LONDON — On a blustery afternoon, a multinational clot of tourists waited for a turn to straddle some of the most ephemeral notions — time and space.
They'd come to the Royal Observatory, perched on a steep grassy knoll in Greenwich on the outskirts of London, to stand by a brass line that runs through a courtyard. A preteen played hopscotch along the 4-inch-wide line, her braids bouncing; a family posed solemnly for a snapshot.
The line in the paving stones at the historic British observatory, now a museum, symbolizes the Prime Meridian of the world, or Longitude 0.
The Prime Meridian is the north-south line from which every place on Earth is measured in terms of its distance west or east. And, thanks to an international agreement, the Prime Meridian is the world's timing lodestone, the source of Greenwich Mean Time (GMT) from which all time zones are calculated.
In a world now so precisely timed, it's hard to remember that even in the mid-1800s there was no internationally standardized measurement of time. Cities and regions kept their own time.
But as the world globalized in the 1800s, with railroads crisscrossing countries and communications opening up, a unified time system was vital.
An international conference in 1884 chose Greenwich as the place from which world time would be standardized.
It was another feather in the cap of the Royal Observatory, which was founded in 1675 when Britain, a seafaring superpower, wanted to improve navigation by finding a way to more exactly measure longitude at sea — which would let mariners figure out where they were from east to west.
The world's lines of longitude, or meridians, run from pole to pole and divide up the world. To calculate longitude (which, when combined with the more easily measured latitude, pinpoints exactly where you are in the world) accurate timekeeping was essential.
Through calculating the difference in time at a fixed point, such as Greenwich, and the local time, the longitude can be determined. (Originally mariners tried to calculate longitude using astronomical calculations — hence the Royal Observatory was created for making charts of the heavens.)
To spur the invention of accurate marine clocks for measuring longitude — timekeepers that wouldn't lose time over months-long sea voyages and could withstand a ship's movement and humidity — the British government created the Board of Longitude. It offered a reward, worth millions of dollars today, for the best marine timekeeper.
The prize eventually was won by a once-unknown English clockmaker, John Harrison, in the late 1700s, after decades of toil.
Harrison's intricate marine chronometers are enshrined at the Royal Observatory (the working observatory moved elsewhere decades ago).
Climb up the steep hill to the museum, tucked amid the broad lawns and groves of Greenwich Park, and get lost in time. Stroll past historic telescopes and through small galleries packed with centuries of clocks, from old-fashioned pocket watches to massive gleaming brass clocks with whirling parts and pendulums.
Harrison's intricate clocks — including his fourth timekeeper that won the longitude prize (called H4 and built in the mid-1700s) are on display.
In Flamsteed House, the observatory's original 17th-century building, cozy rooms with period furnishings show where John Flamsteed, the first astronomer royal, and subsequent astronomers and their families lived.
The building was designed by Sir Christopher Wren, the architect of London's landmark St. Paul's Cathedral and an ardent scientist.
Meander into the Royal Observatory's planetarium for a state-of-the art look at the heavens and interactive displays on space missions, gravity and more.
Finish up in the courtyard, straddling the brass line representing the Prime Meridian that defines time and divides the world into Western and Eastern hemispheres. You can have a foot in both worlds.
IF YOU GO:
The Royal Observatory is at Greenwich, in southeast London. For centuries a separate village along the Thames River, Greenwich now is part of the city. The museum is open daily from 10 a.m. to 5 p.m. Weekend afternoons can be busy; try to visit on weekdays or early in the morning. Admission is free. www.nmm.ac.uk/places/royal-observatory
—Take the Docklands Light Railway (which links to the London Underground, or Tube) to Cutty Sark station. It's roughly a half-hour from central London plus a 10-minute walk (well-signposted) to the museum.
—For a different view of the city, take a riverboat along the Thames one way. There is frequent service between Greenwich and central London piers, and the boats give excellent views of the restored 19th-century warehouses and modern redevelopment along the Thames.
Buy an Oyster card, a London transit pass pre-loaded with credit for fares, or a London Travelcard. They significantly cut the price of the London Underground and other public transport. Get passes in London (see www.tfl.gov.uk for transit information) or buy in advance through the Visit Britain government tourist office, www.visitbritainshop.com/usa/home.html
MORE GREENWICH MUSEUMS, SIGHTSEEING
Greenwich offers enough sights for a full day's visit. The National Maritime Museum includes the massive Maritime Galleries and the Queen's House (as well as the Royal Observatory) —and all are free admission. Take a walk in Greenwich Park and stroll the area's historic streets and bustling street market.
Maritime Galleries: This sprawling museum covers Britain's seafaring history with everything from historic paintings and ship models to a ship-bridge simulator and the uniform that Admiral Lord Nelson, one of Britain's greatest war heroes, wore in the 1805 Battle of Trafalgar, complete with the hole from the bullet that killed him. www.nmm.ac.uk/
Queen's House: The elegant building, designed in 1616 for the wife of King Charles I with ornate marble floors and painted ceilings, showcases centuries of British maritime paintings, Tudor artwork and more. www.nmm.ac.uk/
Greenwich Park: Broad paths wind through the 183-acre park that edges the museums. Enjoy sweeping views of the Thames and central London from its hilltop.
Cutty Sark: The 1869 clipper ship, displayed in Greenwich, has been under restoration for years, including from a fire in 2007. The sailing vessel is expected to reopen to visitors next year. www.cuttysark.org.ukGreenwich Market:Stalls and surrounding small shops offer arts and crafts, antiques and more. In the market courtyard, food vendors dish up West African stews, French crepes, sandwiches and sushi. The market is open Wednesday through Sunday; www.shopgreenwich.co.uk/market/More information: www.visitgreenwich.org.uk/
GREENWICH'S CLAIMS TO FAME
Being home to Greenwich Mean Time and the Prime Meridian gives Greenwich some bragging rights:
—The world is divided into Eastern and Western hemispheres along the Prime Meridian.
—Being the place from which time is measured worldwide, Greenwich is the official starting point for each new day and new year.
—What's called Maritime Greenwich, the cluster of 17th-century buildings along the Thames, has been designated a UNESCO World Heritage Site because of its architectural and scientific significance.