# Lighthouse > A tower and a turning light that mariners can name - Date: c. 550 BCE - Era: Antiquity - Domain: [Ships & Navigation](https://lastcodex.org/en/domains/navigation/) - Rebuild stage: III (Crafts & cities) - Difficulty: 4/5 - URL: https://lastcodex.org/en/inventions/lighthouse/ A lighthouse is a tower that carries a light high enough to be seen far out at sea, marking a coast, a danger or a harbour entrance. In many large lights a lens gathers the lamp into beams that turn and sweep the horizon, and the rhythm of flashes tells mariners which light they see. The oldest excavated lighthouse towers, on the Greek island of Thasos, date from the 6th century BCE; the Pharos of Alexandria, completed about 280 BCE, was the largest and most celebrated of antiquity. ![Lighthouse · A tower and a turning light that mariners can name](https://lastcodex.org/_astro/plate.CH6gDirV_ZvBraS.webp) ## Legend 1. Stone tower 2. Lamp at the focus 3. Bull's-eye lens panel 4. Rings of reflecting prisms 5. Float in a mercury bath 6. Clockwork driven by a falling weight ## What it is A lighthouse raises a light high above the sea so that ships can see it from far away. It marks a headland, a reef, a sandbank or the mouth of a harbour. By day the tower itself is the landmark; at night the light takes over. A navigator who sights it and identifies it on the chart knows where the ship is, or at least which danger lies ahead. Mariners must be able to tell one light from another, so every major light has its own character: a steady light, or a set rhythm of flashes and darkness, sometimes with a colour, repeated over a fixed number of seconds. At the top of a classic tower, inside a glazed lantern, an optical apparatus produces that character. A single lamp burns at the centre, a large lens of glass rings and prisms gathers its light into narrow beams, and a clockwork drive turns the lens so that the beams sweep round the horizon. ## How it works A bare flame sends its light in every direction, much of it up into the sky or down onto the tower, where it is of no use to a ship. There are two ways to gather it into a horizontal beam. A curved mirror of parabolic shape, with the lamp at its focus, reflects the rays out parallel to each other. A lens bends them instead. A single lens large enough for a lighthouse would be enormously thick and heavy, so Augustin Fresnel kept a central lens and replaced the rest by concentric rings of glass, each shaped like the matching zone of the large lens but much thinner. Above and below this belt, rings of glass prisms catch the rays that would go up or down and turn them back to the horizontal by reflection inside the glass. One lamp is enough. ![A ring of Fresnel lens panels in a brass frame around a small central flame, with eight narrow beams of parallel rays leaving it like the spokes of a wheel and a curved arrow showing the rotation](https://lastcodex.org/_astro/v2.tQHnjnXt_1rdMKp.webp) _Each panel sends out one beam; as the lens turns, each beam passing a ship is seen as one flash._ In a flashing light the belt of lenses is divided into panels, each with a round bull's-eye at its centre that gathers the light into one narrow beam. An eight-panel lens sends out eight beams like the spokes of a wheel. The lamp stays still in the middle while the lens turns round it, and each time a beam crosses a ship the watch on deck sees a flash, then darkness until the next beam arrives. The number of panels and the speed of turning set the rhythm. In the first such lens, at Cordouan in 1823, the light brightened for a few seconds once a minute. A weight hanging on a cable inside the tower drove a train of [gears](/en/inventions/gear/), and the keeper wound the weight back up every day. A large lens and its frame can weigh several tonnes, and on rollers they turned slowly. From 1890 the French lighthouse service floated it instead: the turning table rested on a ring-shaped float in a trough of mercury, which carried the weight with almost no friction. At South Foreland in Kent, an optic that had needed eight