How did monsoon winds and a sailor's logbook link India to Rome?

In the first century a sailor, likely an Egyptian Greek, wrote down what a trader needed to know to reach India. His guidebook is the Periplus of the Erythraean Sea, a Greco-Roman periplus written in Koine Greek in the first century. A periplus is a logbook recording sailing itineraries and commercial details about the ports visited, and before maps were in general use it functioned as a combination of atlas and traveller's handbook. This one describes the route from Roman Egyptian ports like Berenice along the Red Sea, the Horn of Africa and the Arabian Sea, including Sindh and southwestern India.

Wind made the route. The Periplus says that a direct sailing route from the Red Sea to the Indian peninsula across the open ocean was discovered by Hippalus. Trade between India and the Greek Ptolemaic Kingdom was started by Eudoxus of Cyzicus in 130 BCE, and opening Red Sea ports and improved knowledge of the seasonal monsoons substantially increased trade. Later, the Roman Empire strengthened direct maritime trade with India and eliminated the taxes extracted by middlemen on land routes.

One port in the story is Muziris, a lost port city on the southwestern coast of India, a major centre of trade between the Chera kingdom and the Roman Empire. According to the Periplus it abounded in ships with cargoes from Arabia and from the Greeks. So much gold was used for this trade that Pliny the Elder complained about the drain of specie to India.

Notice what this shows: India was a hub of the ancient world's trade, not at its edge. And its ports were busy long before: Harappan and Mesopotamian civilizations were already trading by sea as early as 2600-1900 BCE.