28 May 2005

That’s Zulu not Sulu! (but live long and prosper anyhow)

Posted by Dan Satterfield

My wife always laughs when someone asks me the time because I will tend to give the unsuspecting questioner the response…WHERE?

Greenwich Observatory, Greenwich, England, UKTime depends on where you are. (It also depends on how fast you are moving but let’s leave the relativity stuff for another blog)

I’m also asked what the “Z” means on radar images and satellite images we show.

The answer is GMT.

Dan Stands on the Prime MeridianZ stands for Greenwich Mean Time. The official time of science.

Science must have a single reference for time that everyone agrees upon. By world wide agreement that reference is GMT.

GMT is the time in Greenwich, England at Longitude zero. Why Greenwich, and why is it at longitude zero??

Well that is a great story…Read on!
If your at sea on a ship, finding your latitude above the equator is not too difficult. Just look at the North Star (Polaris). At the North pole, Polaris is 90° degrees overhead. At the equator, it is on the horizon. If you go out at night here and measure the angle of Polaris above the horizon you will get 35° degrees. Our latitude here in Huntsville.

You can also do the same with the sun at local noon… as long as you know the date. Mariners have done this for several centuries now!

The difficult thing for ship captains of the past, was not latitude but Longitude. It was nearly impossible to find out accurately how far East or West of “home” they were. Many times the error in position was only found when a ship ran into an Island or continent at night and promptly sunk!

In the early 1700’s, when British sea power ruled the waves, A prize was offered to anyone who could solve the problem of longitude. Enter James Harrison. He solved it and then had to fight for years to get his prize. (An appeal, directly to the King himself was at last successful!) & (A&E screened a film a few years ago based on these events called “Longitude“.)

Harrison realized a key fact. If you divide the spherical earth into 360° degrees and it takes 24 hours for the earth to rotate, then the sun moves 15° degrees per hour.

Say for instance, we are in the Atlantic ocean. 15° degrees longitude west of Greenwich. The sun will reach it high point at noon precisely 1 hour later than it does back in England.

It would be a simple thing to produce a table of exactly when local noon was in Greenwich. This was already known in the 1700’s very precisely. Harrison knew he could solve the longitude problem simply by inventing an accuarate clock!

Easier said then done. Clocks of the day used pendulums and they did not swing to well on a ship riding out a storm on the North Atlantic. Even if they could keep it running, the accuaracy would not be good.

To get an accuarate reading of longitude, a clock was needed that would only be off a few seconds in a year. If the clock ran fast or slow at a certain rate, that would be ok! The correction could be added in.

Harrison built 4 clocks. He called them Chronometers. The famous explorer James Cook took one with him around the world inthe 1770’s and it was found to be very accuarate.

Cook would mark the time when the sun was at noon wherever he was. He would then look at the chronometer to see what time it was in Greenwich. A few calculations and BINGO…he knew where he was on the planet.

In an age of great inventions, Harrison’s Chronometer was one of the greatest.

As Chronometers came into wide use, a single reference point for time was needed. That reference was already there. Greenwich England. Home of the Royal Observatory for the past 400 years!

It is still today the world standard for time. Now they have an atomic clock though!
I visited Greenwich a few years ago and stood on the zero longitude line! Harrison’s clocks are on display there as well!

Gosh it is 0948GMT! I gotta get to bed!