RESEARCH NOTE 03 / LUNAR ORBIT
Why the Moon’s
distance changes.
The familiar average of 384,400 kilometres is useful, but the real Earth–Moon distance is always moving.
The orbit is not a perfect circle
The Moon follows a slightly elliptical orbit around Earth. At its nearest point, called perigee, it is roughly 356,400 kilometres away. Near apogee, the farthest point, the distance can reach about 406,700 kilometres.
Those values vary because the Moon is not moving around an isolated, perfectly spherical Earth. The Sun’s gravity, the shape of Earth, and the gravitational pull of other planets continuously perturb the orbit.
What does “distance from Earth” measure?
Published lunar distances usually measure between the centres of Earth and the Moon. That convention avoids changing the answer depending on a location on Earth’s surface. An observer’s actual distance to the Moon is smaller and varies with Earth’s rotation and the Moon’s position in the sky.
The Neon Archive Moon page uses an astronomical model to estimate the centre-to-centre distance for the current time. It should be read as a useful live estimate, not a direct radar measurement made by the website.
Distance changes apparent size
When the Moon is closer, it appears slightly larger. When it is farther away, it appears smaller. The difference is enough to affect whether a solar eclipse is total or annular: a more distant Moon may not completely cover the Sun.
The popular term “supermoon” generally describes a full Moon occurring near perigee. The change is real, although it is usually subtle without a side-by-side comparison.
Moon phase is controlled by geometry between the Sun, Earth, and Moon. Perigee and apogee describe distance. The two cycles are related only by timing, not by cause.
The Moon is also slowly receding
Laser-ranging experiments using reflectors left by Apollo astronauts show that the Moon’s average distance increases by about 3.8 centimetres per year. Tidal interaction transfers angular momentum from Earth’s rotation to the Moon’s orbit, gradually slowing Earth’s spin and moving the Moon outward.
That long-term drift is tiny compared with the tens of thousands of kilometres between perigee and apogee each month, but it reveals that the Earth–Moon system is still evolving.