Moon Orbits

Moons orbit their parent planets much as planets orbit the Sun. Their paths are shaped by gravity and forward motion, producing a wide variety of stable, tilted, elliptical, and sometimes retrograde orbits.

Earth's Moon is the most familiar example, but the Solar System contains hundreds of confirmed moons with remarkably different sizes, origins, and orbital behaviors.

Earth's Moon completes one orbit in about 27.3 days along a slightly elliptical path. It is tidally locked, meaning it rotates once on its axis in the same time it takes to orbit Earth. As a result, nearly the same hemisphere always faces our planet.

Key Features of the Moon’s Orbit

The Moon remains an average of about 238,855 miles (384,400 kilometers) from Earth. Its orbit is tilted by roughly 5 degrees relative to Earth's orbital plane around the Sun. Because of this tilt, the Sun, Earth, and Moon do not align perfectly every month, which is why eclipses do not occur at every new and full moon.

The Moon is slowly drifting away from Earth at about 1.5 inches (3.8 centimeters) per year because of tidal interactions between the two worlds. Over immense periods, this process gradually lengthens Earth's day and changes the appearance and frequency of solar eclipses.

Diversity of Moon Orbits

Other planets have moons with very different orbital characteristics:

• Jupiter has 101 known moons. Many of its small outer moons follow distant, tilted, and sometimes backward-moving, or retrograde, orbits, suggesting that they were captured after Jupiter formed.
• Saturn has moons ranging from Titan, which follows a nearly circular orbit, to small satellites that help shape its rings through gravitational interactions.
• Mars has two small, irregular moons, Phobos and Deimos. Phobos orbits very close to Mars, while Deimos travels farther away.
• Neptune's Triton follows a retrograde orbit, strongly suggesting that it was captured from elsewhere in the Solar System.

Orbital Resonances and Stability

Many moon systems contain orbital resonances, in which orbital periods are linked by simple ratios. These relationships can stabilize orbital patterns or create repeated gravitational interactions that generate internal heat, drive surface activity, and influence ring structures.

Jupiter's large Galilean moons provide a well-known example. Io, Europa, and Ganymede are locked in a 1:2:4 orbital resonance. Their repeated gravitational tugs help power Io's volcanoes and contribute to tidal heating within Europa, where a global ocean lies beneath the ice.

Why Moon Orbits Matter

Moons can have a powerful influence on their planets. Earth's Moon helps stabilize the planet's axial tilt over long periods, affecting seasons and climate. Its gravity also drives ocean tides that shape coastlines, ecosystems, and human calendars.

By studying moon orbits, scientists can investigate how planetary systems formed, how gravity reshapes worlds over time, and where conditions such as internal heat or hidden oceans may exist.