Ex Mensura, Scientia
A visual aid in determining Moon phase. Companion to Wndsn Telemeters: Low tech, high utility graphical distance & altitude computers from the Wndsn applied science lab.
Drag the strings on the clock, or enter the Moon's age and the time in the form. The place and the date set the season, and the results follow every change.
Next waxing crescent: 2026-11-13 at 20:51 clock time
When the Moon next has the phase of the horizontal string at this place: from the date in the form, or from now without one.
4.11 daysWaxing crescent50.1° east of the Sun2026-11-13 at 20:51 clock time17.9%, waxingFull moon in 10.7 days. The age is the Moon's at that date and time.
−13.1°WSW, azimuth 249.2°The Moon is below the horizon at this time.
21:003.65, the 4th hour of the night20:5115.8 min (only with a date)11:48 in the SE15:34, 14.5° in the south19:20 in the SWThe times of the Moon's day are solar times.
52.50°, longitude = 13.40°+1:002026-11-1307:40, sunset = 16:20 solar time43 min by day and 77 min by night−18.0°, of the Moon = −23.0°The display precision set is .1. To change that, edit the variable &digits=x in the URL where 0 <= x <= 4.
[Load sample values (JSON) (XML) (CSV).]
The Wndsn Universal Moon Clock is a simple equatorium predicting cardinal direction, and approximate average time (in unequal hours) from observed lunar shape and altitude; using the front side for the northern hemisphere and the mirrored reverse side for the southern hemisphere.
The Moon moves 360° east of the Sun in a synodic month of 29.530589 days. So it reaches its highest point about 48.8 minutes later each day, and three hours later from one eighth of its cycle to the next. The first quarter stands highest at sunset, and the full moon at midnight. The age is the number of days since new moon, and the elongation E is how far the Moon stands east of the Sun.
Without a date the clock shows ideal conditions at the equinox. The Sun and the Moon then move on the celestial equator, so the Sun rises at 6:00 and sets at 18:00 solar time. Every hour is sixty minutes long. But with a date the season comes in. The page finds the Sun's ecliptic longitude and the equation of time with the NOAA formulas. It places the Moon on the ecliptic at its elongation east of the Sun, which gives the declinations of both. The day from sunrise to sunset then has twelve unequal hours, and so does the night.
Moonrise and moonset are the moments when the center of the Moon crosses the horizon, without refraction or parallax. And since the model ignores the tilt of the Moon's orbit, its times and directions are approximate. With a date but no age or illuminated part, the age is the Moon's at that date and time, from the main terms of the series for its longitude. Clock time is solar time minus 4 minutes per degree of longitude east, minus the equation of time, plus the UTC offset.
Drag the teal strings: the time along the hours, and the Moon's age up and down the phases. A tap on a phase picks that eighth of the cycle exactly. The arrow keys move a focused string in small steps; hold Shift for larger ones. The circles on the sunset line and at the ends of the day point where the Moon of that row stands at sunset and at sunrise, with north up and east to the right. The button Use my location fills in the place with its UTC offset and sets the date to today. It asks first. And Now sets today's date and the clock time to the present moment. A new date, whether typed or from Use my location, brings the Moon of that date and time, even after the phase string was moved; drag the string again to choose another phase. Every move updates the form and the results. And since the address follows along, a bookmark or a shared link brings back exactly this Moon and this time.
Under the clock the page says when the Moon next has the phase of the horizontal string at this place, in clock time. It counts from the date in the form, or from now when the date is empty. So moving the string to the full moon row answers when the next full moon comes, and Show it puts the form at that moment. The time comes from the same series for the Moon's longitude, within a few minutes.
age0 to 29.53illum, waxwaxing (default) or waning; used when age is missingtime21:30 (default 21:00)clocktime is missinglat-89 to 89, north positive (default 52.5)date2026-10-23; leave it out for the ideal equinox. With no age and no illum, the Moon of that datelon-180 to 180, east positive (default 13.4)utc+1:00 (the default) or -5calc=yes21:00digits0 to 4 (default 1)formatjson, xml or csv for the API; leave it out for this page. In JSON and XML, input holds the Moon, the time, and the place as used, with the defaults filled in, and output holds the rest in five groups: phase_of_moon, hours, position, moon_of_the_day, and season. Numbers are numbers, yes and no are true and false, and a value that does not exist is null (empty in XML), such as a moonrise that does not happen. The CSV has the same columns for every query, and its input column names what the Moon and the time were given by. Invalid input gives notes instead of a result.