Time is something most of us take for granted. We wake up in the morning, rush to the office, carry out our work duties with empathy, indifference or incomprehension and after having enjoyed some leisure time, another day has passed away. Maybe at the end of the day we judge our achievements by stating: "I had a good time", or we complain of having had an awful time or that time passed too fast. When making appointments many of us unconsciously treat time as space: "On that day I have no space open", we state after checking our agenda. The question "What is time?" is often answered with the adagio: time is money. This answer does not clarify what time actually implies but expresses unambiguously the dominant role of money in our acting and thinking. An old Italian saying: times are expensive but modern.

Time Is Distance
Although in ordinary life the question "What is time?" does not concern us much, many philosophers and scientists throughout history have passionately examined the issue. The topics that fascinated and continue to fascinate the brightest among the bright include: does time have a beginning and does it end (i.e. is time finite or infinite); why is time progressing from past to future and not visa versa, whereas we can arbitrarily move back and forth in space; is time a linear or cyclic phenomenon and does time exist without events causing changes in space? The last question already attracted attention in ancient times. To the question "What is time?" Aristotle replied that time is the "number of movements in respect to the before and after, and is continuous...". He proposed the existence of a close connection between time and change, which is now known as the relational theory of time. Relational theory states that the existence of time depends on the space–time relations shown by physical events, or in the words of Aristotle: "there is no time apart from change...". According to absolute theory, time exists even when there are no physical objects and events present in the universe. In contrast, relational theory implies that space–time is nothing but objects, their events and the spatio-temporal relationships between them. Webster's New World College Dictionary (4th Ed.) defines time as: the period between two events during which something exists, happens or acts; measured or measurable interval. This definition implies that the editors of the dictionary not only adopt the relational theory of time, but also associate time with a measurable quantity. Why is measuring time so important in the everyday life of the geomatics professional? That is because today’s distance measurement is done by using electromagnetic waves so that one does measure time intervals when one actually wants to measure distances. The distance is derived from time by multiplication with the speed of light.

Caesium, Hydrogen and Rubidium
This brings us, admittedly via a detour, to the topic of measurement of time with atomic clocks. Without atomic clocks, GPS would not be possible. Three main types of atomic clock can be distinguished depending on the element used: caesium, hydrogen or rubidium. The caesium-133 atom is most commonly used. Atomic clocks do not rely on atomic delay and they are not radioactive. The adjective ‘atomic’ refers to the characteristic oscillation frequencies of atoms. The measurement of vibrations is the principle of all atomic clocks. The major difference concerns, in addition to the element chosen, the way of detecting the change in energy level: caesium clocks separate atoms of different energy levels by magnetic field; hydrogen clocks maintain hydrogen atoms at the required energy level in a container of a special material so that the atoms do not lose their higher energy state too fast, and rubidium atomic clocks, which are the simplest and most compact, use a glass cell of rubidium gas that changes its absorption of light at the optical rubidium frequency when the microwave frequency is just right.

Master Clock
Atomic clocks keep time better than any other clock; they are even more steady than the Earth’s rotation. The caesium atomic clock is the most accurate in the long term: better than 1 second per 1 million years. Hydrogen atomic clocks show a better accuracy in the short term (1 week): about 10 times the accuracy of caesium clocks. The hydrogen maser oscillator provides fractional frequency stability of about 1 part in 1016 for intervals of a few hours to a day. The maser’s high frequency stability is ideally suited for a variety of space applications such as Very Long Baseline Interferometry (VLBI) from space, precision measurements of relativistic and gravitational effects and GPS. The hydrogen maser clock will therefore be Galileo’s master clock (see article by Droz).

As a result of the extremely high accuracy of atomic clocks, the world’s time-keeping system lost its astronomical basis in 1967. Then, the 13th General Conference on Weights and Measures derived the SI second from vibrations of the caesium atom, which is now internationally agreed as the interval taken to complete 9,192,631,770 oscillations of the caesium-133 atom, exposed to a suitable excitation.

Measuring time does not give an answer to the question "What is time?", just as studying bird’s feathers does not give an understanding of how birds fly – but it helps enormously to determine our position in space.