Natural oscillators

Abstract

Observations of the motion of celestial bodies reveal periodic phenomena that are sometimes simple but often highly complex and due to the superposition of a very large number of sinusoidal processes that have different periods and are difficult to isolate, especially if the amplitudes are comparable. These periods can provide standards for the measurement of frequency and time. For example, the rotation of the Earth on its axis can be used to define the sidereal day. The rotation of the Earth around the Sun gives us the tropical year, and the Moon orbits the Earth in a complicated motion that involves several periods, including the 28-day period that defines the month. All these processes have a very clear origin: they are projections of rotational motion on to an axis (the sine is the projection of the radius of a circle on to a diameter). The difficulty lies in the decomposition of the apparent motion into a series of rotations with different periods and amplitudes. Other periodic astronomical phenomena (pulsars, variable stars, cepheids and so on) have a more complicated origin and have given rise to numerous studies.