The Moon orbits Earth because Earth's gravity attracts the Moon while the Moon possesses sufficient sideways velocity to remain in a continuously curved free fall trajectory rather than falling directly into Earth. The Moon is therefore continually accelerated towards Earth, but its motion perpendicular to the Earth Moon line causes it to move around Earth. Without Earth's gravity, the Moon would travel approximately in a straight line according to inertia. Without its transverse orbital motion, it would fall towards Earth.
The actual combination produces an orbit. The Moon's orbit is not a perfect circle. Its average distance from Earth is about 384,400 kilometres, but the distance varies because its orbit is elliptical, with a perigee when it is closer to Earth and an apogee when it is farther away.
The Moon's orbital period relative to the stars is approximately 27.3 days, known as the sidereal orbital period, while the cycle from one similar lunar phase to the next is approximately 29.5 days because Earth is simultaneously moving around the Sun. The distinction between these periods is important when discussing lunar motion.
Earth's gravitational attraction provides the primary force responsible for the Moon's orbit, but the Earth and Moon both orbit their common barycentre rather than one body remaining perfectly stationary. The Earth Moon barycentre lies inside Earth because Earth is much more massive than the Moon, but it is displaced from Earth's centre. Both bodies therefore participate in the system's orbital motion.
In a simplified two body model, the gravitational force between Earth and Moon is GMm divided by r squared, and the corresponding orbital acceleration changes the Moon's velocity continuously. The Moon also exerts an equal and opposite gravitational force on Earth, producing measurable motion of Earth around the barycentre and contributing to ocean and solid Earth tides. The Moon's orbit is consequently a coupled gravitational system rather than a one sided situation in which Earth is fixed and the Moon alone moves.
The Moon's orbital path and speed are governed by conservation of angular momentum and orbital energy. As the Moon moves closer to Earth, its orbital speed increases, while as it moves farther away, its speed decreases. This behaviour follows from the conservation of angular momentum and the conversion between kinetic and gravitational potential energy.
The orbital angular momentum is substantial and changes only through torques exerted by external or internal interactions that transfer angular momentum between components of the Earth Moon system and beyond. Earth's tidal bulges are particularly important because tidal friction transfers angular momentum from Earth's rotation to the Moon's orbital motion. This transfer causes the Moon to recede from Earth at an average modern rate of roughly 3.8 centimetres per year, while Earth's rotation gradually slows. These values vary over geological time and are affected by Earth's oceans, continents, internal structure and orbital configuration. The present Earth Moon system is therefore not static. It is evolving because angular momentum and energy are continually redistributed.
The Moon's rotation is closely connected to its orbit. The Moon rotates once about its own axis in approximately the same time that it completes one orbit around Earth, producing synchronous rotation or tidal locking. As a result, approximately the same lunar hemisphere generally faces Earth. This condition developed through tidal interactions over a long period.
Earth's gravitational field raises tidal deformations in the Moon, and internal dissipation of tidal energy removes rotational energy and drives the Moon towards a state in which its rotation period matches its orbital period. Once synchronous rotation is established, the tidal configuration becomes comparatively stable.
The Moon can still exhibit libration, however, so observers on Earth can see slightly more than half of the lunar surface over time, although never the entire far side from a fixed Earth based vantage point. Libration arises from factors including the Moon's orbital eccentricity, axial tilt and geometric effects associated with viewing from Earth's moving position.
The Moon's orbit is influenced not only by Earth but also by the Sun and other bodies. Solar gravity produces substantial perturbations because the Sun is extremely massive, even though it is much farther away. The Moon's orbit is therefore not a simple fixed ellipse around Earth. Its orbital plane is inclined by about 5.1 degrees relative to the ecliptic, and the orientation of the lunar orbital plane changes over time.
The Moon's orbital nodes regress, completing a cycle of approximately 18.6 years. This nodal motion contributes to long term variations in eclipse geometry and tidal behaviour. The eccentricity of the lunar orbit also changes over time. Solar gravitational perturbations, Earth's nonuniform gravitational field and the gravitational influence of other bodies all contribute to the detailed lunar ephemeris. Modern lunar trajectory calculations therefore use high precision numerical models rather than a single simple equation.
The Moon's orbit is also central to the occurrence of eclipses, tides and the changing lunar phases. Lunar phases arise primarily because the Moon reflects sunlight and the visible fraction of its illuminated hemisphere changes as the Moon orbits Earth. Solar eclipses occur when the Moon passes sufficiently close to the Earth Sun line and its apparent disc covers the Sun from a particular location on Earth.
Lunar eclipses occur when Earth lies between the Sun and Moon and the Moon passes through Earth's shadow. Eclipses do not happen every month because the Moon's orbital plane is tilted relative to Earth's orbital plane. Tides arise primarily from differential gravitational forces across Earth, especially the difference between the gravitational attraction on the near and far sides. The Moon's orbit and Earth's rotation together determine the timing and pattern of tides, although local geography and ocean basin geometry strongly modify observed tidal behaviour.
The long term history of the Earth Moon system provides evidence that its current orbit has evolved. The leading scientific explanation is that the Moon formed approximately 4.5 billion years ago following a giant impact involving the early Earth and a planetary sized body, although details of the impact and subsequent lunar accretion remain active areas of research. The newly formed Moon would have been much closer to Earth than it is today.
Tidal interactions then transferred angular momentum from Earth's rotation into the Moon's orbit, causing lunar recession. Geological evidence, including ancient tidal deposits, provides information about past Earth Moon configurations, although reconstructing deep time requires models and interpretation. The Moon's present distance, synchronous rotation and the slowing of Earth's rotation are therefore connected consequences of a long history of gravitational and tidal evolution.
The Moon orbits Earth because gravity and inertia combine to produce a bound orbital trajectory, while the detailed characteristics of that orbit reflect angular momentum, orbital energy, tidal evolution and perturbations from the Sun and other bodies. The Moon continuously falls towards Earth but possesses enough transverse motion to keep missing the surface. Its elliptical orbit, changing speed, barycentric motion, synchronous rotation, tidal recession and long term orbital evolution all follow from gravitational dynamics and angular momentum transfer. The Moon's orbit is therefore not simply a static circular path. It is a changing dynamical system that links lunar motion with Earth's rotation, ocean tides, eclipses, planetary motion and the long term history of the Earth Moon system.
