Every 29.53 days, the Moon completes a full cycle of phases — from the invisible new moon through the brilliant full moon and back again. These phases have guided agriculture, navigation, and calendars for thousands of years. Today, the Moon Phase Calendar lets you track every phase with precise times calculated from astronomical algorithms.
Why the Moon Has Phases
The Moon doesn't produce its own light — it reflects sunlight. As the Moon orbits Earth, different portions of its sunlit side face us, creating the cycle of phases.
At any given moment, exactly half the Moon is illuminated by the Sun (the half facing the Sun). The phases we observe depend on the angle between the Sun, Earth, and Moon. When the Moon is between Earth and the Sun, we see its dark side — a new moon. When Earth is between the Moon and the Sun, we see the fully illuminated side — a full moon.
The 8 Phases of the Lunar Cycle
New Moon — The Moon is between Earth and the Sun. The illuminated side faces away from us, making the Moon invisible (0% illumination). This marks Day 0 of the synodic cycle.
Waxing Crescent — A thin sliver of light appears on the Moon's right side (in the Northern Hemisphere). Illumination grows from 0% to 50%. The crescent Moon is often visible just after sunset.
First Quarter — Exactly half the Moon's visible face is illuminated (50%). Despite the name "quarter," half the Moon appears lit — it's called "first quarter" because the Moon is one-quarter of the way through its cycle.
Waxing Gibbous — More than half the Moon is illuminated, growing toward full. "Gibbous" comes from the Latin word for "hump."
Full Moon — The entire visible face of the Moon is illuminated (100%). The Moon rises at sunset and sets at sunrise, making it visible all night long. This occurs around Day 14.8 of the cycle.
Waning Gibbous — The illumination begins to decrease from full. The lit portion shrinks from the right side.
Last Quarter — Half the Moon is illuminated again (50%), but now the opposite half from the First Quarter. The Moon is three-quarters through its cycle.
Waning Crescent — A thin sliver remains, visible in the early morning sky before sunrise. The cycle then resets with the next new moon.
The Synodic Month
The complete cycle of phases — from one new moon to the next — is called a synodic month, and it lasts approximately 29.53 days (29 days, 12 hours, 44 minutes). This is different from the sidereal month (27.32 days), which is the time the Moon takes to orbit Earth relative to the stars.
The synodic month is longer because while the Moon orbits Earth, Earth is also moving along its orbit around the Sun. The Moon needs the extra ~2.2 days to "catch up" and return to the same Sun-Earth-Moon angle.
Moon Phases and Tides
The Moon's phase directly affects tides on Earth. During new moons and full moons, the Sun and Moon are aligned, and their gravitational pulls combine to create larger tidal ranges called spring tides. During the quarter phases, the Sun and Moon are at right angles, partially canceling each other's pull, creating smaller tidal ranges called neap tides.
This gravitational interaction is also why the Moon is slowly moving away from Earth — about 3.8 centimeters per year — and why Earth's rotation is gradually slowing down.
Tracking Moon Phases
The Moon Phase Calendar on this site calculates precise phase times using the Meeus astronomical algorithm. You can see daily illumination percentages, exact times for all major phase events, and the current lunar day. For a deeper understanding of the calculation method, see How Lunar Time Is Calculated.