Moon Phase Calculator

At a glance you can see the moon's current phase, how much of its face is lit, and the date of the upcoming full moon. Those details matter when you're timing outdoor plans, working in the garden, or just curious about what the sky is doing overhead. Illumination in particular shapes stargazing conditions, and in 2026 the year's first full moon arrives on January 13th.

Behind the result is an astronomical algorithm. It converts the current date and time into a Julian date, then runs a set of orbital parameters and trigonometric functions to fix the moon's position relative to the Earth and Sun. The lit fraction comes from the angular separation between moon and sun as seen from Earth. To find the next full moon, the algorithm steps forward in time until it detects a full moon, the point where the phase angle reaches 0 or 360 degrees.

The data is precise, but the view from your backyard isn't: cloud cover and local weather can hide what the numbers describe. The next-full-moon date runs on Universal Time (UT), so shift it to your own time zone where that matters. And a new moon is not the same as a pitch-black sky. Faint earthshine can still pick out the disk under good conditions, which catches a lot of observers off guard.