As every traveler soon finds out, it is important to know the local time.【C1】______until the last century, every town and village kept its own time, usually regulated【C2】______an official public clock set in the town hall or village church.【C3】______the sun was directly overhead, the clock【C4】______strike noon. This meant that the time was【C5】______for each degree of longitude(经度). Saudi Arabia and a few Pacific islands still keep this 'sun time'.【C6】______of the world, however, operates according to time zones【C7】______by international agreement. The globe is divided【C8】______24 time zones, one for each hour of the day. Mean time is the time at the Prime Meridian(本初子午线). All the【C9】______zones are measured according to【C10】______time or minus time, reflecting the differences【C11】______the particular time zone and Greenwich mean time. The International Date Line,【C12】______one day changes to the next, is at【C13】______near the 180th meridian. The boundaries between time zones are usually meridians.【C14】______on some maps they appear to be straight lines, they are【C15】______great circles. In many places,【C16】______, the time-zone boundaries have been【C17】______to accommodate national boundaries and other political or economic divisions. Most nations prefer to have【C18】______of their territory in the same time zone. Those【C19】______very large areas, however, need to【C20】______themselves into two or more zones. 【C1】