| Reference | Hertz (Hz) | Microhertz (µHz) |
|---|---|---|
| European mains electricity | 50 Hz | 50000000 µHz |
| Concert pitch A above middle C | 440 Hz | 440000000 µHz |
| An FM radio station | 100000000 Hz | 1 × 1014 µHz |
| A Wi-Fi band | 2.4 × 109 Hz | 2.4 × 1015 µHz |
| A desktop processor clock | 3 × 109 Hz | 3 × 1015 µHz |
The hertz is the SI unit of frequency, equal to one cycle per second. Its symbol is Hz. It is a derived unit whose dimension is simply one divided by time, so a quantity in hertz counts how many times something repeats in a second.
It is named after Heinrich Hertz, who between 1886 and 1889 produced and detected radio waves in the laboratory and so confirmed Maxwell's prediction that light and electromagnetic waves are the same phenomenon. The unit was adopted internationally in 1960, replacing the older and more literal cycles per second, which is still occasionally seen on vintage equipment.
Sound occupies the low end of the scale. Human hearing runs from roughly 20 hertz to 20,000 hertz, though the upper limit falls with age and is usually well below 16,000 hertz by middle life. Musical pitch is frequency: the A above middle C is fixed at 440 hertz by convention, and each octave is a doubling, so the same note an octave higher is 880 hertz.
Mains electricity alternates at 50 hertz across most of the world and 60 hertz in North America and parts of Japan and South America. The split is historical rather than technical, dating from competing equipment standards in the late nineteenth century, and it is why appliances and clocks that depend on mains frequency cannot simply be moved between regions.
Screens and computing use it constantly. A display refreshing at 60 hertz redraws sixty times a second, and higher rates reduce visible motion blur. Processor clock speeds, once quoted in megahertz and now in gigahertz, count the same thing: how many times per second the internal clock ticks.
The unit also applies to anything periodic that is not a wave. Heart rate at 60 beats per minute is one hertz, a pendulum with a one-second period swings at one hertz, and the frequency of a rotating shaft in revolutions per second is expressed identically.
One hertz equals one cycle per second, 1000 millihertz, or one thousandth of a kilohertz.
The microhertz is a unit of frequency equal to one millionth of a hertz. Its symbol is µHz. One microhertz is one cycle per million seconds, which is a little over eleven and a half days, so the unit measures things that repeat on the scale of weeks, months or years.
Asteroseismology is where it is most used. Stars oscillate, and the frequencies of those oscillations depend on their internal structure in the same way the pitch of a bell depends on its shape and thickness. Sun-like stars ring at frequencies of a few thousand microhertz, red giants at a few tens, and the largest evolved stars below one. Space telescopes such as Kepler and TESS measured these frequencies for hundreds of thousands of stars by watching their brightness vary by a few parts per million.
The technique gives quantities no other method can supply. From the oscillation spectrum an astronomer can derive a star's mass, radius and age, and the age of a star is otherwise almost impossible to determine. This is how the ages of planet-hosting stars, and therefore of their planetary systems, are now established.
Gravitational-wave astronomy has claimed the band as well. The planned space-based observatory LISA will be sensitive between roughly 100 microhertz and one hertz, a range containing merging supermassive black holes and the many thousands of close binary white dwarfs in our own galaxy. Ground-based detectors cannot reach these frequencies because seismic noise overwhelms them.
Geophysics uses the unit for the slowest tidal constituents, including the fortnightly and monthly lunar tides and the semi-annual and annual solar ones, which together produce the long-period variations that tide predictions must include. Polar motion, the wobble of the Earth's rotation axis, has a fourteen-month period corresponding to about 0.8 microhertz.
Climate and ocean science reaches lower still. The El Nino Southern Oscillation recurs irregularly every two to seven years, well below a hundredth of a microhertz, and long ice-core records resolve cycles slower again.
The Earth itself supplies a textbook example. Its rotation axis wanders in a small circle with a period of about 433 days, the Chandler wobble discovered in 1891, which is a frequency of roughly 0.027 microhertz. Detecting it took decades of positional astronomy, because a signal that slow can only be separated from drift by observing for many cycles. That is the general constraint of the band: a measurement at one microhertz needs a record of at least a few million seconds, and preferably tens of them, so instruments must be stable for years before their data can be read. The unit therefore belongs to patient sciences rather than to fast ones.
One microhertz equals 0.000001 hertz, one cycle per million seconds, or 1000 nanohertz.