A Circus is a company of performers who put on diverse entertainment shows that may include clowns, acrobats, trained animals, trapeze acts, musicians, dancers, hoopers, tightrope walkers, jugglers, magicians, unicyclists, as well as other object manipulation and stunt-oriented artists. The term circus also describes the performance which has followed various formats through its 250-year modern history. Philip Astley is credited with being the father of the modern circus when he opened the first circus in 1768 in England. A skilled equestrian, Astley demonstrated trick riding, riding in a circle rather than a straight line as his rivals did, and thus chanced on the format which was later named a "circus". In 1770 he hired acrobats, tightrope walkers, jugglers and a clown to fill in the pauses between acts. Performances developed significantly through the next fifty years, with large-scale theatrical battle reenactments becoming a significant feature. The traditional format, whereby a ringmaster introduces a varied selection of acts that mostly perform choreographed acts to traditional music, developed in the latter part of the 19th century and continued almost universally to be the main style of circus up until the 1970s. As styles of performance have developed since the time of Astley, so too have the types of venues where these circuses have performed. The earliest modern circuses were performed in open air structures with limited covered seating. From the late 18th to late 19th century, custom-made circus buildings (often wooden) were built with various types of seating, a centre ring, and sometimes a stage. The traditional large tents, commonly known as "Big Tops" were introduced in the mid-19th century as touring circuses superseded static venues. These tents eventually became the most common venue and remain so to the present day. Contemporary circuses perform in a variety of venues including tents, theatres and casinos. Many circus performances are still held in a ring usually 13 m (42 ft) in diameter. This dimension was adopted by Astley in the late 18th century as the minimum diameter that enabled an acrobatic horse rider to stand upright on a cantering horse to perform their tricks. Contemporary circus has been credited with reviving the circus tradition since the 1980s when a number of groups introduced circuses based almost solely on human skills and which drew from other performing art skills and styles.