minutes per turn on rollers made a full turn in 40 seconds once it floated. Flashes lasting three-tenths of a second, ten seconds apart, became common. Light lists and charts describe each character in a short code. A fixed light shines without a break. A flashing light shows less light than darkness in each period, an occulting light more light than darkness, so that it seems to wink off, and an isophase light equal times of each. Flashes or eclipses can come in groups, two or three together, and the period is the time in seconds before the whole pattern repeats. Pliny the Elder had already seen the problem that this solves: a fire burning steadily at night could be taken for a star. ## How to do it A table model shows how a turning lens makes flashes. You need a small battery LED light, a magnifying glass or a flat plastic Fresnel sheet magnifier, stiff card, a wooden dowel or pencil, a heavy base such as a block of wood, glue, and, if you wish, a small battery motor with a rubber band. ![A table model: a wooden dowel standing in a heavy block, a card disc resting on a card collar on the dowel, a small LED light on top of the dowel and, at the edge of the disc, a round lens held upright in a card frame, with rays spreading from the LED to the lens and leaving it parallel](https://lastcodex.org/_astro/v3.D5QSfrNh_1X3iv1.webp) _The lamp stays still on the dowel while the lens, at its focal distance, turns with the disc._ First find the focal length of your lens. Hold it between a bright window and a sheet of white paper and move it until a sharp, upside-down image of the window appears on the paper; the distance from lens to paper is the focal length. Fix the dowel upright in the base. Cut a card disc a little wider than twice the focal length, make a hole at its centre, and slide it over the dowel on a small collar of card so it can turn freely. Fix the LED to the top of the dowel, above the disc, so that the lamp stays still while the disc turns, just as in a real lighthouse. Glue a card frame at the edge of the disc to hold the lens upright, facing outwards, with its centre level with the LED and at the focal distance from it. Move the lens a little in or out until the beam on a far wall is narrowest. Darken the room and turn the disc slowly by hand. Standing a few metres away, you see a flash each time the beam sweeps past. A second lens on the opposite side of the disc gives two flashes per turn; two lenses side by side give a group of two. To drive it, run a rubber band from a small pulley on a battery motor round the edge of the disc, and slow it down with a larger pulley if the turns are too fast. Time the flashes with a watch and write down the character of your light. Use batteries only and never connect the model to the mains. Put the cells in a proper holder, do not let bare wires touch each other, and take the batteries out when you have finished. Never crush, pierce or heat lithium cells. Keep fingers, hair and loose clothing away from the motor and the rubber band while they run. Do not stare into the LED beam at close range. Keep the magnifying glass out of direct sunlight, as it can focus the sun's heat enough to start a fire or harm the eyes. ## History and evolution Claims for older marks, such as a stone pillar at Sigeum near Troy, dated by some to about 1300 BCE, or the signal fires in Homer, are disputed, because it is hard to show that they were meant to guide ships. The oldest lighthouses whose remains have been excavated stand on headlands of Thasos, in the northern Aegean. In 1989 Tony Koželj and Manuela Wurch-Koželj, working with the French School at Athens, published three round towers on the island's capes as lighthouses, among them the tower of Akeratos on Cape Pyrgos, long known from the inscription it bears. The Centre for Alexandrian Studies dates the tower on Cape Phanari to the 6th century BCE. It was built of grey marble from a quarry beside it, and slabs of sandstone set at its centre carried an open fire on top. The Pharos of Alexandria, finished about 280 BCE under Ptolemy II, was