apparent brightness apparent motion size location
I gave my students this multiple choice question: In the 17th century, to estimate the distance to other planets, scientists first used the technique of viewing the planet from two different locations on Earth's surface. Which characteristic of the planet were the scientists using to calculate the distance from Earth?

Only one answer is correct among these 4 choices:
- apparent brightness
- apparent motion
- size
- location

Could you tell me which one is correct?

location