Śaṅku and Ghaṭī: Shadow and Water

The gnomon read the real Sun, the sinking water-bowl counted the hours in any weather, and each kept the other honest

At Nālandā, a copper bowl with a pin-hole decided when thousands of monks had to stop eating. Learn how the śaṅku (gnomon) read the true Sun by day, how the ghaṭī (sinking water-bowl) counted 24-minute units through cloud and night, and why every water clock had to be reset against a shadow.

The Bowl That Called Noon

It is late morning at Nālandā, some time in the ten years, roughly 675 to 685 CE, that the Chinese monk Yijing spent studying there. In the great monastery, a boy sits beside a copper vessel full of water. Floating on it is a thin copper bowl that holds about two pints. In its bottom is a hole "as small as a pin-hole". Water wells up through it. Slowly the bowl fills, settles lower, and sinks.

Each time it goes under, a drum is struck. Yijing, who later wrote all of this down, counted the signals. Four sinkings after sunrise closed the first watch of the day, with drumbeats and two blasts of a conch-shell. Four more would close the second. That second signal meant noon.

For the thousands of monks at Nālandā, noon was not a small detail. Their rule forbade eating after midday. Yijing records that once the noon strokes had sounded, the monks stopped eating, and anyone found still eating could be expelled. The whole day of a great university hung on a bowl the size of two cupped hands.

So how does a copper bowl know when noon has come? And what happens in the monsoon, when clouds hide the Sun for days? The answer takes two instruments, a stick and a bowl, and the story of how each kept the other honest.

A Stick That Reads the Sun

The older of the two is the śaṅku, the gnomon: a straight stick or pillar set upright on flat ground. The lesson on shadow calculations showed what its shadow can compute. Here we look at the instrument itself, and at how carefully it was built.

The Sūrya Siddhānta opens its chapter on direction, place and time with workshop instructions (3.1 to 3.2). Level a stone surface with water, or use a smooth plastered floor. Draw a circle on it. At the centre, set a gnomon divided into twelve aṅgulas (finger-widths). A level floor and a true upright matter more than any calculation, because a tilted stick gives a lying shadow.