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Before standard time, each town kept its own time by the sun: noon was when the sun stood highest in its sky. Because the Earth turns one degree every four minutes, noon in Bristol came about ten minutes after noon in London.

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Town clocks usually showed local mean time: sundial time averaged over the year, because the real sun runs up to about a quarter of an hour fast or slow across the seasons. The difference grew with distance east or west. In Britain, Oxford was five minutes behind London and Leeds six; in India and North America, cities could be an hour or more apart. That hardly mattered while journeys took days, but trains and the telegraph made it a problem. North American railways switched to five standard time zones on 18 November 1883, and in 1884 delegates from 25 countries meeting in Washington chose the meridian through Greenwich as the world's zero line of longitude. The Ottoman Empire had its own system, alaturka time, which reset clocks to 12 at every sunset; Turkey replaced it with the international 24-hour day in 1926.

Türkçe oku
Saat dilimleri kurulmadan önce her kasaba kendi saatini güneşe göre ayarlardı: Güneş gökte en yükseğe çıktığında o kasabada öğle olurdu. Dünya her dört dakikada bir derece döndüğü için Bristol'de öğle, Londra'dakinden on dakika kadar sonra gelirdi.

Kasaba saatleri genellikle yerel ortalama zamanı gösterirdi. Bu, güneş saatinin yıl boyunca ortalaması alınmış hâliydi; çünkü güneş saati, mevsime göre çeyrek saate kadar ileri ya da geri kalır. Doğuya ya da batıya gidildikçe fark büyürdü. Britanya'da Oxford Londra'nın beş, Leeds altı dakika gerisindeydi; Hindistan'da ve Kuzey Amerika'da şehirler arasındaki fark bir saati bulabiliyordu. Yolculuklar günler sürerken bu pek önemli değildi, ama trenler ve telgraf onu bir soruna dönüştürdü. Kuzey Amerika demiryolları 18 Kasım 1883'te beş standart saat dilimine geçti. 1884'te Washington'da toplanan 25 ülkenin temsilcileri de Greenwich'ten geçen meridyeni dünyanın sıfır boylamı olarak seçti. Osmanlı İmparatorluğu'nun da kendine özgü bir düzeni vardı: her gün güneş batarken saatlerin 12'ye ayarlandığı alaturka saat. Türkiye bu düzeni 1926'da bırakıp uluslararası 24 saatlik güne geçti.

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Next card in the archiveRailways pushed Britain onto one national time. In November 1840 the Great Western Railway began running its trains on London time; by the end of 1847 almost every British railway had followed, and in 1880 Greenwich Mean Time became Great Britain's legal time.Read next →