What this calculator does
The moon cycle averages 29.53 days from one new moon to the next, and the phase on any date follows from how far through that cycle it is. This counts forward from a known new moon and takes the remainder.
The synodic month of 29.530588 days is not the same as the orbital period of 27.32 days. The difference is that the Earth moves around the Sun while the Moon orbits, so the Moon has to travel a little further than one full orbit to return to the same phase. That two-day gap is why the two figures are so often confused.
The formula
The days since a reference new moon are divided by the synodic month of 29.530588 days, and the remainder gives the age of the cycle. The phase name comes from which eighth of the cycle that age falls into, and the approximate illumination is derived from the cycle position. The method gives the phase to within a few hours, which is ample for any non-technical purpose.
| Term | Meaning |
|---|---|
| Synodic month | 29.530588 days, new moon to new moon. |
| Sidereal month | 27.32 days, one full orbit relative to the stars. Not the same as the phase cycle. |
| Age of the cycle | Days elapsed since the last new moon. |
| Illumination | The percentage of the visible disc lit, which peaks at full moon. |
The inputs explained
| Field | What to enter |
|---|---|
| Date | The date to find the phase for. Leave it on today or enter any past or future date. |
When to use it
Planning around moonlight
Astronomy, night photography and night navigation all depend on how much moonlight there will be, which the illumination figure answers.
Fishing and tide planning
Spring tides occur around new and full moon, so the phase is a useful proxy for tidal range.
Checking a historical date
Entering a past date gives the phase then, which is occasionally needed for dating photographs or checking an account of an event.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the phase progress through a cycle?
Four dates spaced eight days apart, covering most of one lunar cycle.
| Date | Moon phase | Age of the cycle | Approx. illumination |
|---|---|---|---|
| 2026-01-01 | Waxing Gibbous | 11.9 days | 91.1% |
| 2026-01-09 | Waning Gibbous | 19.9 days | 72.8% |
| 2026-01-17 | Waning Crescent | 27.9 days | 2.90% |
| 2026-01-25 | First Quarter | 6.4 days | 39.5% |
Questions
How long is a lunar month?
29.530588 days on average from new moon to new moon, which is the synodic month. Individual cycles vary by several hours either side because the Moon orbit is elliptical and its speed changes through the orbit.
Why is the synodic month longer than the orbital period?
Because the Earth moves around the Sun while the Moon orbits. After one full orbit of 27.32 days the Moon is back in the same place relative to the stars, but the Earth-Sun line has shifted, so it takes about another two days to return to the same phase.
How accurate is this calculation?
Within a few hours for the phase, which is more than adequate for planning. It uses an average cycle length and a fixed reference point, so it does not capture the variation caused by the elliptical orbit. Astronomical software accounts for that and is more precise.
Does the phase look the same everywhere?
The illumination is the same worldwide, but the orientation is not. A crescent that appears to open to the right in the northern hemisphere appears to open to the left in the southern, and near the equator it can look like a bowl. The phase is the same; the view is rotated.
For the day of the week and week number of a date, see the day number to date calculator. For sidereal time, see the sidereal time calculator.