Moon Phase Visualiser
The Moon Phase Visualiser works out which phase the Moon is in on a date you choose and shows it visually; it works for past and future dates as well.
The most obvious rhythm noticed by anyone who looks at the sky from time to time is the Moon changing shape. This tool puts that rhythm within reach: it shows you the Moon of today, of your birthday, or of a camping night you are planning. Given that a full moon is sought after for photography and a dark sky for stargazing, phase information is a practical planning aid; if your curiosity runs deeper, the sky, mind and culture section is a good place to continue.
Tool
Frequently asked questions
Why is the Moon invisible at new moon?
At new moon the Moon lies close to the line between the Earth and the Sun; the face turned towards us is not lit, and because it moves across the sky together with the Sun it is lost in daylight. A few days later it reappears as a thin crescent on the evening horizon.
Is the phase the same everywhere in the world on the same day?
Because the phase depends on the Moon–Earth–Sun geometry, yes, it is essentially the same; time differences can shift the calendar day. What changes is not the phase but the angle at which the Moon sits in the sky: in the southern hemisphere the crescent faces the opposite way from ours.
Is it true that the full moon affects human behaviour?
This widespread belief has been tested in research, and no evidence has been found of a strong, consistent effect on sleep or behaviour. There are limited findings that bright full-moon nights may slightly affect sleep; the mechanism there is not mysterious either — it is light.
What is a supermoon, and does it appear in this tool?
A supermoon is a full moon that coincides with the part of the Moon’s orbit closest to the Earth; on those nights the Moon looks slightly larger and brighter. This tool calculates the phase but not the Moon’s distance, so it does not report supermoons separately.
Method and formula
The calculation rests on the fact that the Moon’s phase cycle — the time from one new moon to the next, known in astronomy as the synodic month — averages 29.53 days. A known new moon date is taken as the reference; the difference in days between the date you choose and that reference is divided by 29.53, and the remainder gives the position within the cycle: around 0 is a new moon, the middle of the cycle is a full moon, and the values in between are the crescent and gibbous phases. The percentage of illumination is derived from that position too. The method gives the phase correctly in general terms, but because of irregularities in the orbit it can be out by up to about 18 hours from the true phase time. Location-dependent details, such as exactly where in the sky and at what time the Moon will appear, fall outside the scope of this calculation.
How to read the result
The phase names on the result screen also tell you the direction of the cycle: in the “waxing” phases the Moon grows a little fuller each evening, and in the “waning” phases it thins away again. If you are planning an observation, two practical points help: a full moon is in the sky all night but its brightness drowns out faint objects, while the period around a new moon is the darkest window for deep-sky observing. Because the calculation uses the average cycle, there can be a difference of up to about 18 hours from the true phase times, which on borderline dates can shift the phase name shown by one step.
Limits
- 29.53 days is an average; because of irregularities in the Moon’s orbit, real cycles vary between roughly 29.3 and 29.8 days.
- The tool cannot give the start time of a phase to the minute; because the average cycle is used, the deviation can reach about 18 hours, and on borderline dates the phase name can shift by one step.
- The Moon’s position in the sky, its rise and set times and its visibility above the horizon are not part of the calculation; these depend on where you are.
- Lunar and solar eclipses call for separate calculations; not every full moon or new moon brings an eclipse.
Calculator and tool methodology · Nothing you type is sent to a server; results stay in your browser.