It is this longitude that gives us the reference time, GMT (Greenwich Mean Time), sometimes called UT (Universal Time), or UTC (Coordinated Universal Time), all terms well known to sailors as they systematically use it at sea.
For example, when the Vendée Globe organization organizes a session with a skipper, it always gives him a UT time to give him or her a schedule.
The antimeridian is the meridian opposite that of Greenwich, therefore corresponding to longitude 180°, the full globe being 360°. Time zones are zones of 15° of longitude each, corresponding to one hour. Given that a day has 24 hours, these zones are distributed over the 360° of the terrestrial globe. This antimeridian is also called the midnight meridian. When it is noon in London on this line, it is midnight at longitude 0.
When we move, like the Vendée Globe skippers, from west to east, by crossing this imaginary line, we go back one day.
Although it is theoretically straight, its division has been adjusted by certain countries to avoid cutting territories or islands into two different days, which would considerably complicate the lives of the inhabitants!
And of course almost every Vendée Globe there is nearly always one skipper who gets two Christmas Days!
