descriptive writing exercise: columbia university at dusk

I sat in Columbia for an hour tonight. At eight pm on a Friday night, a steady stream of humanity trickles out of the buildings and onto the main promenade. Knots of people cluster in front of the library and pretty young things wander by like gusts of fluttering butterflies. The lights in the hall slowly make their presence felt, smoothing over the transition as the earth turns away.
A gibbous moon hangs in the sky, face shadowed behind gauzy painted whorls of clouds. The copper green roofs seem brighter, caught by the westering rays, arrayed across the quadrangle like sentinels on duty. Argus of a hundred eyes could not have more viewpoints than the combined windows of these stoic watchmen. The dark is slowly taking over; crickets set up their chirping in the bushes and a squirrel saunters out to look over its property. Halogen lamps extend their rays like cold stars come near and the golden glow of low wattage lights fills one side. In 25 minutes, the place has not become any less busy. Were all these hundreds hidden away in treasure houses all day?
As the noises of the day die down, a fountain chuckles into life, filling the air with susurrations of splashing water. I get up and join the ebbing tide. Night has fallen.