World Time and the Division of Time Zones

Daniel J. Boorstin once said: "The greatest human discovery is time." Time is one of the most fundamental physical quantities, and today's timekeeping has reached an extraordinary level of precision.

Time is invisible and intangible — it can only be perceived through motion.

The entire history of timekeeping is a story of searching for periodic motion, and using fractions of that motion's cycle as the basic unit of measurement: the second.

The flow of shadows, the drift of sand, the movement of water, and the unfolding of events — all have been converted into the music of time, faithfully recording human activity on Earth. Only with clocks and watches can we mark the day of the week, the month, the year — every second, every minute, every hour, every day — liberating humanity from the monotonous, repetitive cycles of nature.

I. The Measurement of Time

(I) Time

Time has no beginning and no end. It can be represented by a straight line. Time is irreversible — it flows forever in one direction, from a past without origin, through the present, and onward into an endless future.

(II) Measuring Time

Time can be measured, and all manner of clocks and watches are instruments for doing so.

The measurement of time takes two forms:

1. Moment Measurement
Measuring a single instant in time (a point on the timeline).
Moment measurement requires both a unit and a reference point. Midnight is the standard reference point for timekeeping. For example, a train departing at 17:30 — "hours" and "minutes" are the units of measurement.

2. Duration Measurement
Measuring the interval between two instants (two points on the timeline).
Duration measurement requires only a unit — no reference point is needed. For example, a new high-speed rail train traveling from Beijing to Guangzhou takes 8 hours from departure to arrival. "Hours" is the unit; the starting point is irrelevant.

(III) Establishing a Time Standard

In ancient times, the standard for measuring time was the movement of celestial bodies — the principle of "rise with the sun, rest at sunset." Just as measuring length requires a ruler, measuring time requires a standard (Figure 1–1–12).

1. True Solar Day
The period between two successive passages of the sun's center over the local meridian at noon is called one day.

Because the Earth's distance from the sun changes as it orbits, the speed of Earth's rotation also varies — meaning each true solar day is slightly different in length. The longest and shortest true solar days differ by several minutes. Therefore, the true solar day is not a reliable time standard.

2. Mean Solar Day
The ancient Egyptians divided one day into 24 hours, one hour into 60 minutes, and one minute into 60 seconds.

In 1820, French scientists convened and decided to use the "average value" of one Earth rotation per year to define the length of a day. This became the universally accepted standard for timekeeping. One mean solar day contains 86,400 mean solar seconds.

3. Ephemeris Second
In 1939, British scientist W.H. Shortt discovered through long-term astronomical observation that the period of Earth's rotation changes measurably each year, making the "mean solar day" based on Earth's rotation an unstable time standard.

In 1956, scientists proposed a new timekeeping standard based on Earth's revolution around the sun. However, since the orbital period also varies slightly each year, they traced back to January 1, 1900 at 12:00 noon, using the length of the tropical year at that moment as the reference. This standard, refined over many years of observation, achieved significantly higher accuracy.

One ephemeris second equals 1/31,556,925.9747 of a tropical year. The ephemeris second came into official use in 1960.

4. Atomic Second
With advances in science and technology, it was discovered that cesium atoms possess an extraordinarily stable vibration frequency. In 1967, the 13th General Conference on Weights and Measures officially decided to replace the ephemeris second with the atomic second. The international standard second is derived from the cesium atomic clock, with an accuracy of 1 second per 3 million years.

When cesium-133 atoms transition between two hyperfine energy levels in their ground state, they absorb or emit electromagnetic radiation. The duration of 9,192,631,770 cycles of this radiation constitutes one international atomic second. The international standard atomic second came into effect on January 1, 1972 at 00:00, ending the era in which all time standards were obtained through complex measurements of celestial motion.

The definition of the "second" was thus decoupled from celestial mechanics. From that point on, timekeeping work in all nations moved from astronomical observatories to metrology laboratories.

From Atomic Precision to the Watch on Your Wrist

Millennia of human ingenuity — from sundials to atomic clocks — have been devoted to one pursuit: measuring time with ever-greater precision and beauty.

Today, that pursuit lives on in fine mechanical watchmaking. If you appreciate the art of timekeeping, explore our collections:

• Men's Full Diamond Octagonal Luxury Watch — where precision engineering meets diamond-set opulence.
• Vintage Leather Apple Watch Band — bridging the timeless craft of leather artisanship with modern wearable technology.