"....L'Enfant carefully planned for the federal city to manifest the star pattern formed by three stars (all of the first magnitude) surrounding Virgo. They are: Arcturus, Spica, and Regulus. These form almost a perfect right angled triangle with Virgo inside the triangle........"
Above image - Virgo triangle in the sky, month of August. Cause the rotation of the sky, this is the location of the triangle respectively on 1st August at hour 23PM, 15th August at 22PM and on 1st September at 21PM. The triangle is approximate. I rotated the manual because the line of horizon is curved as it represents the circular base of the celestial semi-sphere, therefore you must orientate the map in order that the circular horizon has to touch the ray having you as center and pointing towards the portion of sky you are watching. Virgo triangle has been chosen as it represented a good approximation of a perfect squared triangle.