A millennium is a period of a thousand years. From the end of 1999, through the year 2000 and on until the year 2001, people all over the world celebrated the end of one millennium and the start of another. As the Universal Day is based on the time on the Prime Meridian at Greenwich, the Royal Observatory Greenwich was at the centre of Millennium celebrations.

What is the Universal Day?

Each Universal Day begins when it is midnight on the Prime Meridian at Greenwich. Standard Time in most countries is either ahead or behind Universal Time (which is the same as Greenwich Mean Time). Standard Time in countries to the east of Greenwich is ahead by as much as twelve hours and forty-five minutes, but behind by as much as thirteen hours in countries to the west. For instance, when it is 12.00 in Greenwich, the Standard Time in Bangladesh (to the east) is 18.00, but 07.00 in Jamaica to the west.

What is the Prime Meridian?

A meridian line is an imaginary straight line which runs from the North Pole to the South Pole. Meridian lines are often marked on maps or globes and are also known as lines of longitude. Astronomers sometimes set their telescopes up on a north-south line, and at the Royal Observatory Greenwich there are several different meridian lines, each named after an Astronomer Royal who used it.

It was the meridian line named after the astronomer Airy that was chosen as the Prime Meridian, and labelled as zero degrees of longitude. All other lines of longitude are measured from this starting point.

Why was the Royal Observatory built?

Until the 18th century, sailors could only accurately measure their north-south position, or latitude. As a result, voyages were difficult and dangerous. Ships ran aground on rocks because navigators did not know their exact position, or ran out of food and water because journeys took longer than expected. Sailors needed to know their east-west position, or longitude, as well as their latitude, to plan and make safe journeys.

In 1675, King Charles II ordered the building of an observatory at Greenwich to try to find a solution to this problem. The Royal Observatory was designed by Sir Christopher Wren. To save money it was built using second-hand bricks, and cost just over £500.

How could work done at the Observatory solve the longitude problem?

It was known that fifteen degrees of longitude was equal to a time difference of one hour. If there had been a good and reliable clock for sailors to take to sea they could have used this knowledge to work out their longitude. As no such clock existed, Charles II hoped that the study of the stars would allow time difference to be measured.

The Moon moves across the stars at a steady rate, so if a prediction was made of its position in relation to certain stars at set times, sailors could use the information to work out their position. The sky would become their clock. It was to be another hundred years before a clockmaker managed to design and build a real clock that could cope with the rough seas and changes in temperature experienced on board ship during an ocean voyage.

Who studied the stars at the Observatory?

Astronomers, led by the Astronomer Royal, worked at the Observatory. King Charles II appointed John Flamsteed as the first Astronomer Royal. Accurate information about the movement of the Moon and positions of the stars took years to collect, and involved working through the night, so Flamsteed lived at the Observatory as well as working there. He worked at cataloguing the stars from 1675 until his death in 1719.

The second Astronomer Royal to work at the Observatory was Edmond Halley. He carried on Flamsteed's work and correctly predicted the reappearance of the comet which now bears his name.

Did the study of the stars and Moon solve the longitude problem?

Once the astronomers had collected their information, their predictions needed to be published in a form useful to sailors. It took many years to make the necessary observations, so seafarers had to wait until 1767, when the fifth Astronomer Royal, Nevil Maskelyne, was finally able to publish the first Nautical Almanac. The Almanac was a book which contained tables showing where the moon would be in relation to the stars on different days of the year.

Seamen could use the information in the tables to calculate the longitude of their ship. This method of finding longitude was cheap and reliable and soon became popular with ocean-going navigators. Indeed, the information in the Nautical Almanac proved so useful that today it is still published each year.

How did the Royal Observatory become so important in time-keeping?

The Nautical Almanac was soon being used by ships from many countries, so their calculations of longitude were based upon the time at Greenwich. However, the work of the astronomers at Greenwich also helped ordinary people to make sure their clocks and watches were telling the correct time.

How did the work of the astronomers help people other than seafarers with time keeping?

The Earth spins at a steady rate, and as it spins the stars seem to move across the sky. Astronomers knew that a particular star would appear at the same time. The astronomers at Greenwich used information about star positions to make sure that their clocks were telling the correct time.

Until the 19th century, other people had no accurate or reliable way of checking that their clocks and watches were telling the right time. However, in 1833, a red time-ball was placed on the roof of Flamsteed House at the Observatory. It is still in use today. At 12.55 each day, the ball rises half way up its mast. At 12.58, it rises to the very top, then drops at exactly 13.00. In the past, sailors on the River Thames and clockmakers in the City of London could watch the red ball drop and adjust their watches and chronometers if necessary.

In 1852, an electric clock was placed beside the Observatory gates so that members of the public could see the correct time all through the day, not just at 13.00.

Until the early 19th century, clocks in eastern parts of Britain, where the sun rises earlier, were always ahead of clocks in the west of Britain. In Victorian times, as more people began to travel by train, it became essential that everyone agreed an accurate standard for time-keeping. Railway companies in Britain chose Greenwich Time as their measure, as did railways in USA and Canada.

Why is the Universal Day based on Greenwich?

At a conference in 1884, twenty-five countries met to agree a zero meridian for the whole world. The meridian running through Greenwich was a popular choice because most of the world's shipping was already using it. The conference agreed that the Prime Meridian of the World would be the one that ran through Airy's transit telescope at Greenwich.

Do astronomers still work at the Observatory today?

When King Charles II chose Greenwich as the site for the Observatory it was in the countryside. As London grew, Greenwich was swallowed up in the busy city. Pollution, especially light pollution from the glare of the street lights, made observation of the sky increasingly difficult. In 1946, a new Observatory was set up in the grounds of Herstmonceux Castle in Sussex. Just over 40 years later, the Astronomer Royal and other members of staff moved to Cambridge.

The Royal Observatory Greenwich is now a museum where many of the instruments used by the great astronomers of the past are displayed. People come from all over the world to see the famous buildings and to stand astride the Prime Meridian with one foot in the eastern half of the world and one foot in the west.