Short answers, pulled from the story.
Philip Astley founded the modern circus and opened his first permanent venue, Astley's Amphitheatre, in Lambeth, London on the 4th of April 1768. Astley discovered that a 42-foot diameter circle was the precise minimum space required for a horse to canter while an acrobat stood upright on its back. This geometric revelation became the foundational DNA of the modern circus.
Joshuah Purdy Brown became the first circus owner to utilize a large canvas tent in 1825. This decision allowed circuses to travel to remote towns and create a temporary city of entertainment wherever they landed. The transition from static stone arenas to mobile canvas structures began in the 19th century when the touring model superseded permanent venues.
P.T. Barnum and William Cameron Coup revolutionized the industry by introducing circus trains to transport the massive apparatus between cities in the 1870s. This innovation turned the circus into a mobile metropolis that could arrive in a town and vanish just as quickly. The American circus transformed the European model into a larger-than-life spectacle during this period.
Vladimir Lenin declared that the circus should become the people's art-form in 1919 within the Soviet Union. This political mandate led to the nationalization of Russian circuses and the establishment of the State University of Circus and Variety Arts in 1927. The Moscow State Circus began international tours in the 1950s to showcase this new approach.
Cirque du Soleil was founded in Quebec in 1984 and has become the most conspicuous success story in the genre. Estimated annual revenues exceeded US$810 million in 2009 and shows have been seen by nearly 90 million spectators in over 200 cities on five continents. The movement prioritized character development and aesthetic impact over the traditional ringmaster and animal acts.
Isaac A. Van Amburgh entered a cage with several big cats in 1833 to become the first wild animal trainer in American circus history. The earliest involvement of animals was simply the display of exotic creatures in a menagerie, but by the 19th century true animal acts had emerged. The use of animals in circuses faced growing criticism from animal rights activists from the 1960s onward.