far larger, and it became the most celebrated lighthouse of antiquity. Strabo explains why it was needed: the coast on either side of the harbour was low and lacked any landmark, with reefs and shallows offshore. He describes a tower of white stone with many storeys, given by Sostratus of Cnidus "for the safety of mariners". Pliny the Elder reports that it cost 800 talents and that its fires showed ships the shoals and the harbour entrance at night. He adds that similar fires burned in his own day at Ostia and Ravenna. Earthquakes damaged the Pharos, and by the middle of the fourteenth century it lay in ruins. In 1480 the Mamluk sultan Qaitbay built a fort on its site, and divers of the Centre for Alexandrian Studies have since recovered statues from the sea beside it. The Roman tower at A Coruña in north-west Spain, known as the Tower of Hercules, dates from the reign of Trajan (98–117 CE) or was rebuilt then. Of its 55 metres, 34 are Roman masonry, encased in a restoration directed by Eustaquio Giannini from 1789, and it still works as a lighthouse. ![The flat top of a square stone tower by the sea at night, with a large wrought-iron basket of burning coal on a short stand, and a bucket and shovel beside it](https://lastcodex.org/_astro/v1.B0GVq2fg_Zd65Af.webp) _A coal fire burning in an iron basket on top of a tower._ For centuries most lights were open fires of wood or coal, or candles and [oil lamps](/en/inventions/oil-lamp/) behind [glass](/en/inventions/glass/). In 1636 a tower on the Isle of May, at the mouth of the Firth of Forth in Scotland, was given a coal fire on its top. The modern apparatus began with the lamp that Ami Argand devised in the early 1780s: its circular wick, with air rising inside and outside it under a glass chimney, burned almost without smoke. Parabolic reflectors were tried in Sweden from 1738, with poor results because the lamps were not placed at the focus, and on the Mersey in England from 1763. In 1790 Cordouan, on a reef at the mouth of the Gironde, received a rotating apparatus of twelve parabolic reflectors with Argand lamps, turned by clockwork. It proved weak and unreliable. The great change came from Augustin Fresnel, an engineer whom François Arago brought onto the French lighthouse commission in 1819. Stepped lenses had been proposed before, by Buffon in the eighteenth century, but Fresnel had them built from separate pieces of glass, which made large ones possible. In July 1823 the first of his lenses was lit at Cordouan: eight stepped lens panels around a lamp with five concentric wicks, with mirrors and small lenses above to send the upper rays to the horizon. Later, prisms that reflected the light inside the glass replaced the mirrors, and for decades French workshops led the making of lighthouse lenses. The United States lit its first imported Fresnel lens in 1841, but only five were in service in 1853; most American lighthouses were refitted with lenses during the 1850s. Léon Bourdelles, chief engineer of the French lighthouse service, built the first working mercury float in 1890, an idea Fresnel had put forward in 1825. The Swedish engineer Gustaf Dalén made lights that needed no keeper. His acetylene lamps had a flasher that switched the flame on and off in a fraction of a second and a sun valve that lit the gas at dusk and shut it off at dawn, so that a light could run unattended for a year. He received the Nobel Prize in Physics in 1912. Electric lamps have since replaced oil and gas, and many Fresnel lenses have been kept in service around them. ## Key facts - Oldest excavated lighthouses: Capes Phanari and Pyrgos, Thasos, 6th century BCE - Largest ancient lighthouse: Pharos of Alexandria, completed c. 280 BCE - Roman lighthouse still lit: Tower of Hercules, A Coruña, Spain - First Fresnel lens: Cordouan, Gironde estuary, July 1823 - Mercury float: Built by Léon Bourdelles in 1890 ## Requires - [Oil and fat lamp](https://lastcodex.org/en/inventions/oil-lamp/): A wick feeds a small flame from a liquid fuel reserve (20,000 years ago) - [Lime and lime