If one writes down the Newton's equations of motion for the Earth, the forces that act on the Earth are the gravitational forces exerted by all other massive bodies, such as the Sun, the Moon, the planets etc. The largest of these forces is that of the Sun: keeping into account only the Sun's force in the equations of motion, the two-body problem for the Sun-Earth system can be solved, and the orbit of the Earth around the Sun can be determined. However, there are the contributions by other massive bodies that influence the Earth's orbit. What is the most relevant contribution to the Earth's orbit after that of the Sun? A rough estimate based on distances and masses shows that it should be the attraction of the Moon.