Q: When did we first understand that time differences existed between different parts of the world? (via Quora)
A. Even the ancient Greeks knew of this, 2000 years ago. (Relatedly, Eratosthenes used variances in the apparent height of the sun in different cities at local solar noon — mid-day, as determined by sundial — to help prove the Earth was round.)
Local solar noon was the standard for timekeeping for centuries. Even when mechanical clocks were invented in the 1300’s, major cities would use local solar noon to set their clocks.
As a result, the clocks in one city would strike noon at a different time from another city some distance to the east or west; and the further apart the cities, the greater the disparity. This “different noons in different places” was well-known, and didn’t cause problems, due to the slow pace of life and commerce.
But it became more important with long-distance sea navigation in the 1600-1700s. In the absence of landmarks on the open sea, you can easily figure N-S latitude by measuring the height of the sun (or moon or known star…) above the horizon at noon or midnight; but you need to know the time to determine your E-W longitude: Seafarers would compare the local solar time to the time back at home base, and use the difference to determine the E-W separation.
An error in timekeeping would mess up your calculations, and potentially put your ship on the rocks somewhere. So, famously, the British Admiralty even offered a reward to the first person who could design a portable clock accurate enough for long-distance navigation; this led to the invention of the first marine chronometer. (See: https://en.wikipedia.org/wiki/Longitude_rewards) Knowing the exact time difference between locations enabled precision navigation.
But basing everything on local solar time caused new problems in the late 1800’s, with the advent of fast, long distance train travel (mainly in the US). If two trains needed to share the same track, and both left their station at noon, whose “noon” would they use? How would you coordinate the schedules of trains leaving at different, but locally-accurate solar-based times?
Uniform, standardized time zones were the answer, even though this divorced local timekeeping from solar time. By having everyone in a region set their clocks the same, regardless of local solar noon, trains leaving different stations even hundreds miles apart could schedule travel so as not to end up on the same piece of track at the same time.
Today, only those cities that happen to be in the middle of a time zone have clock time more or less match solar time. On the east side of a time zone, the clocks run a bit behind solar time; on the west side of time zones, the clocks run a bit ahead.
Today, that’s still too sloppy for precise science and commerce. For example, on the internet, every packet of data is time-stamped, and that timestamp has to make sense anywhere in the world, regardless of time zone, daylight savings time, local custom, or anything else. So, many applications and industries use Universal Time (https://en.wikipedia.org/wiki/Universal_Time) to coordinate activities around the world.
Ironically, Universal Time is based on — wait for it — solar noon at Greenwich, England!
So, in a way, we’ve gone full circle, and back to the same solar time used by the Greeks, 2000+ years ago. 🙂
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