If a vector points northward, then it generates a meridian of longitude. Following that curve keeps on at the same longitude. A vector pointing eastward points in the direction of a curve whose points are at different longitudes. You mark the points on that curve with their longitudes.

Suppose a person's income were a function of the person's height and you want to graph that function. Label the horizontal axis "height" and the vertical axis "income". The axis labeled "height" is a line along which the height varies. It is not a line of constant height. Likewise the thing labeled "longitude" is a curve along which the longitude varies, not a curve of constant longitude.

If a vector points northward, then it generates a meridian of longitude. Following that curve keeps one at the same longitude. A vector pointing eastward points in the direction of a curve whose points are at different longitudes. You mark the points on that curve with their longitudes.

Suppose a person's income were a function of the person's height and you want to graph that function. Label the horizontal axis "height" and the vertical axis "income". The axis labeled "height" is a line along which the height varies. It is not a line of constant height. Likewise the thing labeled "longitude" is a curve along which the longitude varies, not a curve of constant longitude.

If a vector points northward, then it generates a meridian of longitude. Following that curve keeps on at the same longitude. A vector pointing eastward points in the direction of a curve whose points are at different longitudes. You mark the points on that curve with their longitudes.

If a vector points northward, then it generates a meridian of longitude. Following that curve keeps on at the same longitude. A vector pointing eastward points in the direction of a curve whose points are at different longitudes. You mark the points on that curve with their longitudes.

Suppose a person's income were a function of the person's height and you want to graph that function. Label the horizontal axis "height" and the vertical axis "income". The axis labeled "height" is a line along which the height varies. It is not a line of constant height. Likewise the thing labeled "longitude" is a curve along which the longitude varies, not a curve of constant longitude.