How is the moon phase calculated?

The tool converts the date to a Julian Date, then subtracts the Julian Date of a reference new moon (6 January 2000, 18:14 UTC). Dividing the elapsed days by the mean synodic month (29.530589 days) and taking the remainder gives the Moon''s age in the current cycle. The illuminated fraction is (1 - cos(2*pi*age/SYNODIC)) / 2 — zero at new moon, one at full moon.

What is a synodic month?

The synodic month (29.530589 days) is the time from one new moon to the next — the Moon returning to the same angle between the Earth and Sun. It differs slightly from the sidereal month (27.32 days) because the Earth has moved around the Sun in the meantime, so the Moon must travel a little further to realign with the Sun.

How accurate is the moon phase result?

The calculator applies Jean Meeus harmonic correction terms to the mean synodic month, reducing error to roughly a few hours for dates within a century of J2000. The mean-only approach used in simpler calculators can be off by up to a day; the correction terms here bring that down significantly.

What does illumination percentage mean?

Illumination is the fraction of the lunar disc visible from Earth that is lit by the Sun. Zero percent is new moon (completely dark from Earth), fifty percent is either quarter, and one hundred percent is full moon. The formula is derived from the cosine of the Moon''s phase angle.

A supermoon occurs when a full or new moon coincides with the Moon being near perigee (its closest approach to Earth). The conventional threshold is roughly 363,300 km. The tool flags when the Moon''s approximate distance is inside that threshold. The Moon appears up to 14% larger and 30% brighter than at apogee, and ocean tides are slightly stronger.

Why might the phase look different from a moon-phase app?

Most differences come down to timezone. The tool uses noon UTC on the selected date as the reference instant. If you are far east of UTC, ''tonight'' in your local time corresponds to a different UTC date. Try moving the date one day forward or back to see the transition.

Moon Phase Tonight — Gera Tools

The moon phase tonight tool renders a live lunar disc, shows the exact illumination percentage, marks where today falls on the 29.53-day synodic cycle, and lists when the next four principal phases occur — all computed instantly in your browser without any network call.

How it works

The calculation follows Jean Meeus Astronomical Algorithms (2nd edition), the standard reference for amateur and professional planetary computation.

Step 1 — Julian Date. The selected date (noon UTC) is converted to a Julian Date (JD) — a continuous count of days from 1 January 4713 BC used in astronomy to avoid calendar-system complications.

Step 2 — Moon age. The elapsed days since the reference new moon of 6 January 2000 at 18:14 UTC (JD 2451550.1) are divided by the mean synodic month of 29.530589 days. The fractional remainder is the Moon’s age in the current cycle (0 = new, 14.76 = full, 29.53 = next new).

Step 3 — Illuminated fraction. The fraction through the cycle maps to an illumination via:

illumination = (1 - cos(2 * pi * age / 29.53)) / 2

This follows from the geometry of the Sun–Earth–Moon angle: the lit fraction grows smoothly from 0% at new moon through 50% at the quarters to 100% at full moon.

Step 4 — Phase name. The eight named phases are assigned by dividing the cycle into eighths. The index round(fraction * 8) mod 8 selects the correct label.

Step 5 — Disc rendering. The SVG moon disc draws a right semicircle (the lit side) and closes it with an elliptical terminator whose horizontal semi-axis is R * (1 - 2 * illumination). A positive terminator semi-axis produces a crescent; a negative one produces a gibbous shape. For waning phases the disc is reflected horizontally so the lit side appears on the left.

Step 6 — Earth–Moon distance. A truncated Meeus Chapter 47 series calculates the approximate geocentric distance in kilometres using the Moon’s mean anomaly, the Sun’s mean anomaly, mean elongation, and argument of latitude.

Worked example

On a day when the Moon’s age is 19.5 days (roughly five days after full moon):

Fraction through cycle: 19.5 ÷ 29.53 ≈ 0.66
Illumination: (1 - cos(2π × 0.66)) / 2 ≈ 0.77 → 77% lit
Phase index: round(0.66 × 8) = round(5.28) = 5 → Waning Gibbous
The disc shows a large lit area on the left (waning), with a thin dark crescent on the right

Age (days)	Fraction	Phase	Illumination
0	0.00	New Moon	0%
3.7	0.13	Waxing Crescent	15%
7.4	0.25	First Quarter	50%
11.1	0.38	Waxing Gibbous	85%
14.8	0.50	Full Moon	100%
18.4	0.62	Waning Gibbous	85%
22.1	0.75	Last Quarter	50%
25.8	0.87	Waning Crescent	15%

Formula note

The illumination formula uses the phase angle between the Sun, Earth and Moon. At new moon the phase angle is 0° (Moon between Sun and Earth, far side lit); at full moon it is 180° (Earth between Sun and Moon, near side fully lit). The cosine function maps this continuously: illumination = (1 - cos(phase_angle)) / 2, where phase_angle = 2 * pi * fraction. The Meeus harmonic corrections to the epoch reduce the typical error from around ±1 day (mean-synodic-month-only) down to a few hours.