlier. Flamsteed was, in effect, playing catch-up.

The size of the task facing the astronomer clearly wasn’t lost on Charles II, for the king appointed none other than the celebrated architect Christopher Wren to design the observatory. Wren’s chosen site was the hilltop ruins of Greenwich Castle – and it was here that the foundation stone was laid on 10 August 1675.

With its lofty windows, royal portraits and accurate pendulum clocks, the octagonal ‘star chamber’, completed in 1676, was grand in appearance. Unfortunately, it was also unsuitable for mapping the stars, as it was 13.5° askew from the meridian, an imaginary north-south line connecting the poles. So Wren designed another building nearby. This one would last for the remainder of Flamsteed’s tenure, before being demolished, rebuilt and extended as the Meridian Observatory.

As for Flamsteed, he went on to conceive Greenwich Mean Time (GMT), by which he produced an average (‘mean’) 24-hour day that could be tracked using a mechanical clock year round. And that would have ramifications across the planet.

• Read more | A brief history of astronomy

Dressed for success

1767: How a man in a padded suit transformed the art of navigation

No one could accuse Nevil Maskelyne of not being dedicated to his job. In the dead of night across the mid-1760s, the fifth Astronomer Royal could be found staring up at the sky for hour upon hour, while straining his ears to hear the beats of a nearby clock.

Dressed in an extravagant padded suit made from wool, silk and linen, Maskelyne was prepared for all that a British winter could throw at him. Not even perishing temperatures could stand in the way of the task in hand: to devise a ‘lunar distance method’ to calculate longitude at sea. This technique required mariners to measure the angular distance between the moon and a specific star, then compare it to Greenwich time. And they all used data gleaned from Maskelyne’s meticulous study of the movement of the stars.

This was, however, far from a one-man endeavour. Maskelyne was joined on his freezing vigils by his assistant, David Kinnebrook, and enlisted a network of mathematicians – dubbed ‘computers’ – to work remotely on calculating the predicted position of the moon over the year ahead. The Astronomer Royal even had a team of ‘comparers’ check the computers’ work (which paid off when two of them were found to be copying one another’s work).

All this enabled Maskelyne to produce the Nautical Almanac, the first edition of which was made available on 6 January 1767. It quickly became an essential tool in navigation, reducing the time taken to make longitude calculations from hours to minutes. Updated annually, the Almanac became a required element of navigators’ training, and set the trend for mariners to calculate their position relative to Greenwich.

High standards

1833: A curious metal sphere regulates clocks

It can be seen from miles around – and with good reason. Sitting atop the Royal Observatory is a large red, metal ball that rises to the top of a mast at the same time every day, and drops a few minutes later. For those who haven’t set eyes on it before, it makes for a strange sight. But what is it there for?

To answer that question, we need to rewind to 1764 when John Harrison, the celebrated English clockmaker, sent his fourth marine timekeeper, later known as ‘H4’, on a trial voyage to Barbados. The clock performed exceptionally well, losing a mere 39.2 seconds over 47 days at sea. To Harrison’s delight, it proved that such devices could provide mariners accurate reference times for calculating longitude.

Other clockmakers developed Harrison’s innovative ideas into a standard design that was cheaper and easier to reproduce. By the 1820s, the ‘marine chronometer’ was being used on both commercial and Royal Navy ships. But these chronometers only worked if they were telling the correct time when ships left port. That’s where the Greenwich ‘time ball’ came in.

A time ball is a large spherical device that drops at a predetermined time of day, enabling mariners to set their clocks by it. The first modern example of this technology was erected in Portsmouth in 1829. Greenwich’s, however, would be the most famous. It was installed in 1833 on Flamsteed House, at the heart of the Observatory, providing maximum visibility from the Thames. Ever since, it has risen halfway up the mast at 12.55pm, climbed to the top at 12.58pm and then dropped at precisely 1pm GMT.

Soon time balls were appearing at harbours and observatories around the globe, meaning that mariners could check their chronometers against accurate clocks, even during the longest of voyages.

Timetable drama

1852: Greenwich rides to the railways' rescue

Take a stroll down Corn Street in the heart of Bristol and you’ll come across a clock mounted to the wall of the city’s old corn exchange. There’s nothing unusual about that, you may think. But take a closer look and you’ll notice that this elegant 1820s timepiece has two hands for minutes: a red one for GMT and a black one for local Bristol time. The clock is a curious snapshot of a bygone age, and also a marker of a technological conundrum that the Royal Observatory played a central role in solving.

The mid-19th century was very much the age of the railways – by the 1840s tracks were snaking across the country. This was all well and good. But when, in 1847, the railway companies decided to coordinate their timetables across Britain using GMT, they ran into a problem. Back then, Britain’s cities relied on local times set by sundials, varying by as much as 30 minutes between west and east. So how could the railway companies ensure that these times were perfectly aligned?

Enter George Biddell Airy, the seventh Astronomer Royal. Airy reasoned that a new technology – that of sending impulses via telegraph wires to a series of synchronised dials – could provide a solution. And so he ordered a new timepiece – the Shepherd Motor Clock – to be installed at the Royal Observatory. From 1852, the clock began sending daily time signals to railway stations across London and Kent, along with hourly signals to the Electric Telegraph Company on London’s Strand for national distribution. Train travel would never be the same again and, by 1880, GMT had been embraced as Britain’s legal Civil Time.

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A moment in the sun

1874: Greenwich leads the race to record the transit of Venus

For astronomers everywhere, 9 December 1874 was like Christmas come early. Star gazers counted down the days to this landmark date, while George Biddell Airy ordered expeditions to Egypt, New Zealand and the Indian Ocean to observe the amazing event unfolding overhead.

