Lunation Calendar — Deep Dives — Orbit Diagram

A month of lunar motion becomes one field of geometry.

At the center of the Lunation Chart, Earth sits within the plane of its orbit around the Sun while the Moon’s tilted path rises above and falls below it. The diagram brings together three qualities that change continuously through a lunation—the Moon’s progress through the cycle, its height relative to the ecliptic, and its distance from Earth—so their relationships can be seen at a glance.

Two planes, one orbit

The gold plane is the ecliptic: the plane of Earth’s orbit around the Sun, which also traces the Sun’s apparent annual path through our sky. The Moon does not orbit in this plane. Its blue orbital plane is tilted by about five degrees, carrying the Moon first to one side of the ecliptic and then to the other.

Where the two planes intersect, the Moon’s path crosses the ecliptic at two nodes. At the ascending node ☊, the Moon crosses northward; at the descending node ☋, it crosses southward. These crossings appear in the Orbit Diagram and as Lunar Node Crossing Rare Events. When a node is also near New or Full Moon, the three bodies may align closely enough for an eclipse; the Eclipses Deep Dive follows that alignment further.

An orbit that also keeps time

The diagram is not a snapshot taken at one instant. It spreads the Moon’s whole path through the lunation around Earth, beginning with New Moon and proceeding clockwise in step with the chart’s surrounding bands. A quarter-turn therefore represents one quarter of the lunation’s elapsed time, making it possible to read inward or outward along the same moment. The actual quarter phases fall near those divisions, but not necessarily on them.

This temporal arrangement is an explanatory choice. Viewed from north of the ecliptic, the Moon actually orbits Earth counterclockwise. Nor is the angle around the diagram its exact projected orbital longitude: that angle represents elapsed time within the lunation. The Moon’s calculated distance from Earth and its latitude above or below the ecliptic remain physical quantities, even though time supplies the path around the page.

Above and below the ecliptic

The Moon’s separation from the gold plane shows its ecliptic latitude—how far north or south of the ecliptic it lies. That separation narrows to zero at each node and reaches a northern or southern extreme between crossings. It is the same changing quantity traced through time in the Year Chart’s Eclipse Band.

Ecliptic latitude should not be confused with declination, the Moon’s north-south position relative to Earth’s celestial equator. The two reference planes are tilted to one another. The Moon’s declination extremes are lunistices, explored in the Sun-days and Moon-days Deep Dive.

Nearer and farther

The Moon’s orbit is not a perfect circle, and Earth is not exactly at its center. The Moon’s distance therefore changes continuously. Perigee is the nearest point in this cycle; apogee is the farthest. The Orbit Diagram preserves that changing distance in the shape of the blue path, while Lunar Perigee and Apogee Rare Events mark the two extremes in time.

Changing distance leaves fingerprints elsewhere in the charts. The Moon moves faster when it is nearer Earth and slower when it is farther away, contributing to the unequal spans between principal phases. Distance also changes the Moon’s apparent size: a Full Moon near perigee looks somewhat larger than one near apogee, and during a solar eclipse that difference helps determine whether the aligned Moon covers the Sun or leaves a bright ring. The Eclipses Deep Dive follows that consequence further.

Earth and the observer

Earth appears as a small globe at the center of the diagram. Its axis, equator, and the latitude of the chart location connect orbital geometry to an observer standing on the surface. They provide orientation rather than a second scale: the Earth and Moon are enlarged, and the distances compressed, so that the relationships remain legible.