--- title: Pendulums, Greenwich and railway time, atomic clocks and phones — a lesson of Mova's school · trees and pragmatics description: «Pendulums, Greenwich and railway time, atomic clocks and phones», a lesson of the world course of Mova's primary school, sentence by sentence with dependency trees and pragmatics. It begins: A good clock needs something that repeats at a steady beat and something that counts the beats. --- [Mova](https://md.mova.elamur.ai/free/en/) · [this page in HTML](https://mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/) [home](https://md.mova.elamur.ai/free/en/) / [corpora](https://md.mova.elamur.ai/free/en/corpora/) / [school](https://md.mova.elamur.ai/free/en/corpora/school/) / [World](https://md.mova.elamur.ai/free/en/corpora/school/world/) / Pendulums, Greenwich and railway time,… # Pendulums, Greenwich and railway time, atomic clocks and phones Lesson 123 of the world course of Mova's primary school: 23 sentences, each with its syntax tree and pragmatics. The lessons are written for Mova, and their agreed trees are part of Mova corpus 0.4 (CC BY-SA 4.0). How Mova does on its tests: [the lesson at school](https://md.mova.elamur.ai/free/en/school-run/world/123-pendulums-greenwich-atomic-clocks/). - [0](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/0/) — A good clock needs something that repeats at a steady beat and something that counts the beats. - [1](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/1/) — Galileo noticed in a cathedral that a swinging lamp took the same time for each swing, big or small, timing it with his pulse. - [2](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/2/) — In 1656 Christiaan Huygens built a pendulum clock, and good clocks went from about 15 minutes wrong a day to about 15 seconds: 60 times better. - [3](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/3/) — A clock can even tell you where you are. - [4](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/4/) — The Earth turns 15 degrees an hour, so if your clock still shows London time and the Sun is highest at 2 pm, you are 30 degrees west of London. - [5](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/5/) — Pendulums can't keep time on rolling seas, so this was impossible until John Harrison, a carpenter born in Yorkshire, built sea clocks in the 1700s. - [6](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/6/) — Once, every town kept its own Sun time: Bristol's Corn Exchange clock still has two minute hands, 10 minutes apart, for Bristol and London time. - [7](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/7/) — Trains needed one time, so British railways used London time from the 1840s, and in 1884 countries chose Greenwich as 0 degrees longitude, the Prime Meridian, from which time zones are counted. - [8](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/8/) — Today a quartz crystal in a watch wiggles 32,768 times a second, and atomic clocks count the "ticks" of caesium atoms so exactly they'd lose less than a second in millions of years. - [10](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/10/) — Before that, people set their clocks by shared signals: the red time ball at Greenwich drops at 1 pm every day, the BBC "pips" mark the hour, and in Britain you could dial 123 for the speaking clock. - [11](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/11/) — Galileo noticed that a pendulum takes the same time to swing whether the swing is big or small. - [12](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/12/) — In 1656 Christiaan Huygens built a pendulum clock, far more accurate than any clock before. - [13](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/13/) — A pendulum about 1 metre long takes about 1 second to swing one way. - [14](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/14/) — Good clocks before pendulums could be about 15 minutes wrong in a day, and pendulum clocks about 15 seconds: 900 seconds ÷ 15 = 60 times better. - [15](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/15/) — John Harrison, a carpenter born in Yorkshire, built sea clocks in the 1700s that kept good time on rolling ships, so sailors could find how far east or west they were. - [16](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/16/) — The Earth turns 15 degrees an hour, so each hour of difference between home time and local Sun time means 15 degrees east or west. - [17](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/17/) — The red time ball at the Royal Observatory, Greenwich, has dropped at 1 pm every day since 1833 so ships could set their clocks. - [19](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/19/) — British railways chose London time, called railway time, in the 1840s. - [20](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/20/) — In 1884 countries chose Greenwich as 0 degrees longitude (longitude says how far east or west a place is), the Prime Meridian. - [21](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/21/) — A quartz crystal in a watch vibrates 32,768 times every second. - [22](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/22/) — The first accurate atomic clock was built in London in 1955; today a second is measured by counting over 9 billion tiny ticks of caesium atoms. - [23](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/23/) — Phones get the time from the phone network or from GPS satellites, which carry atomic clocks. - [24](https://md.mova.elamur.ai/free/en/corpora/school/world/123-pendulums-greenwich-atomic-clocks/24/) — Britain's speaking clock (dial 123) began in 1936, and the BBC's six "pips" have marked the hour since 1924; the last pip marks the exact start of the hour. - ← previous: [Clocks before clocks: sundials, water clocks, candles and hourglasses](https://md.mova.elamur.ai/free/en/corpora/school/world/122-clocks-before-clocks/) - next →: [Decades, centuries and millennia](https://md.mova.elamur.ai/free/en/corpora/school/world/124-decades-centuries-millennia/) - ↑ back to [World](https://md.mova.elamur.ai/free/en/corpora/school/world/)