Greenwich is an area of southeast London on the River Thames. The old British Royal Observatory stands there: a building from which scientists watched the moon, the stars, and the clock of the sky. The 0° meridian — the imaginary line that splits east from west — runs through the Observatory grounds. That is why people still say Greenwich Mean Time (GMT) for a world time standard, even though everyday clocks now follow close cousins of that idea. The name comes from Old English Grēnawīċ / Grēnewīċ, “green harbour” or “green settlement,” the same -wich you meet in other old English place names. Though actually English mouths do not say GREEN-witch. The usual British saying is closer to GREN-itch (and Woolwich, Norwich, Dulwich follow the same family habit). There are other Greenwiches in the world, some probably named after this one, but the dictionary sense here is the London place that owns the meridian.