Moon Phase Calculator: Tonight & the Next 10 Days
Enter any date to see the Moon's phase, how much of it is illuminated, and how many days until the next full and new moons — worked out from how many days have passed since a known new moon, divided by the Moon's roughly 29.53-day cycle.
Calculated from the Moon's average orbital period, accurate to within a few hours of exact new and full moon moments — precise enough for everyday viewing plans, but professional almanacs account for small extra wobbles in the Moon's real orbit that this simpler method doesn't.
How a moon phase is worked out
The Moon takes about 29.53 days on average to cycle from one new moon to the next (called a synodic month). Starting from a well-documented new moon in January 2000, the calculator counts how many days lie between that date and the one you entered, divides by 29.53, and keeps only the leftover fraction — that fraction says exactly where in the current cycle your chosen date falls, from 0 (new moon) to just under 1 (the moment before the next new moon).
That position within the cycle, called the Moon's "age" in days, is compared against the standard eight named phases, each covering a set range of the cycle: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter and Waning Crescent, before returning to New Moon.
Illumination and the waxing/waning direction
The illuminated fraction shown is a standard approximation based on where the Moon sits in its cycle: 0% at new moon, 50% at both quarters, and 100% at full moon, following a smooth cosine curve rather than a straight line, since the Moon's edge-on shape near the quarters makes the lit fraction change fastest around new and full moon and slowest around the quarters.
"Waxing" means the illuminated portion is growing (between new moon and full moon), lit from the right in the diagram; "waning" means it's shrinking (between full moon and new moon), lit from the left. This left/right convention is the one used by most calendars and emoji, though it technically flips for an observer south of the equator.
How accurate this is
This method is accurate to within a few hours of the exact moment of each new and full moon — plenty precise for planning stargazing, photography, fishing or a moonlit walk. Professional almanacs go further, modelling small extra wobbles in the Moon's real orbit (it isn't a perfectly even circle) that shift exact phase times by up to a few hours beyond what this simpler average-cycle method captures.
Frequently asked questions
- What is tonight's moon phase?
- Leave the date on today (or open the calculator, which defaults to today) to see the current phase, illumination percentage and days until the next full and new moon.
- How long is a full moon cycle?
- About 29.53 days on average, called a synodic month — the time from one new moon to the next. It varies by up to about half a day cycle to cycle because the Moon's orbit isn't perfectly circular.
- What does the Moon's "age" mean?
- The number of days since the last new moon. An age of 0 is a new moon, about 14.8 is a full moon, and it resets to 0 again at the next new moon.
- How is the illuminated percentage calculated?
- From a standard cosine approximation based on how far through its cycle the Moon is: 0% at new moon, 50% at the first and last quarters, and 100% at full moon.
- Does this calculator also predict tides?
- No. Tides are influenced by the Moon but depend heavily on local coastline shape, so accurate tide times need location-specific data rather than the Moon's phase alone.