 The gnomon was used to study the movements of the sun during the year and helped in determining the new Gregorian calendar for which Danti took place in the commission appointed by Pope Gregorius XIII and instituted in 1582.In his 1585 book "Diversarum Speculationum Mathematicarum" Venetian mathematician Giambattista Benedetti proposed to use a mirror in a 45-degree angle to project the image upright.This leaves the image reversed, but would become common practice in later camera obscura boxes.