“It might be expected that plumb lines, set up in various locations, if extended downward, would all pass through a single point, the center of Earth. But this is not the case.” — Why the Greenwich Meridian Moved, by Stephen Malys et. al., 2015
Both John Harrison’s clocks and lunar distance tables assumed that the prime meridian (zero degrees of longitude) ran through Greenwich Observatory. It took more than 100 years of wrangling for this meridian to be accepted everywhere, but with the need to standardize international time zones, the Greenwich prime meridian was finally adopted in 1884. Although other countries had their own prime meridians — in Paris and Berlin, for example — by then, most of the world’s shipping used charts based on the Greenwich meridian. Fortuitously, this meant that the International Dateline, where it’s midnight at Greenwich noon, runs through the middle of the Pacific Ocean, minimizing disruptions to populated areas. (Standardization came slowly for the French, who didn’t accept the Greenwich meridian until 1911.)
Today, you can visit the Royal Observatory, located on a hill in Greenwich Park in southeast London. Historically, this is a perfect location. An orange ball, visible to ships anchored in the Thames, has been dropping at precisely 1 p.m. every day for hundreds of years. An anachronism now, it used to allow navigators to set their timepieces to “Greenwich Mean Time” (GMT) before setting sail. In the Octagon Room inside the observatory, a “transit telescope” (that only rotates vertically) was used to establish GMT by noting exactly when reference stars crossed its field of view.
GMT is the local mean time at Greenwich counted from midnight: “mean” because Earth’s orbit around the sun is uneven. It’s an ellipse, plus Earth’s axis is tilted, so the sun crosses the Greenwich meridian at a slightly different time — up to 16 minutes either way — from when it’s at its zenith. The “equation of time” adjusts for this discrepancy, hence mean, or average, time. (Nowadays, the acronym GMT has been superseded by UTC, a compromise between the English “Coordinated Universal Time” and French “Temps Universel Coordonné.”)
Visitors to the Royal Observatory can stand on a stainless-steel strip next to the main building, with one foot in each hemisphere, east and west, the site of innumerable selfies. Except … if this is the world’s prime meridian, zero degrees longitude, why do our phones show that we’re actually at 5.3 seconds of longitude west? It wasn’t always so. Not that cellphones — essentially GPS receivers from satellites orbiting 22,000 miles overhead — existed in 1974, but had they, they’d have read exactly zero degrees of longitude.
Our planet isn’t a perfect sphere with uniform density, so plumb bobs don’t always point to the exact center of the Earth. This is the case at Greenwich. Around 50 years ago, geodesists (who study Earth’s size, shape, motion and gravity) realized that the geodetic zero meridian as established by orbital satellites was slightly different from what had been, until then, the prime meridian. The plane swept out by the transit telescope missed the center of the Earth by about 335 feet, so rather than retain the location of the prime meridian and change the time for everyone on Earth by about a third of a second, it was much easier to quietly accept that the true prime meridian, longitude 00°00°00°, was a short distance east of what it had been. Hence the photo of this writer’s phone, on the old prime meridian, showing it’s actually about five seconds west of the “real” one.
I suspect that perfectionist John Harrison, whose timepiece 265 years ago was accurate to five seconds after crossing the Atlantic Ocean, would not have approved.
Barry Evans (he/him, barryevans9@yahoo.com) recommends a visit to London’s Royal Observatory, where both Harrison’s H4 and his three previous attempts are on display.
This article appears in ‘Crash Course’ A Halloween Comic by Liz Valasco and Collin Yeo.