When was the issue of time zones at different longitudes first described?

Question

What are the earliest recorded acknowledgements of the concept that motivates time zones - that the sun and other celestial objects appear in different parts of the sky to people at different longitudes, observing at the same moment?

I would imagine that this would come out as a theoretical consequence of understanding that the world is round before it would be practically observed, since the latter requires either long-distance near-instantaneous communications or some sort of sun-independent clock that you can bring from place to place.

This question was motivated by consideration of a question on Mi Yodeya asking for evidence of such as realization in the Talmud.

Answer 1

The first written account of the phenomenon might be due to Eratosthenes (276-195 BC), the head of Alexandrian Museum and contemporary of Archimedes, with whom he corresponded. Eratosthenes is credited with proposing to use longitude and latitude to specify locations on Earth, an idea developed further by Hipparchus (190-120 BC), "the father of astronomy", and later used by Ptolemy in his maps. Hipparchus appears to be the first to suggest explicitly that local time determines longitude. Interestingly enough, Ctesibius (285–222 BC) invented the first fairly precise and constantly working clock, the clepsydra, around the same time.

However, it is likely that differences in local occurrences of astronomical events were noticed much earlier by seafarers. NOVA mentioned in their special that Phoenicians knew, possibly as far back as late second millenium BC, that "at any one time in the year at any one point on the globe, the sun and stars are found above the horizon at certain fixed "heights"—a distance that mariners can measure with as simple an instrument as one's fingers", and used it for navigation.

Unfortunately, there was little systematic development of navigational uses of local time until the 16-th century. In 1502 Amerigo Vespucci wrote "I maintain that I learned [my longitude]... by the eclipses and conjunctions of the Moon with the planets", and Johannes Werner developed the method systematically in his 1514 commentary to a translation of Ptolemy's Geography. Subsequent proposals for longitude determination based on astronomical observations were given by Galileo and Halley among others. However, the calculations involved were too complicated for regular use, and the issue came to be known as the problem of longitude. Spanish kings even offered a prize for solving it in 1567, and increased it in 1598. The idea of using chronometers, which ultimately resolved the longitude problem, is usually attributed to Gemma Frisius (1508-1555), but it gained little currency until Huygens built precise pendulum clocks in late 17th century.

Answer 2

This is an interesting question because the answer depends on how you actually conceptualise time. In the ancient world, the usual system of time keeping involved “seasonal hours”. This means that the time from sunrise to sunset is divided into 12 equal “hours of the day” and the time from sunset to sunrise is divided into 12 equal “hours of the night”. The time of sunset will be (by definition) “the beginning of the first hour of the night” at all locations on Earth. The "time" of sunset is thus the same everywhere and at every time of the year.

However, for astronomical calculations the ancients also had the concept of “equinoctial hours”, where the time from midnight to midnight (where midnight means the halfway point between sunset and sunrise) is divided into 24 equal hours (as in modern clock time). In this system sunset will be at exactly the same time in all locations on the same latitude, but will be different at different latitudes. This fact was used by Ptolemy when he defined the latitude of different locations not in degrees north or south of the equator, but by indicating the maximum local length of daylight in equinoctial hours (and subdivisions of the same). Similarly, he was fully aware that astronomical phenomena like equinoxes or new moons occur at different times at different longitudes.

This means that Ptolemy had the mathematical and astronomical knowledge that could have enabled him to slice the globe into 24 equal time zones, counting full hours east and west of Alexandria. But there was no practical advantage in doing this, especially since for astronomers it is much more useful to know the exact longitude (in degrees, minutes, seconds…) of a locality. A system of time zones, and of standard time within each zone, becomes of interest only with the invention of the railway and other means of rapid transportation.