Birthplace of modern celestial navigation

To the editor: Capt. Joshua Slocum’s historic solo circumnavigation of the globe, the first British Airways Concorde flight across the Atlantic, today’s seagoing yachtsman peering at the digital readout of a GPS receiver – all of these can be traced to a brass strip in the courtyard of a 330-year-old English observatory.

That bit of metal, polished to a satiny finish by the shoes of visitors who delight in shuffling from one hemisphere to the other, is the defining feature of the Royal Observatory, Greenwich, eight miles east of central London.

For mariners and airmen, however, this north-south sighting line on a hill above the River Thames has a much more important function. Since 1884 it has provided the internationally agreed-upon reference point – the zero meridian of longitude – from which east-west geographical positions and standard time are reckoned.

By order of King Charles II, the complex was built in 1675-76 for the sole purpose of “rectifying the tables of the motions of the heavens, and places of the fixed stars, so as to find out the so-much-desired longitude of places, [and] for perfecting the art of navigation.”

From the time of the ancient Greeks, mariners had calculated their latitude by observing the angular elevation of the sun at noon.

But similar measurements of points east and west were not so easily obtained. They depended on a comparison of ship’s time, computed by trigonometry, with the time at a reference location. The catch was this: Before the age of reliable clocks, a shipmaster could not carry the time of his home port with him. In the 17th and early 18th centuries, the solution to the longitude problem was thought to lie in astronomy.

Mariners everywhere knew Earth rotated on its axis at the rate of 15° an hour. If some means could be discovered of predicting the time of a solar-stellar-lunar event at, say, Greenwich, then the data could be used to figure out the difference in time of the same event at a ship.

Thus, if astronomers were able to map the movement of the moon against a backdrop of sun and stars, navigators could look to the heavens as to the hands of a giant clock. Calculating longitude east or west of Greenwich or of any home port would involve converting the time discrepancy into degrees and minutes of arc.

Enter John Flamsteed, the leading astronomer of his era. In the spring of 1675 he won appointment at Greenwich as first Astronomer Royal. Flamsteed mounted a 140° metal arc on the wall of an outbuilding in the southwest corner of the observatory’s courtyard. Here, night after night year round, he measured and timed the transit of stars. His vigils resulted in a monumental catalogue of declinations and right ascensions – celestial equivalents of our more familiar earthly coordinates of latitude and longitude.

Some 90 years after Flamsteed’s groundbreaking work, the observatory published the first nautical almanac. By tabulating angular distances from moon to sun and selected stars, as measured at Greenwich, it gave sextant-wielding mariners at last a practicable, if cumbersome, way to determine longitude. This lunar distance navigation would only be supplanted in the early 19th century, when advances in chronometer technology enabled mariners to compare Greenwich time directly with local time on their voyages.

In Flamsteed’s day the zero meridian ran through his courtyard shed. As later generations of Astronomers Royal installed more sophisticated transit instruments, the line migrated 20 or 30 feet to the east until it came to rest in the Meridian Building.

For the past 50 years the Greenwich Royal Observatory has attracted, instructed and sometimes mystified the public as a museum of navigation, astronomy and chronometry. Exhibits include handsomely wrought astrolabes, reflecting octants and clockwork planetariums. But the collection’s greatest treasure is an oversized pocket watch dating from 1758. Created by English clockmaker John Harrison, this earliest example of a marine chronometer contains a temperature-compensating device that paved the way for accurate timekeeping at sea. One of the museum’s cupolas displays the celebrated time ball that still drops daily at 1300; before the advent of radio, it gave seaward-bound skippers a final check on their chronometers.

Although hours, minutes and seconds are no longer measured by the stars, but by oscillations of the atom of cesium, the famous brass strip that lies embedded at the building’s door continues to be recognized as marking the meridian of origin of the world’s longitude and the source of Greenwich Mean Time.

– Alan Littell, a frequent contributor to Ocean Navigator, is an author, a journalist and former merchant mariner. He lives in western New York and Athens, Greece.

By Ocean Navigator