What Is a Meridian? The Line Overhead That Defines Noon

Standing anywhere on Earth, the meridian is the imaginary line running from due north on the horizon, up through the point directly overhead, and down to due south. Because the sky appears to rotate about the poles, every celestial object crosses that line once each day, reaching its highest point above the horizon as it does, which is the moment astronomers call transit or culmination. The sun's transit defines local apparent noon, which is when a vertical rod casts its shortest shadow and when the shadow points exactly north or south. Measuring the altitude of an object at transit is the easiest precise measurement in positional astronomy, since the object is momentarily stationary in altitude and is at its highest, which minimises the atmospheric distortion the light has passed through.

The observation is simple and unusually informative:

• •Latitude, from the altitude of the sun at noon combined with the date, which is the basis of the oldest reliable navigation method
• •Local time, since transit defines noon and a clock can be set or checked against it
• •The position of a star in one coordinate, measured directly from the altitude at transit
• •The other coordinate, from the time of transit compared with a reference
• •Longitude, from the difference between local time and the time at a reference meridian, which requires a clock that keeps reference time
• •The obliquity of the Earth's axis, from the difference in the sun's noon altitude between the solstices

The transit instrument, a telescope mounted so that it can swing only in the north to south plane, was the workhorse of positional astronomy for two centuries. Restricting the motion to one plane permits extremely rigid mounting and precise alignment, so the instrument measures altitude and time of crossing with an accuracy a freely moving telescope cannot match. Observatories built for this purpose recorded the transits of thousands of stars, building the catalogues on which navigation, timekeeping and later astrophysics depended. The method requires the instrument to be aligned to the true meridian, which is itself established by observation, and the elaborate procedures for checking and correcting that alignment occupied a great deal of professional effort. The technique has been superseded by satellite methods and the instruments survive in observatories as historical objects.

The gap between transit and the time on a clock is larger than people expect and has two separate causes. The first is time zones, since clock time is uniform across a zone while local noon varies continuously with longitude, so a place at the eastern edge of a zone experiences transit well before the clock says noon and one at the western edge well after, with differences approaching an hour in wide zones and more where zone boundaries are drawn for political convenience. The second is that the sun is not a good clock, since the Earth's orbit is elliptical and its axis is tilted, which makes apparent solar days vary in length through the year, so true noon drifts against uniform time by up to about a quarter of an hour either way. The correction between the two is called the equation of time, and sundials carry it engraved on them for exactly this reason.

Latitude has a natural zero at the equator and longitude has none, so a reference meridian had to be chosen by agreement, and the choice was political as well as technical. Different countries used their own national observatories as the reference for centuries, which meant charts had to be converted and errors crept in. An international conference in 1884 adopted the meridian through the Greenwich observatory, largely because a majority of shipping already used charts based on it, with several countries dissenting and France continuing to use its own for decades. The modern reference is defined by satellite geodesy and sits slightly to the east of the historic line marked at Greenwich, by roughly a hundred metres, which is a source of confusion for visitors whose satellite receivers disagree with the brass strip in the ground.

The circle from due north through the overhead point to due south, crossed once a day by everything in the sky, with the sun's crossing defining local noon. Transit measurements give latitude, time and stellar positions with unusual precision because the object is momentarily stationary and at its highest. Longitude has no natural zero, so the reference meridian was agreed by conference in 1884.