Complex sentence: Greenwich Mean Time (GMT) was established in 1675 when the Royal Observatory was built as an aid to mariners to determine longitude at sea, providing a standard reference time when each city in England kept a different local time.
Split into simpler sentences: Greenwich Mean Time (GMT) was established in 1675 when the Royal Observatory was built as an aid to mariners to determine longitude at sea. It's providing a standard reference time when each city in England kept a different local time.