Select a year, month and day to explore approximate moon age, phases and illumination. New and full moon dates are estimates.
Year and Month
This is a mean-cycle approximation, evaluated at 12:00 UTC each day. Tide labels are phase-based hints, not predictions of local water levels, currents or times.
This Month's New Moon
β
This Month's Full Moon
β
Near new / full moon (approx.)
β
Lunar Age on Selected Date
β
π New Moonπ Waxing Crescentπ First Quarterπ Waxing Gibbousπ Full Moonπ Waning Gibbousπ Last Quarterπ Waning Crescent
The moon changes shape a little every day, right? How does that work? It's not like the moon itself is turning on and off, is it?
π
Right, the moon doesn't emit its own light. It only reflects sunlight. The 'shape' of the moon changes depending on how much of the sunlit hemisphere is visible from Earth. At new moon, the moon is in the same direction as the sun, so the sunlit hemisphere faces away from Earth, making it appear dark. At full moon, the moon is opposite the sun (Earth between them), so the sunlit hemisphere faces us, making it appear round.
π
So what is 'lunar age'? People say things like 'today's lunar age is 14'.
π
Lunar age is the number of days since the last new moon: 0 for new moon, about 7.4 for first quarter (half moon), about 14.8 for full moon, about 22.1 for last quarter, and about 29.5 back to new moon. The synodic month cycle is about 29.530589 days, and this repetition creates the moon's phases. The old lunar calendar was based exactly on this cycle.
π
I often hear that spring tides occur during full moons. Why is that?
π
At new moon and full moon, the Earth, moon, and sun align in a straight line (syzygy). At these times, the moon's gravity and the sun's gravity combine in the same or opposite direction, maximizing tidal forceβthis is the spring tide. Conversely, at first and last quarter, the three bodies form a right angle, and the moon's and sun's gravity partially cancel out, causing neap tides. Coastal fishermen use this for planning their fishing seasons.
π
Why does the crescent moon swell from the right side day by day? Doesn't it ever swell from the left?
π
In the Northern Hemisphere, after a new moon, light always increases from the right (west) sideβthis is called 'waxing'. Because the moon orbits Earth counterclockwise, the sunlit portion increases sequentially from the right. In the Southern Hemisphere, it increases from the left. In Japan (Northern Hemisphere), the rule is 'swelling from the right toward full moon (waxing), waning from the left toward new moon (waning)'.
π
Is the lunar age calculation in this simulator accurate with the formula used?
π
This tool uses a simplified calculation based on the 'mean synodic month' (29.530589 days). The actual moon orbits in Kepler's elliptical orbit, moving faster near perigee and slower near apogee. Additionally, perturbations from the sun's gravity mean actual new/full moon times can deviate from the mean by several hours to about a day. For precise predictions, observatories use theories like ELP-2000 with hundreds of correction terms. For practical confirmation, refer to the National Astronomical Observatory of Japan's calendar calculation room (https://eco.mtk.nao.ac.jp/).
Frequently Asked Questions
What is a synodic month?
It is the period from one new moon to the next, approximately 29.530589 days (29 days, 12 hours, 44 minutes). It is the basis for one month in the lunar calendar (old calendar). Since it does not match the Gregorian calendar month (28β31 days), the dates of new moons and full moons shift each year and month.
Why do full moons and new moons cause spring tides?
At new moon and full moon, the Earth, moon, and sun align in a straight line. The gravitational forces of the moon and sun add up, maximizing tidal force (spring tide). At first and last quarter, the three bodies form a 90Β° angle, and the moon's and sun's gravity are orthogonal, partially canceling out, minimizing tidal force (neap tide). Spring tides peak about 2β3 days after new/full moon (tidal lag).
How many times can the moon be seen in a year?
Since the synodic month (29.53 days) is shorter than a calendar month, there are about 12.37 new moons and full moons per year. Most years have 12, but occasionally 13. A phenomenon with two full moons in one month is called a 'Blue Moon' (some months may have zero). A year with 13 months (adding a leap month in the old calendar) is set to correct this discrepancy.
How do moonrise and moonset times change?
Moonrise and moonset times are delayed by about 50 minutes each day. A new moon rises around sunrise (about 6:00 AM) and sets around sunset (about 6:00 PM). A first quarter moon rises around noon and sets around midnight. A full moon rises around sunset (about 6:00 PM) and sets around sunrise (about 6:00 AM). A last quarter moon rises around midnight and sets around noon.
What is the relationship between the old lunar calendar and lunar age?
The old calendar (in Japan, the 'lunisolar calendar') is a lunisolar calendar where the 1st day (tsuitachi) corresponds to a new moon, and the 15th day corresponds to a full moon. This is why the old calendar's '15th night' (harvest moon) falls on a full moon. However, since actual full moons can deviate from the mean, the 15th day of the old calendar is not always the roundest full moon day.
Select a day and open the detailed phase view to see the approximate age and illuminated fraction.
Compare the calendar and illumination graph.
Worked Example
For 15 January 2024, this mean-cycle model gives a moon age of about 4.3 days at 12:00 UTC.
Practical Notes
New and full moon dates are approximate mean-cycle estimates; actual event times and local dates can differ.
The tide label is based on phase alone. It does not predict local water levels, currents or timing; use official local tide predictions for navigation or fishing.
Moonrise, moonset and observer location are not calculated.