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.
| Unit | Symbol | 1 Microhertz equals | 1 of these equals | Converter |
|---|---|---|---|---|
| Nanohertz | nHz | 1000 nHz | 0.001 µHz | Microhertz to Nanohertz |
| Millihertz | mHz | 0.001 mHz | 1000 µHz | Microhertz to Millihertz |
| Hertz | Hz | 0.000001 Hz | 1000000 µHz | Microhertz to Hertz |
| Kilohertz | kHz | 0.000000001 kHz | 1000000000 µHz | Microhertz to Kilohertz |
| Megahertz | MHz | 1 × 10-12 MHz | 1000000000000 µHz | Microhertz to Megahertz |
| Gigahertz | GHz | 1 × 10-15 GHz | 1 × 1015 µHz | Microhertz to Gigahertz |
| Terahertz | THz | 1 × 10-18 THz | 1 × 1018 µHz | Microhertz to Terahertz |