How the Moon Makes Tides, and Why India's Coasts Don't All Agree

The Moon's gravity acts on the entire Earth, rock included. Water moves because it can. The side of Earth facing the Moon is pulled slightly harder than the centre, so the ocean there bulges toward the Moon. The far side bulges in the opposite direction because the centre of Earth is being pulled away from it. Two bulges, two high tides, roughly every 24 hours and 50 minutes, the extra 50 minutes because the Moon has moved along its orbit while Earth rotated.

The Sun adds its own tidal force, about 46 percent as strong as the Moon's. When the Sun, Moon, and Earth line up at new moon and full moon, both forces stack and produce spring tides, the highest highs and lowest lows of the month. When the Moon sits at a right angle to the Sun at quarter phases, the forces partly cancel into neap tides, the most modest of the cycle. This is not a coincidence of mythology; it is a measurable difference of roughly 20 percent in tidal range between spring and neap.


A uniform ocean covering a smooth spherical Earth would produce tidal bulges of about 0.5 metres everywhere. The real ocean is nothing like that. It has basins, shelves, ridges, and coastlines that funnel, amplify, or dampen the incoming tide.


The critical concept is resonance. Every enclosed or semi-enclosed body of water has a natural sloshing period, the time it takes a wave to travel from one end to the other and back. When that natural period is close to 12 hours and 25 minutes (half the tidal cycle), the basin resonates with the tide. Each incoming tidal pulse adds to the last, and the range grows far beyond what the Moon's pull alone would produce. The Bay of Fundy in Canada, where tides reach 16 metres, is the textbook case. India has its own version.


The Gulf of Khambhat, the narrow wedge of sea between Gujarat's Saurashtra and Kathiawar coasts, regularly sees tidal ranges of 8 to 11 metres at spring tide. The Bhavanagar tide gauge has recorded among the highest readings on the Indian coastline. Three factors combine.

First, the gulf is shaped like a funnel: wide at the mouth, narrowing sharply inland toward Khambhat town. As the tidal wave pushes into a shrinking cross-section, the same volume of water has less space and rises higher, the same reason a river floods more dramatically in a gorge than on a plain.

Second, the gulf is shallow. Shallow water slows a wave down, which means the wavelength compresses and the height increases to conserve energy.

Third, the natural resonant period of the gulf is close enough to the tidal period that partial resonance occurs. The incoming tide doesn't just fill the basin; it reinforces itself. The result is a tidal bore, a wave front that surges visibly upstream into rivers like the Sabarmati and Mahi at certain times of year, a phenomenon documented by the National Institute of Oceanography.

Move south along the west coast and the picture changes quickly. Mumbai's tidal range at spring tide is typically 4 to 5 metres, still significant, enough to expose broad mudflats at low tide and flood the Bandra-Worli causeway approaches at high. Goa's range drops to around 2 metres. By Kanyakumari at the southern tip, where the Arabian Sea meets the Bay of Bengal in open water with no funnelling geometry, the range falls below a metre.

The east coast tells a different story again. The Bay of Bengal is a large, relatively open basin. Chennai and Visakhapatnam see tidal ranges of 1 to 1.5 metres. The Sundarbans delta in West Bengal, with its maze of channels and shallow estuaries, sees locally amplified tides of 3 to 5 metres, enough to drive the dramatic daily flooding and draining that shapes the entire ecosystem and the lives of fishing communities there.

The Andaman and Nicobar Islands, sitting in open deep ocean far from any amplifying coastline, see ranges of under a metre. The Moon's pull is identical over the Andamans and over Khambhat on the same day. The ocean's plumbing does the rest.

India's tidal science is not academic. ISRO's National Remote Sensing Centre maps tidal inundation zones along the Gujarat coast to support port planning at Kandla and the newer Mundra terminal, both of which schedule berths around tidal windows. The proposed tidal energy project at Khambhat, studied by the Ministry of New and Renewable Energy, would, if built, use that 8-to-11-metre range to generate power the way a hydroelectric dam uses a river's fall. The Sundarbans mangrove network, one of the largest in the world, exists because those 3-to-5-metre tides deposit sediment and flush salinity in a rhythm that no other force could sustain at that scale.

The Moon is not doing anything different over Gujarat than over Goa. What changes is the shape of the sea it is pulling on, and India's coastline, running from the shallow funnel of Khambhat to the open deep water off the Andamans, gives that pull every possible outcome at once.