Tonight
Moon Phase
Illumination of the near side
The figure is the moon for this moment in your browser's local time: the usual U.S. phase name, the sunlit percent of the near side, and an approximate age in days.
This figure is an estimate for learning and curiosity. It is not an observatory ephemeris, and it is not a guide for navigation.
What the numbers mean
Illumination is the sunlit share of the Moon's near side in this model, not a brightness reading from your backyard. A first quarter can sit a little above or below 50 percent because the Moon's path is not a perfect circle and the model uses elongation, not a camera measurement.
Moon age counts days since the last new moon found by the same formulas. The mean synodic month used here is 29.530588853 days. People sometimes treat the moon as a mood clock; that is folklore, not a finding from these numbers.
How this is calculated
The calculator uses approximate Sun and Moon longitudes, with the largest periodic terms, to find the Moon's elongation from the Sun. Illumination is then (1 + cos i) / 2, where i is about 180° minus that elongation. The disc follows a Northern Hemisphere sky view: waxing light grows on the right. The eight usual names are on the moon phases page. Every full moon and new moon in a year is on the full moon dates list. Horizon times are on the moonrise page. Distance and almanac names are on the supermoon page.
Questions
Why is illumination not always 50 percent at first or last quarter?
Quarter is a name for the Moon's place in the cycle, near 90° or 270° elongation. The sunlit percent comes from a cosine of the phase angle. Because the orbit is tilted and not circular, those two ideas do not lock to the same number every month.
Which time zone do the clocks use?
Clocks use the time zone you pick from the list, such as New York (Eastern). The page does not look up a city and does not use your IP. UTC is printed next to the local clock, so the same instant can be compared.
How close is this to a published moon calendar?
Principal phases are usually within a small fraction of a day of a public almanac. Daily percents can differ by a few points. Treat the result as a clear estimate, not a substitute for the U.S. Naval Observatory or a navigation almanac.
Does this give moonrise or azimuth?
Horizon times are on the moonrise page. This figure names phase, illumination, and age from elongation. It does not give azimuth or altitude.
Why is the bright side on the right or the left?
The drawing is a Northern Hemisphere view. From much of the Southern Hemisphere the waxing crescent sits on the other side of the disc. This site does not switch the picture by city.
Is this a horoscope?
No. There is no natal chart and no claim that the moon changes sleep or luck. The tools stop at geometry.