Final answer:
The Earth's gravitational force on the Sun and the Sun's gravitational force on the Earth are equal in magnitude due to Newton's Law of Universal Gravitation and his Third Law of Motion, which dictate equal and opposite forces.
Step-by-step explanation:
The gravitational force between two objects is described by Newton's Law of Universal Gravitation, which states that the force is proportional to the product of the two masses and inversely proportional to the square of the distance between their centers. According to this law, the Earth's gravitational force on the Sun and the Sun's gravitational force on the Earth are equal in magnitude but opposite in direction. This is because for every action there is an equal and opposite reaction, as Newton's Third Law of Motion states.
The Sun is much more massive than the Earth, but the gravitational force that the Earth exerts on the Sun is equal to the force that the Sun exerts on the Earth. The reason we only see the Earth moving significantly in response to this force (and not the Sun) is because of the vast difference in their masses, which affects their accelerations. The Earth has a much smaller mass, so it experiences a larger acceleration due to this force, and thus we see the Earth orbiting the Sun.