73

This number is a prime.

The number of rows in the Arecibo Message graphic. The Arecibo Message is a radio message that was sent toward a globular star cluster in Hercules from a radio telescope in Puerto Rico on November 16, 1974. The entire transmission lasted a semiprime number of seconds and provided a way for extraterrestrials to arrange a 73-by-23 array into recognizable data about our life and solar system.