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June 19, 2007

The Home of the Prime Meridian

PrimemeridianIn the accompanying picture, Mary and I are spanning the Prime Meridian, Longitude Zero.  We’re at the Royal Observatory, Greenwich in London – a must-see for visiting nerds. The observatory is all about navigation and not at all about astronomy for its own sake. The main task of the Astronomer Royals who worked (and lived) here was to catalog celestial passages to enable mariners to know where in the world they were.

The history of the Observatory is linked with Britain’s becoming a great naval power and the importance of maritime commerce to an island nation.  If you’re going to sail around a lot, it really helps to know where you are. [note to younger readers:  there actually was a time before GPS.]

Finding your latitude (distance from the equator) is relatively easy.  You can find it, for example, by measuring the height of the sun above the horizon at its zenith (local noon). If you know what day it is, you then know how far north or south you are.

The problem of longitude is far tougher and baffled sailors for centuries.  The theory is simple and was well understood: you can tell when the sun comes up or goes down if you can see the horizon; you can find local noon by observing when the sun reaches its zenith. If you knew how many hours before or after sunrise or sunset or noon at home, local events were occurring, you’d know your longitude. For example, if noon comes four hours late, you must be sixty degrees west of home. Since the earth turns 360 degrees in a day, it turns fifteen degrees every hour.

The problem was knowing what time it was at home.  How do you bring home time with you?  The only accurate clocks of the day were pendulum clocks and they weren’t accurate on a rocking ship. Watches had to be reset at least daily so they were no help. All clocks of the day were affected by temperature changes as their metal parts expanded and contracted. Moreover, a clock that was going to go to sea would need to be sealed against salt water; how do you oil it?

Galileo worked on this problem but didn’t solve it. He wanted to use the moons he’d discovered around Jupiter as a clock that would be visible on a voyage. Trouble was that Jupiter and its moons are only sometimes visible AND that accurate observation of their positions from a moving ship was practically impossible – even with a clever sighting helmet Galileo invented. His method did, however, become the standard for measuring position on land and contributed greatly to accurate map making.

In 1707 four British warships foundered on rocks off the Scilly Isles with a loss of 2000 lives. The boats has misjudged their longitude.  In 1714 Parliament (working swiftly) announced the Longitude Prize - £20,000 pounds to anyone who could solve the problem of determining longitude at sea. This was a few million of today’s pounds so it did attract attention.

There were two main approaches.  Watch and clockmakers concentrated on making a better clock. Astronomers tried to perfect the “lunar method” which involved measuring the angle between the moon and either the sun or some other star – essentially using the moon as a watch hand. In order for this method to work, you’d need three things: an instrument to measure this angle that could be used aboard ship (solution: the sextant), accurate predictions of the path of the moon which turned out to be very hard to predict and is variable from year to year, and voluminous tables of the expected angle between the moon and many other objects for every hour of the day and every day of every year your ship would be at sea.

In the end, both methods were perfected.  The Observatory had a key role in predicting the moon’s erratic movement and charting the stars. It has a somewhat less savory role in trying to deny the prize to clockmaker John Harrison who worked from 1727 to 1759 on a series of clocks designed to keep good time at sea. Along the way he invented the caged ball bearing to solve the friction problem and the technique of using two metals with different expansion rates to compensate for temperature changes. His fourth clock, H-4, grandfather of all chronometers, clearly qualified for the prize.

But the committee which awarded the prize was dominated by astronomers. The Astronomer Royal was a statutory member.  They kept setting new tests an applicant would have to pass as they worked frantically on their own solutions. In the end, it took royal intervention to get Harrison the money he was due – although most of it was a separate appropriation and not the actual prize and prestige that would’ve entailed. No one ever did meet the tightened qualifications for that.

All four of his experimental clocks are at the Observatory as is a version of the story which is a little kinder to astronomers than they deserve.  There is also a great collection of the instruments used by the astronomers to make their observations.

Interesting note: when a new a new telescope was installed in the 1800s, the prime meridian was moved about ten feet to align with the new instrument (the actual location of the meridian is arbitrary but affects everything).  British Admiralty charts are still based on the old location of the prime meridian. This small difference is now within the accuracy of some GPSes. Be careful how close you go to rocks if you’re using an Admiralty chart:-}

A very good book on all this and my source for most of the story above is Dava Sobel’s Longitude.

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