The "long cubit" is a historical unit of length used in various ancient cultures, including ancient Egypt and Mesopotamia. It was a longer version of the standard cubit, which was used for measuring length in these civilizations.
The exact length of the long cubit could vary depending on the region and time period, but it was typically longer than the standard cubit. In ancient Egypt, for example, the long cubit was approximately 52.3 centimeters (about 20.6 inches), while the standard cubit was around 45 centimeters (about 17.7 inches).
These cubits were often used in construction, architecture, and other applications where length measurements were needed. The long cubit was particularly useful for larger and more precise construction projects.
It's important to note that the long cubit is a historical unit of measurement and is not part of modern measurement systems like the metric system or the imperial system.
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.