Tuesday, 29 May 2007

Greenwich - The home of longitude

We have finally made it to the UK, and although jobless and homeless, have managed to have a great time so far, although today's wet and freezing London weather is testing our patience. The UK being a very old country (in the western sense in any case) means that there is an abundance of "culture" and ye olde science that appeals to me - although I must say I haven't found good coffee yet - the Anglos clearly didn't put much effort into coffee science...

I will start a series of stories on the sciencey places I have been, starting with Greenwich.


Greenwich, to the east of the city of London, is home to the Royal Observatory, whose mission is to illustrate for everyone the importance of the sea, ships, time and the stars and their relationship with people.

Greenwich is also home to Greenwich Mean Time and the prime meridian, which is the meridian at which longitude is defined to have zero degrees. Longitude is an important way of defining where on the globe you are - and was of particular importance during the 1600-1800s for seafarers, for whom knowing where they were in the ocean was paramount. Longitude defines the location of a place in a number of degrees east or west of the prime meridian that runs through Greenwich - this is a north-south line that encircles the Earth. Longitude ranges from 0 degrees at the Prime Meridian to +180 degrees eastward and −180 degrees westward on the direct opposite side of the Earth. The photo on the right is me with my bright red scarf straddling the prime meridian.

Latitude, which runs east-west around the Earth, has a natural zero-point - the equator. Longitude does not have this, which is where Greenwich's claim to fame comes in. Whilst meridians through Copenhagen, El Hierro, Jerusalem, Saint Petersburg, Paris, Philadelphia, Pisa, Rome and Washington were used in various parts of the world, Greenwich won the Meridian Idol competition in 1884, when the International Meridian Conference, which must have been the high point in the A-list social calendar of 1884, adopted the Greenwich meridian as the universal zero point of longitude.

Measuring longitude whilst at sea has historically been a very difficult business. Whereas latitude is easy to determine by looking at the positions of the stars or the sun, longitude was often measured using dead-reckoning - that is, knowing how fast you are going and knowing the time at which you started your journey gives you your final position and longitude. This is fraught with inaccuracies, especially in rough conditions.

Giovanni Domenico Cassini developed a method in 1681 that measured longitude by noting the relative positions of Jupiter's natural satellites, which have known orbits. This was difficult, however, for the average sailor. The wrecking of Sir Cloudesley Shovell's British fleet gave rise to the British Longitude Act, which created the Longitude Prize (£10,000 for any method capable of determining a ship's longitude within one degree; £15,000, within 40 minutes, and £20,000 within one half a degree) for anyone who could develop a practical method of determining longitude at sea. The winner of the prize was John Harrison with his H-4 device.

Harrison demonstrated a method of determining longitude on his voyage between Portsmouth in England's south, and Bridgetown in Barbados, by knowing the exact time of day in England using a clock (a new technology at the time), while using astronomical observations to find the exact local time on the ship as it sailed. Knowing the time in England relative to the time on the ship gives you your longitude. For instance, if its midday in Greenwich on the clock you are carrying with you, and midnight where you are, you must be on the exact opposite side of the Earth - that is, at 180 degrees longitude. Daylight saving makes this a little more difficult to work out, but the principle is the same.

Listen to this show here

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