On Dec. 30, 1930, the first-ever photo of the Earth's curvature was taken by Lieutenant Colonel Albert William Stevens, who was an officer in the U.S. Army Air Corps and an aerial photographer.
This photo is one Williams took over South Dakota in 1936 during a record-breaking balloon flight, which also shows Earth's curvature.
(Image credit: National Geographic/Albert William Stevens 1930)
I'm pretty sure the inclusion of any equation should've been a dead giveaway.