Astronomical Moon Phases & Lunar Calendar
Upcoming Primary Lunar Phases
Monthly Lunar Calendar
Percentages indicate lunar surface illumination (0% = New Moon, 50% = Quarter, 100% = Full Moon). Click any date to view astronomical details.
Frequently Asked Questions
Learn how lunar illumination, moon phases, synodic cycles, and orbital distances are computed.
What are the four primary phases of the Moon?
The four primary phases are New Moon (Moon between Earth and Sun), First Quarter (one-half illuminated and increasing), Full Moon (fully illuminated opposite the Sun), and Third/Last Quarter (one-half illuminated and decreasing). These represent specific geometric alignments of the Earth, Moon, and Sun.
How long is a complete lunar cycle (Synodic Month)?
A complete lunar cycle from New Moon to the next New Moon lasts approximately 29.53059 days (29 days, 12 hours, 44 minutes). This is known as a synodic month, which differs from a sidereal month (27.32 days) because Earth continues orbiting the Sun while the Moon orbits Earth.
How is lunar illumination calculated?
Illumination percentage is calculated using astronomical position algorithms based on Jean Meeus equations. The engine determines the Moon's phase angle—the angle between the Sun and Earth as seen from the center of the Moon—to compute the exact fraction of the visible lunar disc illuminated by sunlight.
Does the Moon appear upside-down in the Southern Hemisphere?
Yes! Because observers in the Southern Hemisphere are oriented opposite to observers in the Northern Hemisphere, the Moon appears inverted. In the Northern Hemisphere, the Moon waxes from right to left, whereas in the Southern Hemisphere, it waxes from left to right.