That event was a transit of Venus, a rare astronomical phenomenon in which the planet appears to cross the Sun as a slow-moving black dot. This was the first time that the transit had occurred since 1769, and the first such event since the invention of photography in the 1820s. Little wonder astronomers were excited.

And their excitement was justified. By timing the transit’s key moments as seen from different locations on Earth, star gazers can measure the Sun–Earth distance, known as the ‘astronomical unit’. This yardstick (just under 93 million miles) is still used by scientists today.

Ahead of the 1874 transit, Airy ordered special telescopes known as photoheliographs to project the Sun’s disc onto glass plates. However, his teams struggled with the kit, and the photoheliographs failed to capture the transit with any great accuracy.

Despite those issues, the transit of Venus inspired a renewed interest in sun studies. Greenwich assistant EW Maunder started to compile a daily record of sunspot observations. In 1904, Maunder and his wife, Annie, assembled 30 years of sunspot data into the ‘butterfly diagram’ that still underpins our understanding of the Sun’s 11-year magnetic cycle.

A degree of certainty

1884: The world aligns itself to Greenwich

John Flamsteed had been dead for more than a century. Yet, on 13 October 1884, his great brainchild, Greenwich Mean Time, truly went global. On that day, delegates at the International Meridian Conference in Washington, DC voted in favour of making Greenwich the world’s prime meridian, or 0° longitude.

The decision was influenced by a combination of technological supremacy and pragmatism. Greenwich was one of only four observatories with a suitably accurate telescope – the Airy Transit Circle Telescope – to define a meridian. And more than 70 per cent of shipping companies by tonnage already relied on the Greenwich meridian.

The delegates also agreed that the Universal Day – a 24-hour period to be used as a common reference to record events around the world – should begin at midnight GMT on the Greenwich meridian. This would hardly have come as a surprise: a year earlier, railroads in the US had already agreed to use standard time zones based on hourly meridians from Greenwich.

The delegates assembled in the American capital that October day voted overwhelmingly in favour of the Universal Day, and countries started to define their own standard time zones as hourly intervals before or after Greenwich. The result was the familiar time-zone system we use today.

Seeing double

1893: The Royal Observatory makes its biggest statement yet

Was Greenwich under threat? The International Meridian Conference of 1884 might have confirmed the Royal Observatory’s primacy in the world of astronomy. But, within a year, the eighth Astronomer Royal, William Christie, was voicing his concerns that the principal telescope at Greenwich was being overtaken by newer, better instruments at rival observatories across Europe and North America. Determined to keep Greenwich out in front of the chasing pack, Christie persuaded the Admiralty to fund a new instrument, and that 28-inch diameter lens telescope – still the largest of its type in Britain – came into use in 1893.

Most other telescopes at Greenwich were designed to be fixed into position to ensure the accurate measurement of the stars for timekeeping and navigation. The Great Equatorial Telescope – named after its mount, which kept the instrument moving parallel to the Earth’s equator – was part of a new generation designed for astrophotography. With a clockwork drive that kept the instrument moving in sync with the stars from east to west, it could be used to take photos with very long exposures stretching to several hours, capturing impressive views of faint gas clouds (nebulae).

However, astronomers struggled to configure the heavy glass lens for photography, and so the telescope was reassigned to the study of double stars. By measuring the changing angle between an orbiting pair of stars, the astronomers could now calculate the mass of these distant suns.

All that came to an end in 1971 when the Great Equatorial Telescope was finally retired. However, you can still see this extraordinary instrument – housed in its distinctive onion-shaped dome – at Greenwich today.

• Read more | A brief history of daylight saving time

Pip pip hooray

1924: GMT is piped into homes across Britain

If we want to know the time in 2025, all we have to do is check the phone in our pocket, or glance at the digital watch on our wrists. Across most of the 20th century, of course, that wasn’t an option. If Britons sought an accurate time-check that they could, quite literally, set their clocks and watches by, then they had to turn on their radios and listen out for the familiar strains of the ‘six pips’.

The six pips first entered the public consciousness on 5 February 1924, when the ninth Astronomer Royal, Frank Dyson (below), announced a collaboration with the BBC. The Royal Observatory would start sending electrical impulses down a telephone wire to the newly created broadcaster. These would then be converted into a series of six audio alerts – or ‘pips’ – that would go out on the radio on the hour, every hour.

But while some technologies were underscoring Greenwich’s enduring relevance, others were undermining it. In the 1930s, scientists developed clocks based on the natural vibration of quartz crystals; then, a decade later, the first atomic timekeepers began to appear. Both offered reliability and accuracy that the system of measuring time by the stars couldn’t match.

This wasn’t the only threat to the Royal Observatory’s pre-eminence in the 20th century. As the march of industry impacted visibility across London’s skies, so Greenwich’s suitability for the practice of astrophysics declined. Soon it became clear that the Observatory would have to move. And it did so twice – first to East Sussex in 1957; then to Cambridge in 1990. Eight years later, it shut altogether.

Time had finally caught up with the great old scientific institution. But its huge contribution to the fields of astronomy, timekeeping and navigation cannot be doubted – as the many thousands of people who visit the Royal Observatory museum that now occupies the famous Greenwich complex will no doubt testify. John Flamsteed would surely be proud.

Dr Louise Devoy is senior curator at the Royal Observatory and author of Royal Observatory Greenwich: A History in Objects (October, 2025)

This article was first published in the November 2025 issue of BBC History Magazine