The "actus" was a unit of length used in ancient Rome. It was primarily a measure of distance and was approximately equivalent to 120 Roman feet or about 35.5 meters (116.5 feet) in modern measurements.
The actus was commonly used for various purposes in ancient Rome, including land measurement, surveying, and property boundaries. It was also used in the construction of roads, aqueducts, and other civil engineering projects.
It's important to note that ancient Roman units of measurement, including the actus, were not standardized in the same way that modern units are today. Therefore, the exact length of an actus could vary in different regions and over time.
The use of the actus and other Roman units of measurement has largely been replaced by modern metric and imperial systems in contemporary times, but they remain of historical interest and can still be encountered in the study of ancient Roman culture and history.
In the field of atomic and molecular physics, an "atomic unit of length" is a unit of measurement that is used to express distances at the atomic and molecular scale in a dimensionless way. It is part of a system of atomic units (a.u.) that simplifies calculations involving fundamental physical constants and properties of atoms and molecules.
The atomic unit of length (a.u. of length) is defined in terms of the Bohr radius (a₀), which is a fundamental constant in atomic physics. The Bohr radius is approximately 0.52917721067 angstroms (Å) or 5.2917721067 x 10^-11 meters (m).
In atomic units, the Bohr radius is set to exactly 1 a.u. of length. Therefore, when using atomic units, distances are expressed relative to the Bohr radius, and the value of 1 a.u. of length corresponds to the typical size scale of atomic and molecular structures.
The use of atomic units simplifies many quantum mechanical calculations and allows physicists and chemists to work with dimensionless quantities, making it easier to compare and analyze atomic and molecular properties.