Why is the appearance of a 7.5 minutes map rectangular rather than square?

Answer 1

Consider the earth as a sphere. Then one degree, in the East-West direction (longitude), at the equator would represent 1/360 of the equatorial circumference.

At the North or South pole, however, a one degree EW separation would amount to zero. Even a 180 degree separation would still be 0.

So, as you move further away from the equator, the longitudinal distances become smaller. At the equator a 7.5 minutes map would be a square (except that the earth is not a sphere but is slightly flattened). As you move North, the square becomes a trapezium, with the "vertical" sides tapering very slightly inwards as you move up the shape. When you reach the North Pole, the shape becomes a triangle. In between there are stages where the "verticals" have moved sufficiently towards one another so that the "square" becomes a rectangle.