Telescopes and clocks have a long history together. For
centuries, a star sighting was the most accurate way to find
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.
- the time, if you knew your location
- your location, if you knew the time
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
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
- 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 /
1999 Nov 30