Marking the meridian: The Royal Greenwich Observatory
Our prime meridian goes through Greenwich thanks to a 17th century astronomer's telescope. John Flamsteed was the first Astronomer Royal and made all his observations from a hilltop shed in the grounds of the newly established Royal Greenwich Observatory. An imaginary line drawn from north to south through this telescope became the marker from which celestial measurements of longitude were taken. But Flamsteed's was the only the first of four Greenwich meridians, each defined by a different successive telescope, and each now marked by a silver plaque on the observatory wall.
Meridian 1: based on John Flamsteed's telescope, 1685
Meridian 2: based on Edmund Halley's telescope, 1725 (185cm east of Flamsteed's meridian) Established when Flamsteed's original telescope began to subside into the ground.
Meridian 3: based on James Bradley's telescope, 1750 (11m east of Halley's meridian). Still used by the Ordnance Survey for map-making purposes.
Meridian 4: based on George Airy's telescope, 1851 (5.79m east of Bradley's meridian). Selected as the Prime Meridian of the world exactly 125 years ago this week.
It's the last of these lines which draws thousands of tourists to the summit of Greenwich Park every year. Legs astride the meridian they scramble to take photos of one another in the front courtyard, one foot in the west and the other in the east. You've probably stood here yourself, although if you've not visited recently you may not yet be aware that the public face of the Observatory has had something of a presentational revamp. There's no longer direct admittance to the courtyard, so folk now have to venture inside through a turnstile in the main Equatorial Building. No charge, it's still free, but they've finally got rid of the ridiculous requirement to collect an unnecessary ticket on the way in. Turn left to visit the two-year-old Astronomy Centre and bold bronze conical planetarium[photo]. Both are excellent, but time instead requires us to turn right onto the "Meridian route".
The Observatory's rear courtyard has recently been remodelled, replacing a nondescript patch of grass with a flower border and raised path [photo]. Visitors aren't made aware that they're crossing the meridian here, presumably to avoid congestion, but a metal globe on a plinth marks the invisible line across the garden [photo]. Onward past a rather huge telescope to the information desk, where a beaming girl would quite like to flog you an audio guide for £3.50, then out into a landscaped Garden of Time [photo]. Ahead is Flamsteed House[photo], whose Wren-built Octagon Room would have been perfect for the first meridian telescope had not the building accidentally been aligned 13 degrees off true north. An excellent exhibition of time and timepieces is housed within, including John Harrison's longitude-beating nautical chronometers, although most foreign visitors whizz through here fairly fast. They're after that definitive photo astride the famous brass meridian line in the main courtyard [photo], or maybe an ice cream from the first trailer kiosk in the western hemisphere [photo].
And then visitors pass back inside, this time for a rapid chronological jog through Greenwich's zero degree telescopes. Flamsteed's first, then a room aligned for Halley, then the room in which James Bradley took his measurements. The final chamber is almost completely filled by Airy's Transit Circle, which points outward across the milling snappers in the courtyard towards the stars beyond. If the Greenwich meridian could have an epicentre this would be it, not that most of the passers by seem to recognise the instrument's true significance. A variety of Prime Meridian souvenirs are available nextdoor - perfect if you're ever short of a fridge magnet or two. And don't forget to take the detour up the stairs to the Time and Society Gallery (important, but underwhelming), because this leads on to the Onion Dome perched at the very top of the building. Housed inside is the largest refracting telescope in the UK, a pert 28-incher built in 1893, through which the public are still invited to take a look at least once a month. Had this monster have been in place a decade earlier, then the Greenwich Meridian would probably have passed through these giant crosshairs instead. But it's Airy's line of sight that's been immortalised here at Greenwich and, quite literally, all around the world.
n.b. With the advent of global positioning technology in the 1990s, a new virtual meridian has been introduced. It lies 102½ metresfurther east than the official Greenwich meridian and is the line used for all air and sea navigation. That's why when you stand in the courtyard at Greenwich wielding a handheld GPS device it doesn't show a longitude of precisely 0°0'0".