| Astronomical units (au) | Nanometers (nm) |
|---|---|
| 1 Astronomical unit | 1.495978707 × 1020 nm |
| 2 Astronomical units | 2.991957414 × 1020 nm |
| 3 Astronomical units | 4.487936121 × 1020 nm |
| 4 Astronomical units | 5.983914828 × 1020 nm |
| 5 Astronomical units | 7.479893535 × 1020 nm |
| 10 Astronomical units | 1.495978707 × 1021 nm |
| 20 Astronomical units | 2.991957414 × 1021 nm |
| 25 Astronomical units | 3.7399467675 × 1021 nm |
| 50 Astronomical units | 7.479893535 × 1021 nm |
| 100 Astronomical units | 1.495978707 × 1022 nm |
| Reference | Astronomical units (au) | Nanometers (nm) |
|---|---|---|
| A sheet of A4 paper (long side) | 1.98532 × 10-12 au | 297000000 nm |
| Average adult human height | 1.13638 × 10-11 au | 1.7 × 109 nm |
| A football pitch (length) | 7.01882 × 10-10 au | 1.05 × 1011 nm |
| A marathon | 0.000000282056 au | 4.2195 × 1013 nm |
| Height of Mount Everest | 0.0000000591519 au | 8.849 × 1012 nm |
The astronomical unit is a length equal to exactly 149,597,870,700 metres, or very nearly 150 million kilometres. The symbol is au. It approximates the mean distance between the Earth and the Sun and is the standard measure for distances within the solar system.
The unit was originally defined by that orbital relationship rather than by a fixed number. Earlier definitions tied it to the properties of a hypothetical body orbiting the Sun, which meant its value depended on the gravitational constant and was subject to revision as measurements improved. The International Astronomical Union ended that dependence in 2012 by fixing the astronomical unit as an exact number of metres. The change simplified calculations and removed an inconvenience: a unit whose length shifted whenever a physical constant was refined.
Determining its value was one of the great problems of observational astronomy. Transits of Venus across the Sun's disc, observed from widely separated points on Earth, allowed the distance to be triangulated. Expeditions were mounted for the transits of 1761, 1769, 1874 and 1882, and James Cook's first Pacific voyage was organised around the 1769 event. Radar ranging to Venus in the 1960s eventually settled the figure far more precisely than any optical method.
The unit makes solar system distances legible. Mercury orbits at 0.39 au, Mars at 1.52 au, Jupiter at 5.2 au and Neptune at 30.1 au. The Kuiper Belt extends to roughly 50 au. Voyager 1, the most distant human-made object, has passed 165 au. Light takes about 499 seconds to cover one astronomical unit, so the Sun is a little over eight light minutes away.
Beyond the solar system the unit becomes unwieldy, and astronomers switch to light years and parsecs. One parsec is 206,265 au.
Spacecraft navigation depends on the unit being exact rather than approximate. Trajectories to the outer planets are computed over distances of tens of astronomical units, and an error in the length of the unit itself would propagate into every position calculation. Fixing the value in 2012 removed that source of drift from the ephemerides used for mission planning.
One astronomical unit equals 149,597,870.7 kilometres, or about 92.956 million miles.
The nanometre is one billionth of a metre. It is formed by applying the SI prefix nano, meaning 10-9, to the metre, and is written nm. The prefix derives from the Greek nanos, meaning dwarf.
This is the working scale of modern optics and electronics. Visible light spans roughly 380 nm at the violet end to 750 nm at the red end, which makes the nanometre the standard unit for describing colour in physical terms. A laser pointer emitting at 532 nm is green; one at 650 nm is red. Ultraviolet light falls below 380 nm and infrared above 750 nm.
Biology uses the unit constantly. The DNA double helix is about 2 nm across. A typical virus measures between 20 and 300 nm. Cell membranes are around 7 nm thick. These dimensions sit below the resolution of conventional light microscopes, which is limited by the wavelength of the light itself to roughly 200 nm.
Semiconductor manufacturing made the nanometre familiar outside science. Process nodes have been labelled 90 nm, 45 nm, 14 nm, 5 nm and smaller. The figure no longer corresponds to any single measurable feature on the chip, having become a marketing designation rather than a physical dimension, but the underlying structures genuinely are nanometres across. A modern transistor gate is a few tens of atoms wide.
Nanotechnology takes its name from the unit and conventionally covers structures between 1 and 100 nm. Materials often behave differently in this range because surface effects begin to dominate bulk properties.
Measuring at this scale requires instruments that do not rely on visible light. Electron microscopes resolve features below one nanometre by using electrons, whose effective wavelength is far shorter than that of light. Atomic force microscopes work differently again, dragging a sharp tip across a surface and recording its deflection. Both were essential to the development of nanotechnology, since a field cannot advance far while its subject matter remains invisible.
One nanometre equals 10 ångströms, 1000 picometres, or 0.001 micrometres. A sheet of paper is roughly 100,000 nm thick.