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London, the capital of the United Kingdom, has about 9 million residents near 51.5 degrees north latitude. It is the reference point for Greenwich Mean Time, running at UTC+0 in winter, and observes daylight saving, advancing to British Summer Time at UTC+1 from spring to autumn. As a result the city sits level with Coordinated Universal Time for part of the year and one hour ahead for the rest. Its relatively high northern latitude produces long summer days and short winter ones. As the historical origin of the prime meridian, London occupies a central place in the global system of time zones, from which offsets east and west are measured.