We are often taught that the Earth orbits the Sun and the Moon orbits the Earth. However, this is a simplification of the complex orbits of celestial bodies.
The Sun, with a mass 1,048 times that of Jupiter, is the largest star in the Solar System, and every planet or object is influenced by its gravitational pull. Yet, this is not as straightforward as we might think. The Earth also exerts a very small gravitational force on the Sun.
The Moon does not technically orbit the Earth (Image source: IFL Science).
According to NASA, Kepler’s Third Law describes the relationship between the masses of two bodies orbiting each other. When a smaller star orbits a larger star, both revolve around a common center of mass, known as the barycentre.
In cases where one celestial body is much larger and heavier than the others, the center of mass for that system will lie within the larger body. When two celestial bodies have comparable masses, the center of mass will be located outside of both, and they will orbit around that point.
Scientists have used this principle to measure the motion of stars around their center of mass with larger planets, which aids in the exploration of distant planetary systems.
In reality, the barycentre of objects in the Solar System is typically close to the Sun; however, due to the gravitational influence of massive gas giants like Jupiter and Saturn, the barycentre rarely lies inside the Sun.
Therefore, the Earth does not currently orbit a point inside the Sun because the center of mass lies outside of it. Our planet is orbiting that point in space rather than the Sun itself.
“Planets do not orbit the Sun alone due to the primary gravitational influence of Jupiter,” explains astronomer O’Donoghue. “This means that the planets must orbit a new point in space.”
The same principle applies to smaller bodies such as planets and their moons. The Earth and the Moon orbit a point 5,000 km from the Earth’s center, and this changes as the Moon gradually moves farther away from the Earth.
While these facts may have little impact on our daily lives, they serve as a reminder that things are more complex than what we are taught in school.
