Mars in Opposition and Conjunction

Here is a nice animation showing Mars motions relative to Earth's motion. With Earth closer to the Sun than Mars, Earth moves faster, in accordance with Kepler's Third Law. When you run this animation, stop it when Mars is in opposition (Mars, Earth, Sun lined up), and when it is in conjunction (Mars, Sun, Earth lined up).What is the distance between Mars and Earth when Mars is in opposition? What is the distance between Mars and Earth when Mars is in conjunction? (You can "run the clock" forwards or backwards.) Use this information to determine Mars average orbital radius in AU (average the two numbers in kilometers, and divide by 150,000). You should find that this "seat of the pants" method for determining Mars average orbital radius is not exactly the same number you'll find in your book or online. Why is that?