We can’t see the North-South longitudinal lines, but they are there, and at the historical “zero degrees longitude” or “prime meridian,” all 24 meridians of longitude can be determined. The prime meridian is also used as the basis for the world’s standard time zones.
Technology has taken giant steps forward since Sir George Airy marked the imaginary line with his telescope in 1851. In 1884, Airy’s zero degrees longitude point of reference was officially adopted at a meeting of the International Meridian Conference held in Washington D.C.
But the prime meridian used by satellite navigation systems lies 102 meters (334 feet) east of Airy’s line. Many people thought this difference in location was due to readings by the Global Positioning Satellite (GPS) or the earlier Navy Navigation Satellite systems. But Airy’s point of reference for the prime meridian was just as correct as the GPS coordinates used by modern satellites because both are consistent with the science and technology of the time.
So how is the prime meridian now determined? We now use an assortment of technologies, from incorporating variations in the Earth’s rotation to using atomic clocks, lasers bounced from Earth to the moon and back and GPS satellites. Calculations take into account the variations on the Earth’s surface as well as the gravitational pull that affects the center bulge of the planet.
The instruments used today are a far cry from the photographic zenith tube, a telescope that used the reflection of light off a basin of mercury to determine the imaginary vertical line running toward the core of the planet. Astronomers in the 1800s believed the Earth was a perfect sphere with no variations.
“Better technology came along and phased out the optical methods,” says Ken Seidelmann, an astronomer at the University of Virginia. Seidelmann and a team of scientists were the ones to determine the prime meridian had shifted, not because of variations in the Earth’s surface, but because of better instrumentation, according to Discovery-Seeker.
In 1984, the Bureau International de l’Heure (International Time Bureau), (BIH), redefined the prime meridian based on new calculations of the Earth’s core and satellite data. Based on the measurements, the Earth’s center of mass was calculated with an accuracy of about the size of a dime.
The BIH had to make a decision, did they change the longitudes of the Earth or the time zones to match the new calculations? They choose to shift the map rather than the time zones. “They chose not to have a discontinuity in time and let the longitudes correct themselves,” Seidelmann said.
