Millihertzian

Millihertzian is a term used to describe phenomena or frequencies that occur in the millihertz range. A hertz (Hz) is a unit of frequency equal to one cycle per second. Therefore, one millihertz (mHz) is equal to one-thousandth of a hertz, or one cycle per thousand seconds. This corresponds to an extremely low frequency.

In practice, millihertzian phenomena are characterized by very long periods. For instance, a signal at 1 mHz would have a period of 1000 seconds, which is approximately 16.7 minutes. Signals at even lower frequencies, such as 0.1 mHz, would have periods of 10,000 seconds, or about 2.8 hours.

These low frequencies are often observed in natural geological and astronomical processes. For example, the Earth's free oscillations, which are seismic vibrations that can persist for long periods after a large earthquake, can have frequencies in the millihertz range. Tidal forces, which are caused by the gravitational pull of the Moon and Sun, also operate on very long timescales and can be considered in the millihertz domain. In astrophysics, processes like the pulsation of very large stars or the orbital periods of distant celestial bodies might also fall into this frequency range. Studying millihertzian signals requires sensitive instruments capable of detecting subtle, long-duration variations.