August 10

On August 10, 1675, the foundation stone was laid for the Royal Observatory, Greenwich, near London. Charles II charged the Astronomer Royal with finding “the so much desired longitude of places for the perfecting of the art of navigation." Time observations at the site established Greenwich Mean Time (GMT) as a global standard. Also called Universal Time, GMT is now calculated from extra-galactic radio sources. In 1851, the Prime Meridian, the basis of longitude, was established at Greenwich (51° 28′ 40.12″ N, 0° 0′ 5.31″ W). It passes through the observatory, marked by a steel strip in the courtyard (pictured). It is also marked by a powerful green laser shining north across the London night sky.