Moon Phase Calculator

The Moon goes through 8 distinct phases during its 29.53-day lunar cycle:

• New Moon - The Moon is between Earth and Sun, with the illuminated side facing away from us (0% visible)
• Waxing Crescent - A small sliver of the Moon becomes visible on the right side (1-49% visible)
• First Quarter - Half of the Moon is illuminated on the right side (50% visible)
• Waxing Gibbous - More than half is visible as we approach Full Moon (51-99% visible)
• Full Moon - The entire face of the Moon is illuminated (100% visible)
• Waning Gibbous - The illumination begins decreasing from the right (99-51% visible)
• Last Quarter - Half of the Moon is illuminated on the left side (50% visible)
• Waning Crescent - A small sliver remains visible on the left before returning to New Moon (49-1% visible)

A complete lunar cycle (also called a synodic month) takes approximately 29.53 days. This is the time it takes for the Moon to return to the same phase, such as from one Full Moon to the next Full Moon. During this period, the Moon completes all 8 phases as it orbits around Earth.

Moon phases are calculated based on the Moon's position relative to Earth and the Sun. Our calculator uses a known New Moon date as a reference point and calculates how many days have passed since then. By dividing the elapsed days by the lunar cycle length (29.53 days), we can determine where the Moon is in its current cycle and what phase is visible from Earth.

Moon illumination percentage indicates how much of the Moon's visible surface is lit by the Sun from our perspective on Earth. At New Moon, illumination is 0% because the lit side faces away from us. At Full Moon, illumination is 100% because we see the entire sunlit hemisphere. The illumination gradually changes between these extremes as the Moon orbits Earth.

The Moon doesn't actually change shape - it's always a sphere! What changes is how much of the sunlit portion we can see from Earth. As the Moon orbits our planet, the angle between the Sun, Moon, and Earth changes, altering how much of the illuminated side is visible to us. This creates the illusion of the Moon "growing" (waxing) and "shrinking" (waning) throughout its cycle.