1. In the first half of the 19th Century the parallax of Mars was measured and found to be approximately 0.01" when this planet was at its closest point to the Earth. The diameter of the Earth was known to be approximately 13,000 km. The diameter of the Earth was accurately known as early as 200 B.C. It is interesting to note that astronomers knew the Earth was a sphere and its diameter 1700 years before they accepted the fact that the Sun was the center of the Solar System, a. Use the data in the introduction section, and equation 1, to determine the distance between Earth and Mars when Mars is at opposition (i.e., when it is at its closest point to the Earth). Hint: The Earth-Mars distance is the difference between the Mars-Sun distance and the Earth- Sun distance. Mars-Sun distance Earth-Mars distance Earth-Sun distance Distance from Earth to Mars at opposition (in AU) = b. Use the parallax of Mars at opposition (0.010 from the text for question 1), the Earth's diameter, and parallax formula (Object Distance = 57.3 [Earth Diameter/Parallax Angle]) to calculate the 19th Century estimate to the distance to Mars in kilometers