Every measurement starts somewhere — and that somewhere is usually an officially designated zero.
The name Zero Oficial — 'official zero' — points to a surprisingly rich subject: the designated zeros from which measurement begins. Roads, maps, elevations, temperatures and computer clocks all start counting at a zero that somebody, at some point, had to define and defend. This site is an independent reference guide to those zeros: where they are, who set them, and why they still matter.
An official zero is a reference point fixed by an authority or by international agreement so that measurements can be compared. Distances need a starting marker, heights need a level surface, temperatures need a floor, and clocks need an epoch. In each case the zero is at least partly arbitrary — nature rarely marks it — so its authority comes from convention: everyone agrees to count from the same place.
The Spanish phrase 'cero oficial' captures the idea neatly: the zero that counts because it has been officially declared. This guide surveys the best-known official zeros across geography, geodesy, physics, computing and the history of mathematics, and explains how each one was chosen.
Many countries mark an official point from which national road distances are measured. Spain's Kilómetro Cero is a small plaque in Madrid's Puerta del Sol, in front of the Royal House of the Post Office; distances along Spain's radial national highways are counted from it. France keeps its Point zéro des routes de France on the square in front of Notre-Dame Cathedral in Paris, and Russia's marker sits by the Resurrection Gate at the edge of Red Square in Moscow.
The idea is ancient. Rome's Milliarium Aureum, the gilded milestone erected under Augustus near the Forum, served as the nominal origin of the Roman road network. Washington, D.C., dedicated its own Zero Milestone in 1923 just south of the White House, intending it as the starting point for national highway mileage. Not every country uses a single monument: some measure from city halls, central post offices, or provincial boundary posts, which is why 'kilometre zero' can mean different things on different maps.
Longitude needs an arbitrary zero; unlike latitude, there is no natural east–west equivalent of the equator. For centuries each seafaring nation used its own meridian — Paris, Ferro, Cadiz and others — which made charts hard to reconcile. In October 1884 the International Meridian Conference in Washington, D.C., voted to adopt the meridian passing through the Airy Transit Circle at the Royal Observatory, Greenwich, as the prime meridian: 0° longitude for the world.
Modern geodesy has quietly refined that decision. Satellite measurements showed that the historic Airy line does not coincide with the centre-of-mass reference used by global positioning, so since the 1980s the IERS Reference Meridian — roughly 102 metres east of the observatory's brass strip — has served as 0° in GPS and international reference frames. Visitors at Greenwich still straddle the old line, which is close to, but not exactly, the modern zero.
Heights are quoted 'above sea level', but the sea is not level: tides, currents, winds and variations in Earth's gravity make mean sea level differ from place to place. Each country therefore defines a vertical datum — an official zero — usually tied to long tide-gauge records. The Amsterdam Ordnance Datum (Normaal Amsterdams Peil, NAP), rooted in tide observations begun in the seventeenth century, underpins height systems in the Netherlands and parts of neighbouring countries.
Russia and several neighbouring states use the Kronstadt datum, based on a tide gauge in the Gulf of Finland. Because national zeros differ, 'the same' elevation can shift by decimetres across a border, and engineers on cross-border projects must convert between datums. The geoid — the equipotential surface of Earth's gravity field that best fits mean sea level — is the modern attempt to give the whole planet one physically meaningful zero.
Celsius and Fahrenheit set their zeros at convenient but arbitrary points: freezing water in one case, a brine mixture in the other. The Kelvin scale instead starts at absolute zero — 0 K, equal to −273.15 °C — the state in which a system's thermal motion reaches its quantum-mechanical minimum. The third law of thermodynamics implies that absolute zero can be approached ever more closely but never exactly reached.
Laboratories routinely come within billionths of a kelvin using laser cooling and evaporative cooling; matter that cold forms Bose–Einstein condensates and powers the most precise atomic clocks. A small official detail: since a 1967–68 decision of the General Conference on Weights and Measures, the unit is written simply 'kelvin', without the word 'degree' — the zero of this scale, at least, needs no qualifier.
Placeholder zeros appeared in Babylonian cuneiform by around the third century BCE, and Maya calendar keepers used a zero of their own, but India turned the symbol into a full number. Brahmagupta's Brahmasphutasiddhanta (628 CE) stated rules for arithmetic with zero and negative quantities. Through Arabic scholarship, including the works of al-Khwarizmi, the digit travelled west and entered European practice with Fibonacci's Liber Abaci in 1202.
Computing added its own official zeros. The Unix epoch — 00:00:00 UTC on 1 January 1970 — is the instant from which most operating systems count time in seconds; GPS time counts from 6 January 1980. Each is arbitrary but agreed, proof that even in the digital age measurement still begins only when someone declares: start here.