Exercise 28 Page 526

In Euclidean geometry, whenever two perpendicular lines meet, there are $4$ right angles formed.

However, in spherical geometry, a line is a circle on the sphere whose diameter is equal to the diameter of the sphere. Therefore, lines in spherical geometry will meet twice — once at the front and another time at the back of the sphere.

In spherical geometry we have a total of $8$ right angles formed by two perpendicular lines.