Gnomon.

The Torre Calatrava was a telecommunications tower specially designed and built for the 1992 Olympics in Barcelona. Towering over Montjuïc, the tower and Plaça de Europa are architecturally integrated so that the entire space acts as a huge sundial: the cast shadow of Santiago Calatrava's futurist pylon indicating the hour by sprawling across the plaza and its pillars.

The skies over Barcelona were like a chalkboard; streams of contrails in all directions a constant graffito across the heavens - a signpost for human migration, marking time on a much larger scale.