Reference · Astronomy
What is a lunation?
Kevin Blaxton — software builder; astrology student for 40+ years · figures computed by the OrbHeaven engine
Published · Last revised
A lunation is one full cycle of the Moon’s phases — the synodic month, about 29.5 days, from one New Moon to the next. It is driven by the changing angle between the Moon and the Sun as seen from the Earth: as that elongation grows from 0° to 180° and back, the lit fraction of the Moon we see waxes to full and wanes away again.
What is a lunation?
It is the synodic month: the time for the Moon to return to the same phase, about 29.53 days. It is longer than the Moon’s orbit around the Earth (27.3 days) because the Earth and Moon are also moving around the Sun, so the Moon must travel a little further each cycle to catch up with the shifted Sun. The word also names a single New or Full Moon within that cycle.
The cycle of phases
The phase is set by the Sun–Moon elongation. At New Moon the two share a longitude (0°) and the near side is dark; the lit fraction then grows through the waxing crescent and first quarter to Full at 180°, where the whole near side is lit; then it shrinks back through the last quarter and waning crescent to New again. The lit fraction is a simple function of the elongation — one half of one minus its cosine — so the quarters are exactly half lit.
The strip shows the eight principal phases with their illuminated fractions, from New through Full and back.
How OrbHeaven finds the New and Full Moon
By searching for the instant the Sun–Moon elongation reaches 0° (New) or 180° (Full). The engine finds, for example, the New Moon of 29 January 2025 and the Full Moon of 13 January 2025 in the same cycle. Those instants are exact; the fuller treatment of each, with the eclipse geometry, is in The New Moon and The Full Moon.
A short history
The lunation is the oldest unit of the calendar: the month is the Moon’s cycle, and lunisolar calendars — Babylonian, Hebrew, Chinese and others — count their months from New Moon to New Moon, inserting extra months to keep step with the solar year. The regularity of the synodic cycle made it the first reliable clock beyond the day.
What the tradition reads
Interpretation, labelled as such: astrology reads each New Moon as a beginning and each Full Moon as a culmination, and the waxing and waning halves as times for building and for releasing. These readings are a tradition laid on the plain fact of the changing light, and are kept separate from the astronomy of the cycle.
Common misconceptions
- Why is a lunation longer than the Moon’s orbit?
- The Moon orbits the Earth in about 27.3 days, but in that time the Earth has moved around the Sun, so the Moon needs about 2.2 more days to return to the same phase — giving the 29.5-day synodic month.
- Is a lunation the same as a New Moon?
- A lunation is the whole cycle of phases, but the word is also used for a single New or Full Moon within it. Both uses are common.
References & further reading
- [1] Meeus, J., Astronomical Algorithms, 2nd ed. (Willmann-Bell, 1998) — the synodic month and lunar phases.
- [2] Richards, E. G., Mapping Time: The Calendar and Its History — lunisolar calendars built on the lunation.
- [3] OrbHeaven engine (astronomy-engine) — the New/Full Moon instants and the phase illumination; re-derived in moon-phases.test.ts and lunation-demo.test.ts.
Suggested citation
Blaxton, K. (2026). What is a lunation. OrbHeaven Reference. https://orbheaven.com/reference/lunationsRevision history
- First build. New shared moon-phase strip (full cycle; illuminated fractions re-derived and engine-checked in moon-phases.test.ts); synodic vs sidereal month; cross-links The New Moon and The Full Moon; calendar history and tradition kept separate. No photo, no interactive.
About a minute. Birth time optional, and no account until after you have seen it.
More in the reference library
What is a new Moon?
A new Moon is the instant the Sun and Moon share the same ecliptic longitude — a conjunction, with the Moon between Earth and Sun and its lit side turned away. This explains the geometry, distinguishes the exact instant from the visible crescent, works a real example from the engine, and shows why most new Moons are not solar eclipses.
What is a full Moon?
A full Moon is the instant the Sun and Moon reach about 180° apart in ecliptic longitude — an opposition, the Earth between them and the near side fully lit. With the geometry, a worked example from the engine, and why most full Moons are not lunar eclipses.
What is the waxing Moon?
The waxing Moon is the half of the lunar cycle from New to Full, when the lit part of the disc grows each night. This explains the astronomy — the rising elongation and illuminated fraction — and separates it from the traditional reading.