Astronomical Unit
An astronomical unit (AU) is a unit of distance used in astronomy to describe the average distance between the Earth and the Sun. It equals about 149.6 million kilometers (or about 93 million miles). The AU is useful for measuring distances within our solar system, such as the distance from Earth to other planets. For example, Mars is about 1.5 AU from the Sun. Using the AU makes it easier to understand and compare distances between planets without dealing with very large numbers. The symbol for astronomical unit is "AU".
Chain
A chain is a unit of length traditionally used in land surveying. The most common version is the Gunter’s chain, which equals 66 feet or 22 yards, approximately 20.1168 meters.
The chain was introduced by English clergyman Edmund Gunter in the 17th century to simplify land measurement. One chain consists of 100 links, each 0.66 feet long. It became a standard unit in British and American land surveying and was instrumental in defining land areas, especially in the U.S. Public Land Survey System.
Key Conversions:
1 chain = 66 feet
1 chain = 22 yards
1 chain ≈ 20.1168 meters
10 chains = 1 furlong
80 chains = 1 mile
Though now largely replaced by metric units and modern surveying tools, the chain still appears in older land deeds, maps, and rural property descriptions.