If you use your smartphone’s GPS to find Earth’s prime meridian – the north-south line marking zero degrees longitude – at Greenwich Observatory, you’ll notice something a bit odd. You won’t be standing on the monument that marks where the prime meridian should be; you’ll actually be standing 102 meters (334 feet) to the east. What’s going on?
This discrepancy between where the prime meridian is marked and where it actually is was noticed in the 1960s when satellites first started to be used to pinpoint locations. The original position was marked using a basin of mercury; the surface of the liquid was thought to be parallel to Earth, with a decent level of accuracy. But with GPS we can be much more accurate, although it has taken until now to explain why there is such a noticeable difference.
Scientists at the University of Virginia have found that it is the oblate shape of Earth that is the cause. GPS takes into account the slight bulge of our planet and its uneven gravity, and thus the lines of longitude are not split evenly across the globe as in astronomical coordinates. Basically, the location of the prime meridian at Greenwich Observatory is wrong. The research is published in the Journal of Geodesy.
“I think it would be a good idea to put a marker at the site of the offset,” lead researcher Ken Seidelmann from the University of Virginia told IFLScience. “It could give a clear explanation for people who do come with a GPS receiver and see it’s not right. It would show where GPS zero is. It’s another tourist attraction.”
It should be noted that the prime meridian at Greenwich Observatory isn’t really used for much, other than tourism. “Observations are not going to be made at the prime meridian site,” said Seidelmann. Nonetheless, according to Dr Marek Kukula, the Royal Observatory's public astronomer, talking to The Indepedent, staff are frequently asked about this anomaly. IFLScience has asked if the observatory plans to install a marker.
It’s not just Greenwich that is wrong, though. The scientists found that other markers for lines of longitude around the world were also incorrect. And the second major finding of the paper was that these other offsets are not the same, but actually vary from place to place, which adhered to the team’s gravity model and confirmed their theory. The biggest offset was in South America, specifically Chile, where astronomical instruments are high in the mountains.
While there aren’t many scientific implications from the research, it’s certainly still pretty interesting. And it puts to bed an intriguing, if not essential, puzzle.