Astronomy & Time

Moon Phase Calendar

A lunar calendar that computes the Moon-Sun angle for every night, draws the phase disc from the illuminated fraction, and uses Astronomy Engine to find the new, quarter and full moon event instants in your local time.

Open the calendar Southern Cross Clock Sunrise star map

Computed, not illustratedCited lunar geometryIllumination, phase trend, moon age, elongation, N/Eq/S orientation and NASA/USNO/Astronomy Engine citations in one calm working calendar.

SunMonTueWedThuFriSat

Showing the sky as seen from the Southern Hemisphere, where the lit limb appears opposite to Northern views.

The formula

k = (1 - cos e) / 2

k is the illuminated fraction of the disc. e is the geocentric elongation: the Moon's ecliptic longitude minus the Sun's. At 0 degrees the Moon is new; at 180 degrees it is full.

Worked example - selected night

On Tue, 23 June 2026, the computed Moon-Sun elongation is 104 degrees. That gives 62% illumination and a waxing gibbous.

Assumptions & limits

Principal phase times come from Astronomy Engine's Moon phase search, with event instants shown in your device's local timezone.
Daily discs use the geocentric Moon-Sun elongation at local noon, so they are calendar-scale phase views rather than a full sky-position simulation.
Illumination is the lit area of the disc, not perceived brightness.
The N/Eq/S toggle changes the displayed orientation, not the actual phase. Exact orientation in the sky still depends on time of night and observer latitude.

Sources

NASA Science - Moon phases USNO - Dates of primary phases of the Moon USNO - Fraction of the Moon illuminated Astronomy Engine - lunar phase search

How it works

The calendar is built from elongation

The page computes the Moon's phase as ecliptic longitude relative to the Sun, then turns that angle into illuminated fraction. Principal phase times use Astronomy Engine's Moon phase search, while the visible method keeps the calendar checkable.

Phase

New, quarter and full labels are assigned from the computed Moon-Sun angle instead of a decorative icon set.

Orientation

The N/Eq/S toggle changes the display orientation so northern, equatorial and southern views can be compared.

Citation boundary

The page cites public astronomy explainers and phase data services, and names the engine used for event timing.

Citations

Source basis for the moon calendar

NASA ScienceMoon phase explanation, phase sequence and illuminated-side context.USNO Moon PhasesOfficial primary phase data service for new, quarter and full Moon dates.USNO Illuminated FractionFraction-illuminated reference for comparing phase and illumination.Astronomy EngineOpen-source Sun/Moon/planet calculation engine used for phase search timing.