(2 votes from 2 institutions)
Over the course of eleven months we determined the position of Mars on 45 occasions using a handheld cross staff and two to five bright reference stars of known right ascension and declination on each occasion. On average the observed positions are within 12 arc minutes of the true positions. Given that we took data prior to the start of retrograde motion and well past the end of retrograde motion, we can easily derive the date of opposition to the Sun. We were able to derive the date of perihelion, the orbital eccentricity, and the semi-major axis size of Mars' orbit. We obtain a value of the eccentricity of 0.086 +/- 0.010, which is to be compared to the modern value of 0.0934. Values as low as 0.053 or as high as 0.123 can be rejected at a high confidence level. A simple dataset can be obtained with cardboard and a ruler that demonstrates the elliptical shape of Mars' orbit.