How accurate is this compared to an actual astronomical almanac?

It's accurate to well within a day for any given date, since it's based on the moon's average synodic period -- but the moon's orbit is actually elliptical, not perfectly circular, so the real time between phases varies slightly (by a matter of hours) from one cycle to the next. That small variation is exactly why this method isn't precise enough for eclipse prediction, which depends on the moon's actual position, not just its average cycle timing.

What does "moon age" mean?

It's simply how many days have elapsed since the most recent new moon before your chosen date -- 0 days at a new moon, roughly 14.8 days at a full moon, and counting back up to just under 29.5 days as the next new moon approaches.

Why does the illuminated side switch from right to left partway through the cycle?

This reflects the real waxing/waning behavior of the moon as seen from the Northern Hemisphere -- during the first half of the cycle (waxing, phase fraction under 0.5) the right side grows brighter, and during the second half (waning) the left side is what remains lit as the moon shrinks back toward new. The tool switches which side it lights based on exactly that fraction.

Can I scrub through an entire month quickly instead of picking dates one at a time?

Yes -- the day slider lets you drag through every day of the currently displayed month, and the previous/next month buttons jump a full month while keeping the same day-of-month where possible (clamping to the last valid day for shorter months), which makes it easy to watch a full lunar cycle progress without re-entering a date each time.

Moon Phase Calculator

Pick a date to see the moon's phase, illumination, and age — the moon graphic below is drawn live from the numbers, not a picture.

How Moon Phase Calculator Works

The moon cycles through its full set of phases roughly every 29.5 days, and rather than looking up a chart or checking an app, this calculator works out the phase, illumination percentage, and "moon age" for literally any date directly from a well-known reference new moon and the moon's average orbital period.

Formula & Method

The number of days between your chosen date and a known reference new moon (January 6, 2000) is divided by the synodic month -- 29.530588853 days, the average time between one new moon and the next -- and the remainder (as a fraction of that cycle) determines where in the cycle your date falls. Illumination is derived from that fraction with (1 − cos(2π × fraction)) / 2, which models the sunlit portion as it grows and shrinks smoothly across the cycle, and the phase name (New Moon, Waxing Crescent, First Quarter, and so on) comes from checking which segment of the cycle the fraction falls into. The moon graphic itself is two overlapping CSS shapes -- a lit disc and a dark (or light) ellipse whose width encodes the illumination percentage -- not an image.

Worked Example

A date exactly 14.77 days (half a synodic month) after the reference new moon lands at phase fraction 0.5, which the calculator reports as a Full Moon with 100% illumination -- the reference point the whole cycle is built around.

Frequently Asked Questions

How accurate is this compared to an actual astronomical almanac?: It's accurate to well within a day for any given date, since it's based on the moon's average synodic period -- but the moon's orbit is actually elliptical, not perfectly circular, so the real time between phases varies slightly (by a matter of hours) from one cycle to the next. That small variation is exactly why this method isn't precise enough for eclipse prediction, which depends on the moon's actual position, not just its average cycle timing.
What does "moon age" mean?: It's simply how many days have elapsed since the most recent new moon before your chosen date -- 0 days at a new moon, roughly 14.8 days at a full moon, and counting back up to just under 29.5 days as the next new moon approaches.
Why does the illuminated side switch from right to left partway through the cycle?: This reflects the real waxing/waning behavior of the moon as seen from the Northern Hemisphere -- during the first half of the cycle (waxing, phase fraction under 0.5) the right side grows brighter, and during the second half (waning) the left side is what remains lit as the moon shrinks back toward new. The tool switches which side it lights based on exactly that fraction.
Can I scrub through an entire month quickly instead of picking dates one at a time?: Yes -- the day slider lets you drag through every day of the currently displayed month, and the previous/next month buttons jump a full month while keeping the same day-of-month where possible (clamping to the last valid day for shorter months), which makes it easy to watch a full lunar cycle progress without re-entering a date each time.

Date

Scrub Through

Uses the average 29.53-day lunar cycle from a known reference new moon — accurate to well within a day, but not precise enough for eclipse prediction, which depends on the moon's actual elliptical orbit.

— Moon Phase