During the night, on our flight, we crossed the International Date Line – which runs a zig-zag pattern north and south down the middle of the Pacific Ocean. The International Date Line isn’t a straight longitude line because then some of the Pacific island nations would be on two different dates where it divided them! When we crossed the International Date Line we went from September 30th to October 2nd. What happened to October 1st?
To help explain this, you can get a world globe or make your own globe with an orange and a marker. Orient your orange with the top (where it connected to the tree) as the North Pole, and the bottom as the South Pole. Draw in some continents on your orange – they don’t have to be perfect – and then draw six equally-spaced longitude lines connecting the top of the orange to the bottom of the orange. Each of the six lines will represent four hours of time.
When you fly from New York to California, you would travel from East to West and you would set your watch back 3 hours. Why? The sun appears to rise in the east because of the way the earth spins on its axis. Turn your orange to the RIGHT in front of your face (you are the sun!) and see how some of the continents you are facing have sunlight and the ones you can’t see are in the dark. Every 24 hours the earth spins once on its axis so we have a full-cycle of day and night every 24 hours.
Pretend it is noon on the longitude line that is closest to where you live on your orange. As you travel west to the next longitude line you drew it is now only 8 am there. Keep going and it is 4 am and then midnight of the previous day. Hmm. If you keep going west, you will get home the day before you left! So people figured out that must change the date somewhere on the globe. The British had already determined that Greenwich, (pronounced "grennitch”) England was the 0 degree longitude line because they had an astronomical observatory there and they needed a reference point for the maps they were creating. Conveniently, the International Date Line, which needed to be opposite the 0 degree longitude line, ended up in the middle of the Pacific Ocean where it wouldn’t affect too many people’s lives going from one date to the next.
So the International Date Line changes the date so that as you travel west you "lose” a day, and then when you travel east across the line, you gain the day back. Pretty clever, eh?