X-unit
The X-unit (symbol: xu) is a very small unit of length used primarily to measure wavelengths of X-rays and gamma rays. It was introduced in the early 20th century for precision measurements in X-ray crystallography.
1 X-unit ≈ 0.1 picometers (pm) or 10⁻¹³ meters
It allows scientists to express extremely small wavelengths typical of X-rays, which are on the order of atomic spacing
The exact definition has varied historically, but the unit remains useful for comparing wavelengths in X-ray spectroscopy and crystallography
Though largely replaced by the picometer or ångström in modern usage, the X-unit played a crucial role in early developments of atomic-scale measurement.
Gigameter
A gigameter (Gm) is a unit of length in the metric system, equal to 1 billion meters (1 Gm = 10⁹ meters). Gigameters are used to measure very large distances, often within the solar system. For example, the average distance from Earth to the Sun is about 150 gigameters, or 1 astronomical unit (AU). While not commonly used in everyday life, gigameters are helpful in astronomy and space science to describe distances between planets or other celestial bodies.