mortar](https://lastcodex.org/en/inventions/lime-mortar/): Burnt stone that turns back to stone (12,000 years ago) - [Glass](https://lastcodex.org/en/inventions/glass/): Melted sand you can see through (c. 2,500 BCE) ## Sources - [Strabo, Geography 17.1.6 (trans. H. L. Jones)](https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Strabo/17A1*.html), LacusCurtius, University of Chicago - [Pliny the Elder, Natural History 36.18, The Pharos (trans. J. Bostock and H. T. Riley)](https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.02.0137:book=36:chapter=18), Perseus Digital Library, Tufts University - [Ancient lighthouses (Thasos, Cos and others)](https://pharos.cealex.org/ancient-lighthouses/), Centre d'Études Alexandrines (CNRS) - [Phares de Thasos (Bulletin de correspondance hellénique 113.1, 1989, pp. 161–181)](https://doi.org/10.3406/bch.1989.4716), Tony Koželj and Manuela Wurch-Koželj, École française d'Athènes - [Pyrgos (Thasos), ancient lighthouse, after the Barrington Atlas 51 D3](https://pleiades.stoa.org/places/501586), Pleiades gazetteer of ancient places - [First lighthouse](https://www.guinnessworldrecords.com/world-records/717337-first-lighthouse), Guinness World Records - [Pharos of Alexandria](https://www.britannica.com/print/article/455210), Encyclopaedia Britannica - [Pharos (news brief, January/February 1996)](https://archive.archaeology.org/9601/newsbriefs/pharos.html), Archaeology, Archaeological Institute of America - [Tower of Hercules](https://www.britannica.com/print/article/262815), Encyclopaedia Britannica - [Curiosities of the Tower](https://www.coruna.gal/the-tower/en/discover-the-tower/curiosities-of-the-tower), Concello da Coruña - [Tower of Hercules](https://whc.unesco.org/en/list/1312/), UNESCO World Heritage Centre - [Isle of May, old lighthouse (beacon tower of 1636)](https://canmore.org.uk/event/846574), Canmore, Historic Environment Scotland - [The Canadian Lighthouse, chapter 'Apparatus: Lights and Optics' (Canadian Historic Sites 9)](https://parkscanadahistory.com/series/chs/9/chs9-1e.htm), Edward F. Bush, Parks Canada - [Borda and Lenoir's device at Cordouan](https://cordouan.culture.gouv.fr/en/borda-and-lenoirs-device-cordouan), Phare de Cordouan, Centre des monuments nationaux - [Cordouan's first lens](https://cordouan.culture.gouv.fr/en/cordouans-first-lens), Phare de Cordouan, Centre des monuments nationaux - [Phare de Cordouan](https://structurae.net/fr/ouvrages/phare-de-cordouan), Structurae - [1823–2023 : bicentenaire de la lentille de Fresnel](https://daniel-hennequin.univ-lille.fr/publis/2023CGSFP-fresnel.pdf), Daniel Hennequin and Philippe Verkerk, Université de Lille, CNRS - [Fresnel bicentenary closing seminar, Le Verdon-sur-Mer, October 2023](https://iala.int/content/uploads/2023/07/Plaquette_Seminaire_FRESNEL_2023-VFRUK.pdf), IALA-AISM and Phares de France - [Lighthouses of Massachusetts, Thematic Group Nomination (National Register of Historic Places)](https://npgallery.nps.gov/pdfhost/docs/NRHP/Text/64000282.pdf), National Park Service - [The Fresnel lens: flashing lenses and lens rotation](https://uslhs.org/node/1614), United States Lighthouse Society - [South Foreland in the 20th Century](https://lightontheforelands.info/Light%20On%20The%20Forelands%20-%20South%20Foreland%20In%20The%2020th%20Century.pdf), Light on the Forelands - [Light List, General Information (2026–27)](https://www.linz.govt.nz/sites/default/files/2026-06/hydro_202627-almanac_light-list-general-information_pdf.pdf), Land Information New Zealand - [Recommendation E-110: Rhythmic Characters of Lights on Aids to Navigation (ed. 4, 2016)](https://iala-aism.org/e-110-ed-4-rhythmic-characters-of-lights-on-aids-to-navigation_16dec2016-2), IALA-AISM - [Gustaf Dalén, biographical](https://www.nobelprize.org/prizes/physics/1912/dalen/biographical/), The Nobel Prize - [Gustaf Dalén](https://lindahall.org/gustaf-dalen), Linda Hall Library Fact-checked on 2026-10-17. --- The Last Codex · https://lastcodex.org/en/inventions/lighthouse/ · © The Last Codex. 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