Meridian Longitude ★

The resolution of the longitude problem is one of the great sagas of scientific history. In the 18th century, the British Parliament’s Longitude Act spurred inventors like John Harrison to develop the marine chronometer. Because longitude is essentially a measurement of time—one hour of time difference equals 15 degrees of rotation—keeping accurate time at sea allowed sailors to compare their local solar time to the time at a fixed reference point. This breakthrough transformed the meridian from a theoretical line into a practical tool for survival.

While modern geodesy uses a 3D ellipsoid model (like WGS84) rather than a perfect sphere, the underlying graticule of remains the standard. As we map other planets—Mars, Venus, the Moon—we arbitrarily assign them a prime meridian (for Mars, it passes through the Airy-0 crater). The same logic of converging north-south lines applies across the solar system. meridian longitude