Running along and close to the 180 is a dashed line called the International Date Line, something that regularly intrigues international travelers. Suppose on our previous flight to Japan we left the United States on Sunday and headed over the Pacific Ocean. As soon as we arrive at that date line, it suddenly becomes Monday. If you arrive at it at 2 p.m. local time on Sunday, it instantly changes to 2 p.m. Monday. Similarly, if returning home you arrive at that line at, say, 11 a.m. Friday, it suddenly becomes 11 a.m. Thursday as you fly eastward towards home. What's it all about and why do we have to gain or lose a whole day -- 24 hours -- at one time at one location?
The reason is because each day on Earth must have two boundaries. One of these we all know - the Midnight Line. This is not fixed on Earth, but moves incessantly westward as the Earth rotates eastward. This Midnight Line's location on Earth is directly opposite the sun -- opposite the longitude on Earth that is experiencing noon. If you could enter a super fast rocket traveling thousands of miles per second at the time you're experiencing noon and head directly north to the North Pole without stopping there, you would arrive at the opposite side of the Earth which at the time is experiencing midnight. This Midnight Line is but one boundary of each day.
The other is our International Date Line. Like the Prime Meridian, this location can be at any longitude. So it was again agreed internationally to place this Date Line at the 180th meridian, not because there is any special significance to its being directly opposite Greenwich, but because it runs right down the middle of the Pacific Ocean where there are the fewest land masses. However, the line does jog east and west of the 180 meridian to some extent to avoid splitting the day across single political entities such as the Aleutian Islands, which are part of Alaska.
To give you a clear cut idea of why the Date Line is necessary, suppose your home location is experiencing midnight, east of which is Wednesday -- the new day -- while the old day Tuesday is west of the midnight meridian. Now enter your imaginary super-rocket and again go up to and past the North Pole down the other side of the Earth where it's noon. But non of which day - Tuesday or Wednesday? See the problem? Se we need a date boundary that is fixed on the Earth in addition to the Midnight Line date boundary; the fixed one is the International Date Line.
When it is midnight along the Date Line, the entire Earth is experiencing the same day -- that is the only time this occurs. Now as the turning Earth causes the Midnight Line to head westward towards Asia, it "unreels" the new day, let's say Wednesday, from the Date Line. When the Midnight Line reaches Greenwich, the half of the Earth West of Greenwich all the way to the Date Line is still in Tuesday, while the other half is in Wednesday. As the Earth keeps turning, Tuesday keeps shrinking on the Earth while Wednesday keeps expanding until the Midnight Line again reaches the Date Line. Now it is Wednesday all over the Earth, but as the Midnight Line keeps heading west, Thursday is born, while Wednesday is shrinking.
A most interesting consequence of all this -- which might even become a real mind-challenging parlor game -- is that a particular day, or calendar date, remains on the Earth for 48 -- not 24 hours! Can you picture why this is so? Could your "sharp" friends and acquaintances? With the information just given, plus your globe, you can show the reason for it.
This International Date Line phenomenon is a real puzzler to many, but not a few people -- including veteran international travelers -- have trouble picturing why there are different time zones as we travel east and west, which causes that infamous "jet lag" so often experienced. With the aid of the globe, visualizing it is very easy. Around the time the Greenwich meridian and International Date Line was set up, the world was also divided into 24 time zones, each of which represented 15 of longitude. The idea was that each 15 would run down the center of a zone which would experience the same time; this is the origin of the Eastern Central, Mountain and Pacific time zones we use in the United States. Originally the time zone boundaries ran pretty much 7 1/2 east and west of the meridian of the zone, but since then there were major "gerrymanders" in these zones -- by far the worst being outside the United States -- because these time zones were, like legislative districts, created politically. Many atlases contain time zone maps of the world, with all their present convolutions. However, as a general rule, for each 15 of longitude you head east, it becomes 1 hour later; the opposite is true heading west.