Marking the meridian: The Royal Greenwich Observatory
Millions of tourists have stood precisely here, in the courtyard at the Royal Greenwich Observatory astride the famous brass meridian line. Cameras at the ready, left leg in the western hemisphere, right leg in the eastern hemisphere, click. The meridian passes directly through the observatory, and is precisely defined by the centre of the crosshairs of George Airy's 1851 transit telescope. Above the telescope on the outside of the building there's a clock counting the days since the Millennium, a silver plaque and a tiny hole out of which a green laser shines along the meridian after dark, visible for many miles to the north. The red line down the face of the building marks the precise longitude at which time begins. But it's not the original Greenwich Meridian.
Flamsteed House was built in 1675 on the highest ground within Greenwich Park, the perfect location for an observatory with unobstructed views of the sky over London. Unfortunately the windows of Christopher Wren's magnificent Octagon Room were found to be pointing slightly off the north-south axis, and so the first Royal Astronomer John Flamsteed made all his observations from a shed at the bottom of the garden instead. It was the telescope in this shed that established the first of four Greenwich meridians, each defined by a different 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 120 years ago today.
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". However, for the last 120 years it's been Airy's Prime Meridian that is more properly recognised here at Greenwich and, quite literally, all around the world.