How to Use the Moon Phase Calculator
The Moon's appearance changes as the amount of sunlit surface visible from Earth shifts through a repeating cycle: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent before returning to new moon. This full cycle is called a synodic month, and it averages about 29.53 days. This calculator figures out where your chosen date falls within that 29.53-day cycle, then shows the moon's shape, its illumination percentage, and how many days have passed since the most recent new moon.
The math works by measuring the time elapsed between a known reference new moon (January 7, 1970, 20:35 UTC) and your chosen date, then taking the remainder after dividing by the 29.53058867-day synodic period. A remainder close to zero means new moon, while a remainder near the halfway point (about 14.77 days) means full moon. Illumination is approximated with a cosine curve that smoothly transitions from 0% to 100%, close enough to real astronomical values for everyday planning.
Knowing the moon's brightness on a specific date is handy for camping, fishing, stargazing, or night photography trips. Around the full moon, bright moonlight washes out fainter stars, while around the new moon, dark skies are ideal for viewing the Milky Way or meteor showers. It's also a fun way to check what the moon looked like on a birthday or other memorable date.
Frequently Asked Questions
We calculate the time elapsed since a known reference new moon, then take the remainder after dividing by the synodic month (about 29.53 days). A remainder near 0 means new moon, and near the midpoint means full moon.
On average 29.53058867 days. Because the relative positions of Earth, the Moon, and the Sun vary slightly, actual observed phases can differ from this average by up to about a day.