The International Date Line, an imaginary line, functions as a line of demarcation separating two consecutive calendar dates. The time difference on either side of this line is 24 hours. So, the date changes as soon as one crosses this line. As one travels from the western to the eastern hemisphere, the date becomes one day later, while it is one day earlier as one travels across it from the eastern to the western hemisphere.
The International Date Line passes through the mid-Pacific Ocean and roughly follows a 180 degrees longitude north-south line on the Earth. The 180° meridian passes through the mid Pacific, which is devoid of big land masses and mostly runs through the sparsely populated islands on the Central Pacific Ocean. Though the International Date Line follows the 180º Meridian of Longitude for most of its distance, it is not a straight line and it goes Zigzag in some places to avoid islands and countries from experiencing two different dates on either side of the Line.
Despite its importance and internationally accepted name, the International Date Line has no legal international status and countries are free to choose the dates that they observe.
Great Circles & Great Circle Routes
A Great Circle is any circle that circumnavigates the Earth, passes through its center, and always divides the Earth in half. Thus, among all the lines of latitude only the Equator is a great circle and each line of longitude combined with it diametrically opposite the meridian is a great circle. The shortest distance between any two points on the Earth lies along a great circle.
Great circles are easily identified on a globe based on the lines of latitude and longitude. As each meridian has a corresponding line on the opposite side of the Earth, all the lines of Longitude, or Meridians, are of the same length and represent half of a great circle. When combined, they cut the globe into equal halves, representing great circles. The Prime Meridian at 0° is half of a great circle, while the International Date Line at 180º on the opposite side of the globe, also represents half of a great circle. If these two are combined, they will create a full great circle which cuts the Earth into equal halves.
The Equator is the only line of latitude characterized as a great circle, as it passes through the exact centre of the Earth and divides it in half. But, none of the Lines of Latitude north and south of the Equator is a great circle, because their length decreases as they move toward the poles and they do not pass through Earth's center. Hence, these parallels are considered as small circles.
Since they are the most efficient way to move across the globe, great circle routes are still used for long distance travels. They are most commonly used by ships and aircraft where wind and water currents are not a significant factor. Thus, in the northern hemisphere, planes traveling west normally follow a great circle route that moves into the Arctic. However, when traveling east, instead of the great circle route, it is more efficient for these planes to use the jet stream, a rapidly moving current of air, which is usually thousands of miles long and wide, but is relatively thin.