# Magnetic Compass > A needle that points north - Date: c. 1088 - Era: Middle Ages - Domain: [Ships & Navigation](https://lastcodex.org/en/domains/navigation/) - Rebuild stage: III (Crafts & cities) - Difficulty: 2/5 - URL: https://lastcodex.org/en/inventions/compass/ A magnetic compass is a magnetized needle, free to turn, that lines itself up with the Earth's magnetic field and so shows north in any weather. Described in China in the eleventh century and at sea in Europe by about 1190, it let sailors hold a course out of sight of land and under cloud. ![Magnetic Compass · A needle that points north](https://lastcodex.org/_astro/plate.DaMA-MEg_mYavX.webp) ## Legend 1. Gimbal rings 2. Bowl 3. Compass card 4. Magnetized needle 5. Pivot pin 6. Lodestone ## What it is A compass is a small magnet that can turn freely. Left alone, it settles along the lines of the Earth's magnetic field, one end towards magnetic north and the other towards magnetic south. A navigator who knows where north is can read every other direction from it, at night, in fog or out of sight of land. The plate shows a mariner's compass of the sixteenth century. The needle is glued under a paper card painted with a wind rose, so the whole card turns with it. The card balances on a pin in a wooden bowl, and the bowl hangs in gimbals so it stays level when the ship rolls. ## How it works ![A rough lodestone with a steel needle resting across it and a second needle beside it](https://lastcodex.org/_astro/v1.Xy2J0o9d_qSSCH.webp) *Stroked along a lodestone, a steel needle becomes a magnet in its own right.* The Earth behaves roughly like a huge bar magnet. Its field is weak, about half a gauss at the surface, but a light needle on a good pivot needs very little force to turn. The needle swings until its own field lines up with the horizontal part of the Earth's field, then stops. The needle has to be magnetized first. In iron and steel, tiny regions called domains each act as a magnet, but they point every which way and cancel out. Stroking the needle in one direction with a lodestone, a naturally magnetic piece of magnetite, lines many of them up, and the needle keeps that order. Hard steel holds it best. Soft iron loses it quickly, and before the mid-eighteenth century sailors had to touch up their needles with a lodestone kept on board. ![A needle threaded through a short piece of reed, floating in a bowl of water](https://lastcodex.org/_astro/v2.nbwF_cGz_Z15DI6g.webp) *An early method: a needle on a float, turning freely on still water.* Magnetic north is not the geographic pole, and the angle between them, the declination, changes from place to place and slowly over the years. The field is also tilted: in the northern hemisphere it pulls the north end of the needle downwards, in the southern hemisphere the south end, most steeply near the magnetic poles, so compass makers balance the needle with a small weight. On a ship, iron fittings and cargo pull the needle aside too, an error called deviation. On the plate, the pointed pin and the small cap on the needle keep friction low. The card lets the helmsman read the heading against a fixed mark on the bowl. The glass cover keeps out wind, and the gimbals stop the card from tipping when the ship rolls. ## How to build it **Make the needle.** Use a steel sewing needle or a thin strip of hard steel about 5 cm long. Iron that is too soft will not stay magnetized. **Magnetize it.** Stroke the needle from the middle to one end with one pole of a lodestone or magnet, lifting the stone away in a wide arc at the end of each stroke. Repeat twenty or thirty times, always in the same direction. **Test it on water.** Push the needle through a short piece of cork or dry reed and float it in a bowl of still water, away from iron. Find which end points north and mark it. North of the tropics, check it against the pole star at night or the shortest shadow at local solar noon, which then points north; south of the tropics the noon shadow points south. **Give it a pivot.** Press a small cone into a scrap of thin brass to make a cap and fix it to the centre of the needle. Set a sharp needle point upright in the bottom of a wooden bowl and rest the cap on it. Balance the needle with a dab of wax until it lies level. **Add a card and a cover.** Glue the needle under a light card marked with the points of the compass, its north mark over the north end of the needle. Paint a fixed mark, the lubber line, inside the bowl and line it up with the bow of the boat: the heading is read on the card against it. A pane of glass over the bowl keeps draughts away. **Hang it.** For a model of a ship's compass, mount the bowl in two rings pivoted at right angles to each other. A home-made compass is a demonstration: do not navigate by it before checking it against a proven compass. ## History and evolution People knew in both China and Greece more than two thousand years ago that lodestone attracts iron. Han-dynasty texts mention a south-pointing device used in divination, perhaps a lodestone spoon, though historians disagree about what it was. ![A needle hanging level from a single silk thread fixed with a dab of wax](https://lastcodex.org/_astro/v3.IPW8H5iY_2NVFa.webp) *Shen Kuo judged a needle hung from a single silk thread the best mounting.* The military manual *Wujing Zongyao*, compiled around 1044, describes a thin iron fish heated red-hot, cooled lying north to south and then floated on water, where it pointed south. Around 1088 Shen Kuo described needles magnetized by rubbing them on lodestone, floated on water or hung from a silk thread. He also noted that the needle points slightly east of due south, an early record of declination. Between 1111 and 1117, Zhu Yu wrote that ship pilots watched the needle when clouds hid the sun and stars. In Europe the English scholar Alexander Neckam described sailors using a magnetized needle around 1190. A Persian text of 1232 gives the first mention in the Islamic world. In 1269 Petrus Peregrinus described a pivoted dry compass, and around 1300 the dry box compass with a card spread through the Mediterranean. Liquid-filled compasses, which damp the card's swinging, took over in the nineteenth and early twentieth centuries, and the gyrocompass, which finds true north without magnetism, followed in the first decade of the twentieth century. ## Key figures - Iron fish floated on water: Wujing Zongyao, c. 1044 - Needle compass and declination described: Shen Kuo, c. 1088 - First recorded use at sea: China, between 1111 and 1117 - First European mention: Alexander Neckam, c. 1190 - Strength of the Earth's field: About 0.5 gauss (50 microtesla) ## Requires - [Iron Smelting](https://lastcodex.org/en/inventions/iron-smelting/): Wider adoption around 1200 BCE; ore, charcoal and a bloomery (c. 1,200 BCE) - [Steel](https://lastcodex.org/en/inventions/steel/): Iron with just enough carbon (c. 1,000 BCE) ## Leads to - [Celestial Navigation](https://lastcodex.org/en/inventions/celestial-navigation/): A ship's position read from the sky (c. 1460) ## Sources - [History of the compass](https://en.wikipedia.org/wiki/History_of_the_compass), Wikipedia - [Compass](https://en.wikipedia.org/wiki/Compass), Wikipedia - [Shen Kuo: first Renaissance man](https://www.aaas.org/shen-kuo-first-renaissance-man), AAAS - [Lodestone, c. 1700–1720 (NAV0710)](https://www.rmg.co.uk/collections/objects/rmgc-object-42922), Royal Museums Greenwich - [Dry card compass, 1838 (ACO0005)](https://www.rmg.co.uk/collections/objects/rmgc-object-7085), Royal Museums Greenwich Fact-checked on 2026-10-11. --- The Last Codex · https://lastcodex.org/en/inventions/compass/ · © The Last Codex. Texts, plates and figures: CC BY-NC-ND 4.0.