Greenwich, England

Telescopes and clocks have a long history together. For centuries, a star sighting was the most accurate way to find

the time, if you knew your location
your location, if you knew the time

The British built telescopes to do star sightings, and put one in a borough of London called Greenwich. Then they gridded out the whole planet with lines of latitude and longitude.

Lines and circles

The origin of latitude lies naturally at either the equator or at a pole; the British chose to put it at the equator. In contrast, the origin of longitude is arbitrary: you can put it anywhere you want. Since the British were drawing the lines, they drew the line of 0° longitude through Greenwich—through the focal point of their telescope.

As long as the telescope at Greenwich was in use, it defined the origin of longitude. Today, the observatory is a museum, and the line of 0° longitude is marked by a brass strip set into the pavement.

Seconds and miles

The flip side of this whole business was timekeeping. If you're on a ship at sea, a star sighting will tell you your latitude, which is good. A star sighting will also tell you your longitude, provided you know what time it is, which is bad, because they generally didn't.

The problem was that until the 19th century, the only timebase with sufficient stability for nautical use was the pendulum, and pendulums don't work on the rolling seas. Uncertainty in time becomes uncertainty in space with a conversion on the order of 10 miles for each minute, and many lives and ships were lost for it.

Old and new

The museum is full of telescopes, navigation instruments, and clocks, both old and new. Two displays caught my eye

A GPS receiver, battery powered, LCD display, running, showing
```
0° 0.0145' W longitude 
```
or about 35 feet off of the prime meridian. (Well, that's where it was...)
A 4 foot cast iron cylinder, with glass caps, a pendulum inside, a vacuum pump and assorted other machinery outside. A plaque explained that this was the standard high-precision timebase used in the Soviet Union through the 1970s? 1980s? by TV stations, and radio stations, or anyplace else that needed to know the correct time.
The plaque was tactful about it, but what it basically said was that the Soviets used these monsters because they could build a vacuum chamber, and they couldn't build a quartz clock...

Notes

brass strip: Near the observatory, the brass strip gives way to an LED display that scrolls a continuous newsfeed along the prime meridian, courtesy of Dow Jones.
1970s? 1980s?: I forget the exact date; it was quite recent.

Steven W. McDougall / resume / swmcd@theworld.com / 1999 Nov 30