Figure 3.4.2, You can see how the international date line bends and turns to incorporate political boundaries. (“Standard Time Zones of the World,” CIA World Factbook, 2007. https://www.cia.gov/cia/ publications/factbook/ reference_maps/time_zones.html)

International Dateline

Now let's look at Figure 3.4.2, the international date line, which is at 180º longitude. When you cross this line, the date and day change. If you cross the line from the west to the east, then you gain a day. If you cross the line from the east to the west, then you lose a day.

Find the international date line on this map. Now imagine that you are lounging on a cruise ship, a foot or two east of the line. This is tricky because from this vantage point the east is on the left side of the map! Your friend is on the other end of the ship, a foot or two west of the line (right side). If it were 12 noon on December 25, or Christmas day, on the east side (left side) of the line where you are sitting, it would be 12 noon on December 24, or Christmas Eve, on the west side (right side) of the line where your friend is sitting!

Notice that the prime meridian and the international date line, as well as other time zone lines, are not always straight; sometimes they veer away from longitude lines. This is because time zones sometimes follow political boundaries for convenience. As you can imagine, it would make things complicated if the line went down the middle of communities.

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