How is the moon phase calculated without a live ephemeris?

The tool converts your date to a Julian Day Number and measures how far it is past a known reference new moon. Dividing that by the synodic month length of about 29.53 days gives the lunar age, which maps directly to the phase.

How accurate is the illumination percentage?

It is a close approximation using a cosine model of the phase angle. It is typically within a percent or two of a full ephemeris, which is more than enough for planning whether the sky will be dark or washed out.

Why does astrophotography care about moon phase?

A bright moon floods the sky with scattered light, drowning faint nebulae and galaxies. Deep-sky imagers schedule their sessions around new moon, when illumination is near zero and the sky is darkest.

What is the synodic month?

The synodic month is the time from one new moon to the next, averaging 29.53 days. It is longer than the orbital period because the Earth also moves around the Sun, so the Moon must travel a little extra to line up again.

Does this account for my time zone?

The calculation uses noon on the selected calendar date as a reference point, so the phase shown is correct for that day worldwide. Phase changes slowly, about 12 degrees per day, so a few hours of time-zone offset rarely changes the named phase.

Moon Phase & Illumination Calculator

The Moon’s appearance follows a predictable 29.53-day cycle from new moon to full and back. This calculator converts any calendar date into a Julian Day Number, compares it to a known new moon, and reports the lunar age, the named phase, and the percent of the disc that is illuminated.

How it works

The date is first turned into a Julian Day Number (JDN), a continuous count of days used in astronomy. The lunar age is then the JDN distance past a reference new moon, reduced into one synodic month:

age   = (JDN − reference_new_moon_JDN) mod 29.53059
frac  = age / 29.53059          (0 = new, 0.5 = full)
illum = (1 − cos(2π × frac)) / 2 × 100%

The phase angle 2π × frac sweeps from 0 at new moon to π at full moon. The cosine model maps that angle to the lit fraction of the disc, which peaks at 100 percent at full moon and falls to roughly 0 at new moon. The named phase comes from which eighth of the cycle the age falls into.

Example and tips

A date 7.4 days after a new moon sits near the first quarter, about half illuminated and waxing. For deep-sky astrophotography, target the nights within three or four days of new moon, when illumination is below roughly 25 percent. The few days around full moon are best reserved for the Moon itself, bright planets, or simply staying indoors and processing old data.