Shedding light on the art of timekeeping
We have always lived in a world illuminated by Light and marked by the relentless ﬂow of Time. In spite of the diﬃculty of ﬁnding a universal deﬁnition for them, Light and Time are essential elements of our existence. They govern countless aspects of our practical life and accompany us in various cultural and sentimental experiences. The earliest people on the planet naturally entrusted the organization of their activities in the light coming from celestial bodies. The periodic character of the most basic astronomical motions was immediately recognized: the Sun rise and set (Earth’s rotation around its axis), the appearance of the highlighted portion of the Moon (Moon’s revolution around the Earth), and the weather periodical behavior that seemed to be related to the movement of the Sun with respect to the stars (Earth’s revolution around the Sun). The units of days, months, and years accordingly followed. The main disadvantage of Nature’s clocks resided in that the scale unit was too large for many practical purposes. Consequently, natural oscillators soon began to be supplemented by those constructed by mankind. Around 3500 BC Egyptians already divided the time of the day into shorter sections by observing the direction of the shadow cast from obelisks or sundials by the Sun, depending on its position in the sky . It is amazing to note that the ancient and honored Earth-Sun clock met many of the most demanding requirements that the scientiﬁc community today exacts from an acceptable standard: ﬁrst, it is universally available and recognized; second, it involves neither responsibility nor operation expenses for anyone; third, it is pretty reliable and we cannot foresee any possibility that it may stop or lose the time. In spite of all these nice features, however, this clock does not represent an extremely stable timepiece. According to our current knowledge in astronomy, ﬁrst, Earth’s orbit around the Sun is elliptical rather than a perfect circle, which means that Earth travels faster when it is closer to the Sun than when it is farther away. In addition, Earth’s axis is tilted with respect to the plane containing its orbit around the Sun. Finally, Earth spins at an irregular rate around its axis of rotation
and frequency domain
and even wobbles on it. The latter eﬀect is due to the circumstance that, as Earth is neither perfectly symmetrical in shape, nor homogeneous, nor ideally rigid (its mass distribution constantly changes over time), its rotation axis does not coincide exactly with the ﬁgure axis. For the same reason, even natural disasters of exceptional importance may perturb the clock mechanism. For example, it has been calculated that the recent earthquake in Japan (March 2011) has moved Earth’s ﬁgure axis by a few milliarcseconds or, in other words, rearranged Earth’s mass bringing more of it a bit closer to the rotation axis. This should have slightly increased Earth’s rate of spin, thus shortening the length of the day by less than 2 microseconds . Such a small variation has no practical eﬀect in daily life, but it is of interest for precision measurements of space and time.