| Microhertz (µHz) | Gigahertz (GHz) |
|---|---|
| 1 Microhertz | 1 × 10-15 GHz |
| 2 Microhertz | 2 × 10-15 GHz |
| 3 Microhertz | 3 × 10-15 GHz |
| 4 Microhertz | 4 × 10-15 GHz |
| 5 Microhertz | 5 × 10-15 GHz |
| 10 Microhertz | 1 × 10-14 GHz |
| 20 Microhertz | 2 × 10-14 GHz |
| 25 Microhertz | 2.5 × 10-14 GHz |
| 50 Microhertz | 5 × 10-14 GHz |
| 100 Microhertz | 1 × 10-13 GHz |
| Reference | Microhertz (µHz) | Gigahertz (GHz) |
|---|---|---|
| European mains electricity | 50000000 µHz | 0.00000005 GHz |
| Concert pitch A above middle C | 440000000 µHz | 0.00000044 GHz |
| An FM radio station | 1 × 1014 µHz | 0.1 GHz |
| A Wi-Fi band | 2.4 × 1015 µHz | 2.4 GHz |
| A desktop processor clock | 3 × 1015 µHz | 3 GHz |
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.
The gigahertz is a unit of frequency equal to one thousand million hertz. Its symbol is GHz. It is the range of modern wireless communication and of the clock inside every current computer, and almost nothing in it was in everyday use before the 1990s.
Wi-Fi occupies two main bands, at 2.4 and 5 gigahertz, with a newer band near 6. The 2.4 gigahertz band is crowded because it is licence-free almost everywhere and shared with Bluetooth, cordless phones and microwave ovens, which operate at 2.45 gigahertz because water molecules absorb energy efficiently there. The 5 gigahertz band offers more channels and higher rates but is absorbed more strongly by walls, so it covers a smaller area.
Mobile telephony spans the range. Earlier generations used bands below 1 gigahertz for their long reach into buildings and across countryside, while newer allocations at 1.8, 2.6 and 3.5 gigahertz carry more data over shorter distances. The higher a frequency, the more bandwidth is available and the less it penetrates, and every generation of network design is a negotiation between those two facts.
Computer clock speeds crossed into gigahertz around the year 2000 and stalled near four a few years later. The limit is thermal: power dissipation rises steeply with frequency, and beyond about four gigahertz a conventional silicon processor cannot shed the heat. Performance gains since then have come from adding cores, widening instruction issue and improving memory hierarchies rather than from raising the clock.
Satellite links, radar and radio astronomy all work here. Weather radar typically runs between 2.7 and 5.6 gigahertz, air traffic control radar in similar bands, and the microwave background radiation that fills the universe peaks near 160 gigahertz, well above the range of terrestrial communication.
Millimetre-wave systems at 24 gigahertz and above are used for automotive collision-avoidance radar and for the highest-capacity mobile network bands. Their very short wavelengths allow small antennas and enormous data rates but are blocked by almost anything, including rain and foliage.
One gigahertz equals one thousand million hertz, 1000 megahertz, or 0.001 terahertz.