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How to Use the Moon Phase Calculator
- View today's phase — The main display at the top shows the current moon phase name, emoji, illumination percentage, and moon age in days. This updates automatically each day.
- Check upcoming events — The stats grid below the main display shows the dates of the next full moon and next new moon, along with countdown timers showing how many days remain until each event.
- Browse the lunar calendar — The monthly calendar shows the moon phase emoji for every day of the month. The current day is highlighted for easy reference.
- Navigate between months — Use the Prev and Next buttons to move backward or forward through months. You can browse any month in any year to plan activities around the lunar cycle.
How Moon Phase Calculation Works
Our moon phase calculator uses a well-established astronomical algorithm based on the lunar synodic period. The synodic period is the time it takes for the Moon to complete one full cycle of phases, from new moon to new moon, which averages 29.53059 days. By using a known reference point, the New Moon that occurred on January 6, 2000 at 18:14 UTC, the algorithm calculates the moon age for any given date.
The calculation works by measuring the number of days elapsed between the reference date and the target date, then finding the remainder when divided by the synodic period. This remainder is the moon age, which indicates how many days have passed since the most recent new moon. The moon age is then mapped to one of the eight standard phases by dividing the synodic period into eight equal segments.
Illumination percentage is derived from the moon age as well. At new moon (age 0), illumination is 0%. It increases linearly to 100% at full moon (age ~14.77 days), then decreases back to 0% by the next new moon. While real lunar illumination follows a slightly more complex curve, this linear approximation is accurate enough for practical purposes.
Moon Phase Reference Table
| Phase | Symbol | Illumination | Description |
|---|---|---|---|
| New Moon | 🌑 | 0% | Moon is between Earth and Sun, not visible from Earth |
| Waxing Crescent | 🌒 | 1-49% | Thin sliver of light appears on the right side, growing larger |
| First Quarter | 🌓 | 50% | Right half of the Moon is illuminated, marking one quarter of the cycle |
| Waxing Gibbous | 🌔 | 51-99% | More than half illuminated and growing toward full moon |
| Full Moon | 🌕 | 100% | Entire face is illuminated, Earth is between Moon and Sun |
| Waning Gibbous | 🌖 | 99-51% | More than half illuminated but shrinking after full moon |
| Last Quarter | 🌗 | 50% | Left half of the Moon is illuminated, three quarters through the cycle |
| Waning Crescent | 🌘 | 49-1% | Thin sliver of light on the left side, shrinking toward new moon |
Frequently Asked Questions
What is the moon phase today?
The current moon phase is displayed at the top of this page, showing the phase name, illumination percentage, and moon age in days. The display updates automatically each day based on precise astronomical calculations using the synodic period and a known reference new moon date.
When is the next full moon?
The next full moon date is calculated and displayed in the stats grid above the calendar. It also shows a countdown of how many days remain. Full moons occur approximately every 29.53 days, so you can expect roughly one full moon per calendar month, with occasional months having two (the second being called a "blue moon").
How are moon phases calculated?
Our calculator uses the known New Moon reference date of January 6, 2000 at 18:14 UTC and the synodic period of 29.53059 days. The algorithm measures the elapsed time since the reference date, divides by the synodic period, and uses the remainder to determine the current moon age and phase. This method is accurate for practical purposes across centuries.
What is a supermoon?
A supermoon occurs when a full moon coincides with the Moon being at or near its closest point to Earth in its elliptical orbit, known as perigee. During a supermoon, the Moon appears approximately 14% larger and 30% brighter than when it is at its farthest point (apogee). Supermoons typically occur three to four times per year.
How do moon phases affect tides?
The Moon's gravitational pull is the primary driver of ocean tides. During full moons and new moons, the Sun, Moon, and Earth align, producing stronger spring tides with higher highs and lower lows. During quarter moons, the gravitational forces partially cancel out, resulting in weaker neap tides with less extreme tidal variation.
Does the Moon change apparent size during its phases?
The Moon's phases do not change its actual apparent size. However, the Moon's elliptical orbit means its distance from Earth varies between about 356,500 km at perigee and 406,700 km at apogee, causing its apparent diameter to change by about 14%. This size variation is independent of the phase cycle and depends solely on orbital position.
