Question

When Venus passes between the Earth and the sun, it is visible as a tiny black dot on the sun's bright disk. Why is Mars never visible in this same way?

Answer 1

Mars never transits the Sun because its orbit is outside that of the Earth.

Explanation:

When a planet passes between the Sun and the Earth and is visible as a dot crossing the solar disc it is called a transit. Only Mercury and Venus can transit the Sun because their orbits are inside the Earth's orbit. All of the other planets, including Mars have orbits completely outside that of the Earth and so can never pass between the Sun and the Earth.