The brass board on the wall disagrees with the internet

Search for the height of the Alappuzha lighthouse and you will be given four numbers: 27 metres, 28 metres, 30 metres, and 32.9 metres. They are quoted as though someone has simply measured badly.

They are not measuring the same thing

Inside the station hangs a black board lettered in gold — the Directorate General of Lighthouses and Lightships’ character board, the standard technical placard every Indian light carries. Photographed and released into the public domain, it is legible at full resolution, and one line settles the argument:

**ELEVATION OF FOCAL PLANE : 32.9 METRES.**

The focal plane is the height of the light itself above sea level. It includes whatever ground the tower stands on and the height of the lantern within it. It is not, and never was, the height of the building. The tower is given as 28 metres.

So the honest formulation is: a 28-metre tower whose light sits 32.9 metres above the sea. Printing 32.9 as the tower’s height — which a great many pages do — is not a small rounding difference. It is quoting a navigational figure as an architectural one.

What else the board says

The rest of it is worth reading, because it is the light’s entire job description in about twelve lines.

Character: white, single flash, every 15 seconds — 0.8 seconds of light, 14.2 seconds of dark. Four panels of 90° each, rotating at one revolution per minute, which is exactly what produces a flash every fifteen seconds. Optic: 4th order, focal distance 250 mm. Main source 150 W at 220 V, with a 1 kW standby lamp and an emergency electronic flasher behind it. Effective beam intensity: over 333,000 candela. Divergence 5° horizontally, 3.4° vertically. Luminous range 16 miles in average conditions, 24.5 in clear ones. Geographical range 16.6 miles.

And at the foot, two dates: ELE. LIGHT COMMISSIONED ON 4-8-1960, and ESTD 1862.

Why the flash pattern is the point

The specification that matters most to the people the light was built for is the one that looks most arbitrary: white, single flash, every fifteen seconds.

A light that simply shines tells a navigator nothing. Every lighthouse on a coast has a distinct character — the colour, the number of flashes, the interval — precisely so that a ship offshore at night can identify which light it is looking at by timing it against a chart. Fifteen seconds, one white flash, is Alleppey’s signature and nobody else’s on this stretch.

The four 90° panels turning at one revolution per minute are how that signature is produced mechanically: the optic sweeps a beam past you four times a minute, and a minute has sixty seconds. The number on the board is not a design flourish. It is arithmetic.

There is also a small irony in the geographical range. At 16.6 miles it is almost exactly equal to the 16-mile luminous range — meaning that on an average night this light is limited as much by the curvature of the earth as by its own brightness. Making the lamp stronger would not help. Only building higher would, and that is what the 32.9-metre figure is really about.