If you think navigating was difficult in the days before GPS, take a look at this. It’s the world’s first astrolabe, a bronze disc that Vasco da Gama’s navigators used to sail from Portugal to India around 1500, just a few years after Columbus found America. In 1503, da Gama was returning home after his second voyage to India. He left several ships behind to explore (and exploit) the area, but they sank in storms. Several years ago, researchers found this 7-inch astrolabe among many artifacts on one of those ships, Esmeralda, off the coast of Oman. At first, they…