How the calculation works
The Moon takes on average 29.53 days (the "synodic month") to complete a full cycle of phases, from one new moon to the next. Starting from a precisely known reference new moon (January 6, 2000, 18:14 UTC), it's enough to calculate how many days have passed to the target date and see where in the 29.53-day cycle it falls.
Algorithm validation
Before publishing this calculator, the algorithm was tested against 6 real, documented new moon and full moon dates (including the July 2023 supermoon and the 2020 Halloween blue full moon), matching within a few hours in every case — within the expected precision for this kind of simplified calculation.
Limitations
This is a simplified deterministic calculation (it doesn't account for minor orbital perturbations), accurate enough for general purposes, but not for high-precision professional astronomical use.
Frequently asked questions
Does this work for any date, past or future?
Yes, the algorithm is deterministic and works equally well for any date, with no practical range limit.
Does the time of day matter for the result?
This calculator uses UTC noon of the chosen date as a reference, so the result is the approximate phase for that day, not for a specific hour.
Why isn't "new moon" always exactly 0% illumination?
Because the illumination model used is a continuous sinusoidal approximation of the illuminated disk fraction — near an actual new moon, the calculated illumination gets very close to zero, but rarely exactly zero at noon.