Moon Phase Calculator

The synodic period — from new moon to new moon — is 29.53058867 days, approximately 29.5 days or about 4 weeks. This is why a lunar month is nearly the same as a calendar month ('month' derives from 'moon').

Moon phases result from the changing geometry between the Moon, Earth, and Sun. As the Moon orbits Earth, we see different portions of its sunlit half. The Moon itself does not change — only our viewing angle changes.

This calculator is accurate to approximately ±1 day. It uses a simplified synodic period model from a known reference new moon. Precise astronomical calculations account for the Moon's elliptical orbit, solar perturbations, and other factors. For exact new/full moon times to the minute, consult the US Naval Observatory or JPL ephemerides.

Spring tides (strongest tides, largest tidal range) occur at new and full moons when the Sun, Moon, and Earth align. Neap tides (weakest) occur at quarter moons when the Sun and Moon are at right angles to Earth. The lunar cycle thus has a strong 29.5-day signal in tidal patterns.

A supermoon occurs when a full moon coincides with the Moon's closest approach to Earth (perigee), making it appear up to 14% larger and 30% brighter than an average full moon. Supermoons occur because the Moon's orbit is elliptical, not circular.

The most common modern definition: the second full moon in a calendar month. Since the lunar cycle is ~29.5 days and most months are 30-31 days, this happens about every 2.5 years. Hence the expression 'once in a blue moon' for something rare.

Some research suggests a modest correlation between the full moon and slightly reduced sleep duration (3-4 minutes less on average). The mechanism is unclear but may involve moonlight exposure or evolutionary responses. The effect size is small and not universally replicated.

A lunar calendar tracks time based on lunar cycles rather than solar years. The Islamic Hijri calendar is purely lunar (12 months of 29-30 days = 354-day year). The Hebrew and Chinese calendars are lunisolar, adding an extra month periodically to stay aligned with seasons.

Enter today's date and check the 'Days to Full Moon' output. A full moon occurs every 29.5 days; if the current phase shows 0-7 days to full, you will see a full moon very soon. Check a precise astronomical calendar for exact times.