Question

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

Answer 1

The crescent phases of Venus and mercury make it clear that they are inner planets to Earth.

Explanation:

By the revolution period of planets we can understand that position of planets from Kepler's 2nd and 3rd law.
Pictjure credit ESO .Org./you tube.

Answer 2

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

Explanation:

Kepler's third law relates the orbital period `P` in years to the semi-major axis distance `a` is astronomical units (AU).

`P^2=a^3`

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.