Actually, the Earth's tilt doesn't change from season to season. It stays tilted the same amount, but when it is on one side of the Sun, the northern half (USA)is farthest away from the sun (winter) and the southern half (Australia)is closest to the Sun (summer). Six months later, when the Earth has completed another half of its' orbit, the reverse is true. Twice each year, once in the spring and once in the fall, the northern half and the southern half are equal distances away from the Sun. Clear yet?