Question

How do we know earth is the third planet away from the sun?

Answer 1

By the photographs taken by astronomers. And also by satellites

Answer 2

We can determine the distances of the planets from the Sun by their orbital periods.

Explanation:

We can measure the orbital periods of the planets by observing their positions with respect to the fixed stars.

Mercury and Venus both have orbital periods of less than a year and so are closer to the Sun than the Earth.

Mars and the outer planets all have orbital periods greater than a year and so are further away from the Sun than the Earth.

Therefore as Earth has the third longest orbital period it is the third planet from the Sun.