Cholas invented latitude sailing. Latitude sailing is a technique where a ship first steers north or south to reach the latitude of the destination, then travels east or west along that parallel.
Once the sighted latitude matched that of the target port, the ship would hold a due-east or due-west course to arrive. This method avoided the difficult problem of longitude (which required accurate timekeeping), since latitude could be obtained from star altitudes.
The invention of the kamāl, a knotted-board device, was a breakthrough: it enabled a quantitative latitude measurement and “the earliest known latitude sailing”
The Rapalagai is a traditional Indian navigational instrument, essentially identical to the kamal but invented before Kamal, used for determining latitude at sea.
The term “Rapalagai” comes from South Indian languages (notably Tamil and Malayalam) and translates to “night board” or “night instrument,” reflecting its primary use for celestial navigation at night
Description and Use
The Rapalagai consists of a small rectangular wooden board with a string attached to its center. The string is marked with knots at regular intervals.
To use it, a sailor holds one end of the string (often between the teeth) and stretches the string so that the board is held at a distance from the eye. The lower edge of the board is aligned with the horizon, and the upper edge is aligned with a target star, typically Polaris (the North Star). And as per Chola tradition even Ardra nakshatra too was used (working on understanding this)
The number of knots between the teeth and the board gives a direct measurement of the angle between the horizon and the star, which corresponds to the observer’s latitude
The Rapalagai was especially valued because it was simple, robust, and unaffected by the rolling of the ship’s deck, unlike more complex instruments such as the quadrant or astrolabe!
Historical Context
The Rapalagai (or later kamal) was widely used by Indian and Arab navigators in the Indian Ocean from at least the 10th century and possibly earlier. It was accurate enough to allow sailors to navigate among the many islands and trading posts of the Indian Ocean with a margin of error often less than 11 nautical miles.
Vasco da Gama’s successful voyage to India famously relied on an Indian pilot, Kanha, who used a Rapalagai to guide the Portuguese fleet, demonstrating its effectiveness and international recognition.
Summary
Rapalagai is a simple, yet highly effective navigational tool used by Indian sailors to measure the altitude of stars and determine latitude at sea. Its design—a small board and a knotted string—made it accessible and reliable for open-ocean navigation, and it played a crucial role in the maritime history of the Indian Ocean world
(Picture courtesy http://iau.org ) Check this link for details https://bit.ly/3SMuZ3l