crossmeridian

Crossmeridian is a term used in various contexts, often referring to the act or process of crossing the prime meridian or the International Date Line. The prime meridian, also known as the Greenwich Meridian, is the zero-degree line of longitude that runs through Greenwich, London. The International Date Line is an imaginary line on the Earth's surface that generally follows the line of longitude 180 degrees east or west of the prime meridian.

When a person or vehicle crosses the prime meridian from west to east, they are moving into the Eastern Hemisphere. Conversely, crossing from east to west places them in the Western Hemisphere. This is primarily a geographical distinction.

The International Date Line has a more significant practical implication. Crossing the International Date Line from east to west causes the calendar date to advance by one day. Conversely, crossing from west to east causes the calendar date to fall back by one day. This is to keep the calendar date roughly consistent with the solar day across different longitudes. For example, if it is Monday on the west side of the line, it will be Sunday on the east side.

The concept of "crossmeridian" can also be applied metaphorically in fields like physics or astronomy, though its usage in these contexts is less common and may refer to crossing a specific reference meridian in a celestial sphere or a theoretical framework. In everyday language, it most directly relates to the geographical crossing of these significant lines of longitude.