 Above the Earth, we can measure geodesics and their deviation using test particles. For example, if we place three test particles vertically above the Earth's atmosphere and separate them by a small amount, we can see what happens when they fall freely along their geodesic lines towards the center of the Earth. Because the particles closer to the Earth feel a slightly stronger gravitational attraction than the particles further up, they will accelerate faster. The distance between them will increase. This shows that the curvature is negative along this line in space above us. If we place three test particles horizontally along an east-west line with the same starting separation, we can see what happens when they fall freely along their geodesic lines towards the center of the Earth. Because the particles are moving to the same point, the distance between them will decrease. This shows that the curvature is positive along this line in space. The same would be true for the same reason if we started with a horizontal north-south line. Interestingly, if we sum the three curvatures for the three spatial dimensions, we get a total curvature of zero. While true above the Earth where there is no matter, this would not be the case inside the Earth.