The Moon orbits the Earth, and the Earth orbits the Sun. Humans have known this for a long time. We figured this out through observation and by noticing patterns in the apparent movements of both celestial bodies across the sky. The model we used to have for the motion of celestial bodies was that they orbited in circles. Kepler showed this was not the case in his first law of planetary motion: everything orbits in an ellipse. This resource looks specifically at the orbit of the Moon around the Earth. The simulation that you’ll see below enables you to change a few settings so you can observe how the Moon orbits.
This resource is a good follow-on to the phases of the Moon resource and is part of a sequence that builds toward understanding how eclipses work.
Terms
Barycentre: The Moon and the Earth actually orbit each other around a common point, which is called the barycentre.
Earth’s axial tilt (obliquity): Earth’s rotation axis is shown tilted by 23.44° relative to the ecliptic, the plane of Earth’s orbit around the Sun.
Moon’s axial tilt: The Moon’s rotation axis is tilted by about 1.54° to the ecliptic, or approximately 6.68° relative to the normal of its own orbital plane.
Lunar orbital inclination: The Moon’s orbit is inclined by 5.145° to the ecliptic, allowing students to see that the Moon does not orbit Earth in exactly the same plane that Earth orbits the Sun.
Elliptical lunar orbit: The Moon follows an elliptical orbit with an eccentricity of 0.0549, so its distance from Earth changes continuously throughout each orbit.
Changing Earth–Moon distance: The simulation displays the instantaneous Earth–Moon distance as the Moon moves between the nearer and farther parts of its elliptical orbit.
Earth–Moon barycentre: The barycentre shows the common centre of mass around which both Earth and the Moon actually move.
Earth’s barycentric motion: When the barycentre display is enabled, Earth can be seen making a small orbit around the Earth–Moon system’s centre of mass rather than remaining perfectly stationary.
Moon’s orbital motion: The Moon completes one sidereal orbit of Earth in approximately 27.32 days, with its orbital speed varying naturally around the ellipse.
Earth’s rotation: Earth rotates about its tilted axis once every sidereal day, approximately 23 hours 56 minutes.
Synchronous lunar rotation: The Moon rotates once during each orbit around Earth, causing approximately the same lunar hemisphere to remain directed toward Earth.
Visible rotation axes: Colour-coded axes show the orientations of Earth’s and the Moon’s spin axes, making their different obliquities easy to compare.
Orbital path: The Moon’s inclined elliptical orbital path is displayed in three dimensions, so you can examine its orientation around Earth from different viewpoints.
Top, side and 3D views: Preset viewpoints allow the orbital inclination, axial tilts and overall geometry of the system to be examined from different perspectives.
Relative-size view: Earth and Moon are shown at the correct size ratio while their separation is compressed to make the system easier to examine.
True-scale view: Earth, Moon and their centre-to-centre separation are displayed at the same proportional scale, revealing just how distant the Moon is compared with the sizes of the two bodies.
Real surface textures: Earth and Moon use mapped surface imagery to make their rotation and orientation easier to recognise visually.
Interactive time control: Students can pause the simulation, change its speed or move directly through the lunar orbital period to investigate particular configurations.
Next Steps
Now that we’ve covered the basics of the Moon’s orbit, we can start looking at how eclipses are caused, and the next major concept to understand is nodes.
