The estate began as 200 acres belonging to Saint Peter's Abbey in Ghent, until the Crown took it back in 1427. Henry VI gave it to his uncle Humphrey, Duke of Gloucester, who built a riverside house called Bella Court and a small castle on the hill, known as Duke Humphrey's Tower. Bella Court grew over generations into the Tudor Palace of Placentia, where Henry VIII was born and where his daughters Mary and Elizabeth spent their childhoods. Henry stocked the park with deer and rode out from these gates to hunt. In 1433 Greenwich became the first of London's royal parks to be enclosed, and James I later wrapped the whole estate in a twelve-foot brick wall, two miles long, which still defines the modern boundary.

Duke Humphrey's Tower fell into disrepair. In 1675 Charles II, advised by Christopher Wren, chose its hilltop site for a new astronomical observatory. The decision changed everything. Astronomers here spent two centuries refining the measurement of stars and time, and in 1884 an international conference in Washington formalised what mariners had already accepted: the line passing through Airy's transit telescope at Greenwich would be the world's Prime Meridian. The famous red Time Ball on the Observatory roof still drops at one o'clock each day, as it has since 1833, originally so that ships on the Thames could set their chronometers. The IERS Reference Meridian, defined by satellites, now runs 102 metres east of the painted line, but tourists still queue to straddle the older boundary.

Walk through the gates from the river and you are on the lower park, a green plain dotted with the children's playground, a boating lake, the Queen's Orchard, and the Pavilion Cafe. Then the hill rises sharply. A steep climb past gnarled sweet chestnut trees, planted in the 17th century and now corkscrewing skyward, brings you to the upper park: a flat extension of Blackheath dotted with the rose garden, the cricket pitch, and Queen Elizabeth's Oak, a hollow shell of a tree associated with the young Princess Elizabeth. From the plaza outside the Observatory, where the statue of General Wolfe gazes north, the city unspools in a single sweep. Canary Wharf glints in the foreground. The Shard rises to the west. The O2's white peaks crouch downstream. Simon Jenkins called this view one of the finest in England.

Greenwich Park has not always been ornamental. During the First World War, 139 plots were carved out on the northern lawns where local families grew vegetables for seven shillings and sixpence a year. The allotments returned in 1939, and anti-aircraft guns were positioned in the flower garden, with tree tops cut down to clear their lines of fire. Beneath the soil east of One Tree Hill, archaeologists have recently excavated a community-built air-raid shelter, alongside Saxon burial mounds and the remains of a magnetic observatory built by John Pond in 1817. The park is layered with such things: Roman foundations, medieval boundaries, royal indulgences, twentieth-century rationing.

In 2012 the park hosted the Olympic equestrian events, despite a petition of 12,000 signatures from neighbours who argued the lawns could not bear it. They could. The runners of the London Marathon are still corralled here every spring before crossing the start line at Charlton Way, and the autumn Big Half finishes here too. A 12 million pound restoration called Greenwich Park Revealed is now restoring the Grand Ascent, a series of giant grass steps that climb the slope, and a new learning centre by Vanbrugh Gate carries the name of Ignatius Sancho, the eighteenth-century writer and abolitionist who lived in Greenwich. Underneath, the chestnuts go on swirling, the Time Ball goes on dropping, and the line in the courtyard goes on cleaving the world in two.