Look at the picture above. Like I said before, the 180°E and 180°W are the same line so most of the time we just call it longitude 180° and we don't even bother to add E/W sign after at 180°. 90°N is the North Pole, while 90°S is the South Pole. Even though 90°N and 90°S are longtitude, they are not circles but just two points because they are the two ends of the earth, just like their names -- poles. Confused? Look at this:
Now you see the 90°N and 90°S are just two points right? Technically 90°N and 90°S are the shortest longitude on the Earth. Geographically, the 90°N is located in the middle of Arctic Ocean, while the South Pole 90°S is located in the middle of the continent Arctica. The 0° line (see the picture above), in fact, is the equator. The equator is the longest longitude (sounds weird but true) on the Earth. Above and below the equator we have 23.5°N and 23.5°S, and these two lines are important because between these two longtitudes we have tropics -- tropic of cancer. Be advised, the word "cancer" does not mean "tumor" here. Look that up in your dictionary! Compared to longitude, latitude does not have so many special lines and points, just remember all latitude lines are of the same length!The only special latitudes are the 0° and 180°. The International Date Line in the Pacific Ocean is defined based on longitude line 180° as what I have mentioned before, so that's the line we use to decide dates. For example, now is 12:00PM September 9th, 2011 in New York City (located at West Hemisphere, west side of International Date Line , but it is 12:00AM September 10th, 2011 in Beijing,China (located at East Hemisphere, east side of International Date Line). And countries located closest to 180° latitude are the earliest to have the next day. In other words, the people there see the sun rises first everyday in the Earth. New Zealand is the country that have the first sun shine on Earth everyday. Countries like Japan also have their mornings pretty early, and that is probably one of the reasons why they have a red sun on their flag. The 0° latitude is called Prime Meridian. The famous Royal Greenwich Observatory, RGO is build there. Also, there is a laser projected from the observatory marking the Prime Meridian line. That's how we get the Greenwich Mean Time (GMT).