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Moon Phase and Illumination: How Moon Age Relates to Percent Lit

The moon's phase and its illuminated percentage both track a single underlying number β€” its age since new moon.

The synodic month: about 29.53 days

A full cycle from new moon to new moon (called a synodic month) takes about 29.53 days on average, which is why moon phase calendars do not line up perfectly with a fixed 30-day month.

What "moon age" means

"Moon age" simply means how many days have passed since the most recent new moon, and it is the number that drives both the phase name and the approximate illumination percentage.

A simple illumination approximation

A common simple approximation treats illumination as following a cosine curve over the cycle: roughly 0% at new moon, 50% at first and last quarter, and 100% at full moon, with a smooth curve in between rather than a straight line.

The eight named phases

The cycle is commonly divided into eight named phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent β€” "waxing" means growing more illuminated, "waning" means shrinking.

Why the approximation is not exact

The moon's orbit is elliptical rather than perfectly circular, and Earth's own position relative to the sun shifts slightly through the cycle, so precise illumination figures from astronomical almanacs deviate a little from the simple cosine approximation, especially near new and full moon.

Practical uses of moon phase

Moon phase and illumination matter for astrophotography and stargazing (a bright moon washes out faint stars), and are referenced informally in fishing and hunting folklore, even though the scientific backing for those particular claims is much weaker than the astronomy itself.

Why "monthly" moon phases do not match the calendar month

The synodic month (29.53 days) is shorter than most calendar months, which is why the date of a full moon drifts earlier from month to month and why some months contain two full moons β€” the second one popularly nicknamed a "blue moon." Calendar months were set by cultural and administrative convention, not by the moon's actual cycle, despite the word "month" itself deriving from "moon."

How the simple illumination formula actually works

Treating the moon's illuminated fraction as (1 βˆ’ cos(2Ο€ Γ— moon age Γ· 29.53)) Γ· 2 gives a reasonable approximation: at moon age 0 (new moon) it returns 0%, at moon age ~14.77 (full moon) it returns 100%, and it smoothly interpolates the quarters in between. It is a simplified model, not the precise calculation astronomers use, but it is accurate enough for casual reference.

Frequently Asked Questions

Why do different apps sometimes show a slightly different date for the full moon?

The exact moment of full moon is a specific instant in time (when the moon is precisely opposite the sun), and depending on time zone, that instant can fall on different calendar dates in different parts of the world, even though it is the same astronomical event everywhere.

Does the moon phase actually affect tides?

Yes β€” during new and full moon, the sun and moon's gravitational pull align, producing stronger "spring tides" with a bigger difference between high and low tide. During the first and last quarter, their pull partially cancels out, producing weaker "neap tides."