Throughout the last few thousand years the mariners and trade routes of the Indian Ocean have moved to a unique rhythm based upon the prevailing seasonal weather patterns. These are known individually as a monsoon, derived from the Arabic mawsim, meaning a fixed time of year. Two main monsoons can be identified: blowing from the north-east in the winter and the south-west during the summer with a variable weather season in between.
These two monsoons have very different characteristics from each other, despite occurring over the same body of water. The north-easterly monsoon of the winter is characterised by dry, steady, relatively gentle winds which encourage sailing throughout its duration. Meanwhile, the south-westerly summer monsoon is wet, violent and characterised by storms and strong wind with sailing only feasible at the beginning and end; in the late spring and early autumn. Unlike in the Mediterranean and Northern Europe, then, sailing in the Indian Ocean tended to avoid the summer months of June, July and August.
The switch in overall wind direction resulting from the monsoon patterns means that it is possible to sail on the Indian Ocean with a constantly favourable wind, if done in conjunction with the monsoon rhythms. Using favourable winds as much as possible was important. Ancient and medieval Indian Ocean sailing vessels could only sail to windward in lighter winds and calm seas, but were efficient when sailing with the wind. They could average as much as 11 kph on extended voyages, with an even higher top speed in very good conditions.
The use of the monsoon in this manner is inferred in Roman period written sources. One of which is the Periplus Maris Erythraei, a Greek text written in the mid-1st century AD by a merchant with intimate knowledge of the Red Sea, Persian Gulf, Bay of Bengal and Arabian Sea. The Periplus lists the ports of trade, the distances between them and the products that could and should be traded at each port. In this sense it is much more of a trading gazeteer than a description of navigational methods.
Monsoon and Ibn Majid
Our best information for using the monsoon comes from a master navigator called Ibn Majid. He came from what is now the United Arab Emirates/Oman and sailed and wrote in the 15th century. As well as describing the Arab methods of navigating using the stars, Ibn Majid recounts the routes around the Indian Ocean and lists the times of the year when vessels should depart certain ports in order to arrive safely at their destination. The constant nature of the monsoon over recent millennia means that his timetable or departures can also be used to understand earlier eras, e.g. the Roman one.
From Ibn Majid’s work, we can construct a seasonal timetable whereby ships departed from ports in the Gulf like Siraf and southwestern India during the autumn, sailing to East African ports like Zanzibar on the north-easterly monsoon and returning during the spring on the first winds of the south-westerly monsoon. Vessels from Red Sea ports like Aylah would sail south in late summer, using the tail-end of the south-westerly monsoon to sail to south-western Indian ports, returning again in December and January when they would have the favourable winds of the north-easterly monsoon. The voyage between the Red Sea and East Africa could be made using a combination of the two monsoons and a stopover at a port such as Aden in modern Yemen.
Voyages even further eastward, to south-east Asia and China, probably via the straits of Malacca, also fitted within this timetable. Vessels could leave southern India in late December, arriving in the China Sea in April or May with an arrival in Canton for the summer. The return voyage would depart in the autumn and cross the Bay of Bengal in January. A vessel sailing from a Gulf port might take a year and a half to complete the round trip to China and back.
In all the examples above, and for ancient as well as medieval eras, voyages could be made directly, or by stopping to trade at ports along the way. In this way the two monsoons provided the mariners of the Indian Ocean with a means to sail from place to place with a degree of relative certainty and reliability, arriving in specific ports at specific times, and leaving them during designated periods depending on the next destination. This regular timetable derived entirely from the combination of available sailing technology, in conjunction with the predictable monsoon weather systems. It provided a contrast to the seafaring of more northern seas where the technology was broadly similar, but where the weather was far more unpredictable.