Moon phase

Pick a date to see the moon phase, its illumination and the next new and full moons.

How Moon phase works

Moon Phase calculates the phase of the moon for any date you choose, together with the illumination percentage, and the dates of the next full moon and new moon, all using local astronomical formulas inside your browser. There is no API call to an external ephemeris service; the calculations use the synodic period of the moon and the Julian date of a known reference new moon to determine the current age and phase. Nothing you enter leaves your device.

The illumination percentage is derived from the moon's phase angle using a standard formula from Jean Meeus's Astronomical Algorithms. The phase name (new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent) is determined by the fraction of the synodic cycle elapsed since the last new moon. For dates in the near future or recent past, the results match published almanac data to within a few hours.

Written by Bastien Sulyan

How to use Moon phase, step by step

Select the date you want to check using the date picker, or leave it at today's date.
Read the phase name and illumination percentage displayed.
Note the next full moon and next new moon dates shown below.
Navigate forward or backward by day to watch the phase progress through the cycle.

Common use cases

A gardener who follows lunar planting calendars checks the moon phase for the coming week to decide when to sow or prune.
A wildlife photographer planning a night shoot looks up the moon phase to gauge how much natural light will be available at the shooting location.
A fiction writer researching a historical date checks what phase the moon was in on a specific night to make their scene accurate.
A sailor planning an offshore passage consults the tool to identify nights around the full moon when tidal ranges will be at their maximum.

Frequently asked questions

How accurate is the moon phase calculation?

The calculation uses the synodic period (approximately 29.53059 days) anchored to a verified reference new moon and applies a Julian date offset to arrive at the current phase. A real lunation is not constant, so an actual new or full moon can fall up to about 14 hours either side of the mean instant this model computes, and the gap widens for dates far from the reference epoch. That is plenty to answer what phase tonight is; it is not an ephemeris.

What does the illumination percentage represent?

It represents the fraction of the moon's visible disk that appears lit from Earth on the chosen date. At new moon the illumination is 0%, at full moon it is 100%. At first and third quarter the illumination is approximately 50%, but the lit half is on opposite sides of the disk.

Does the tool use a live data feed from an astronomy API?

No. All calculations are performed by a local JavaScript implementation of the lunar phase formula. The page does not make any outbound request to a third-party ephemeris or astronomy API. The results are computed from the date you enter and the mathematical model embedded in the page.

Can I look up the moon phase for a date centuries in the past?

The synodic period formula works in principle for any date, but accuracy decreases for dates far from the reference epoch because the simplified model does not account for all the perturbations in the moon's orbit. For historical research requiring high precision, a dedicated astronomical ephemeris tool is more reliable.

Does the phase shown account for my location on Earth?

The moon phase, illumination and phase name are the same for all observers on Earth at a given moment; they do not depend on your location. The exact minute of moonrise and moonset do depend on latitude, but those quantities are not currently shown by this tool.

How are the next full moon and new moon dates determined?

Starting from the current phase angle, the tool advances the synodic cycle forward until the full moon threshold (illumination crossing 99.5%) and the new moon threshold (illumination falling below 0.5%) are each reached, and reports those dates. The calculation is iterative and runs in under a millisecond in the browser.

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