Question

Why as the degree of longitude increases the lenght decreases?

Answer 1

Both Latitude and Longitude can be though of imaginary/invisible geometric planes that slice through the Earth. The trace of these planes where they intersect the surface of the Earth form the lines of latitude and longitude you find on the globe (or a map). The difference is that: the set of all latitude planes are parallel to each other and parallel to the Earth’s equator (think of them as being horizontal with respect to a globe); the set of all longitude lines intersect along a single line or axis and pass through the Earth’s North and South Pole (think of them as vertical with respect to a globe).

Thus the parallel latitudes never intersect and never grow closer to each other, but the intersecting longitude lines, which all pass through and contain the Earth’s north-south rotational axis, are farthest apart at the equator but closer together as you move towards the poles.

Therefore if you were standing at the North Pole (or south) longitude is meaningless, but not 0 degrees, which is a specific longitude. Effectively you'd be at “every longitude” at the same time. Take one step off the pole towards the equator however, and you'd be on a specific line of longitude. Conversely if you’re on the equator, at say 75-degrees East longitude, if you walk east or west your longitude changes but your latitude stays the same, and if you step off the equator to the north or south your latitude would change but not your longitude.