Moon Phase Calculator — Free Online Moon Phase Tool

How to Use the Moon Phase Calculator

1. Select a date: Choose any date using the date picker. The calculator defaults to today's date, giving you the current moon phase instantly. You can select past dates to see historical moon phases or future dates to plan around upcoming lunar events. The algorithm works for any date within centuries of the present.
2. View the moon phase visualization: The circular visual indicator at the top shows an approximate representation of the Moon's appearance for the selected date. The illuminated portion is shown in gold and the dark portion in deep blue. Below the visual, the phase name (such as "Waxing Gibbous" or "Full Moon") is displayed prominently along with the exact illumination percentage.
3. Check the illumination progress bar: The horizontal bar below the visual shows the illumination percentage as a filled bar, making it easy to see at a glance how close the Moon is to full or new.
4. Review cycle details and upcoming events: The statistics section shows days since the last new moon and the cycle completion percentage. The upcoming events panel displays the projected dates for the next new moon and the next full moon, helping you plan observations, photography, or outdoor activities.

Try selecting dates one week apart to see how the Moon progresses through its phases. A full cycle from new moon to new moon takes approximately 29.5 days, so checking every 3-4 days shows a noticeable change in illumination and phase.

Moon Phase Calculation Formula

Variables Explained

• Known New Moon: A reference point — the new moon of January 6, 2000, at 18:14 UTC. By measuring elapsed time from this known event and dividing by the cycle length, we can determine the position within any subsequent cycle.
• Synodic Month: The average length of one complete lunar cycle: 29.53059 days. This is the time from one new moon to the next, accounting for Earth's orbital motion around the Sun.
• Synodic Position: The number of days elapsed within the current cycle (0 to 29.53 days). A position of 0 indicates a new moon, while approximately 14.77 indicates a full moon.
• Phase Angle: The synodic position converted to an angular value (0 to 360 degrees) representing the Moon's orbital position. This is used to calculate illumination via the cosine function.
• Illumination: The percentage of the Moon's visible disk that is illuminated by sunlight, calculated using the cosine of the phase angle. This produces the characteristic non-linear illumination curve: slow changes near new and full moon, rapid changes near the quarters.

Step-by-Step Example

Calculate the moon phase for February 20, 2026:

1. Reference new moon: January 6, 2000, 18:14 UTC
2. Days elapsed: from January 6, 2000 to February 20, 2026 = approximately 9,542 days
3. Complete cycles: 9,542 / 29.53059 = approximately 323.07 complete cycles
4. Fractional cycle: 0.07 × 29.53059 = approximately 2.07 days since last new moon
5. Phase angle: (2.07 / 29.53059) × 360 = approximately 25.2 degrees
6. Illumination: (1 - cos(25.2°)) / 2 × 100 = approximately 4.8%
7. Phase name: Waxing Crescent (between 1.85 and 7.38 days)

The Moon on February 20, 2026 is a thin waxing crescent with about 4.8% illumination, visible as a slender sliver in the western sky shortly after sunset.

Practical Examples

Example 1: Nathan's Astrophotography Planning

Nathan is an astrophotographer in New York planning his next deep-sky imaging session. He needs to shoot during the new moon window when the sky is darkest for capturing the Orion Nebula. Using the moon phase calculator, he checks the phase for upcoming weekends in March 2026:

• March 7: Waxing Gibbous (86% illuminated) — too bright
• March 14: Waning Gibbous (68% illuminated) — still too bright
• March 21: Waning Crescent (9% illuminated) — excellent conditions
• March 28: New Moon (0% illuminated) — perfect, darkest skies

Nathan selects the weekend of March 28 for his deep-sky session. With a new moon, light pollution from the Moon is zero, providing the darkest possible sky background. He also notes the March 21 weekend as a backup date, since the thin crescent sets early in the evening, leaving most of the night dark enough for imaging.

Example 2: Maria's Garden Planning

Maria practices lunar gardening, a traditional agricultural method that aligns planting activities with moon phases. According to this tradition, leafy vegetables should be planted during the waxing phase (from new moon to full moon) when increasing moonlight supposedly encourages leaf growth, while root vegetables should be planted during the waning phase (from full moon to new moon). She checks April 2026:

• April 3: New Moon — start of the waxing phase, plant lettuce and spinach
• April 10: First Quarter (50%) — good for transplanting seedlings
• April 18: Full Moon — switch to root vegetable planting
• April 25: Last Quarter (50%) — plant carrots and potatoes

Maria uses the calculator to plan her entire spring planting calendar around the lunar cycle. While scientific evidence for lunar gardening effects is limited, the practice provides a structured planting schedule that naturally spreads garden work across the month and has deep cultural roots in many agricultural traditions around the world.

Example 3: Captain Torres' Fishing Expedition

Captain Torres runs a charter fishing business in Miami and many experienced anglers prefer fishing around the new moon and full moon, when spring tides create stronger water movement that can trigger feeding activity. He checks the moon phase calendar for the first two weeks of May 2026 to plan premium fishing charters:

• May 2: New Moon (0%) — strongest spring tides, peak feeding expected
• May 9: First Quarter (50%) — neap tides, moderate conditions
• May 17: Full Moon (100%) — spring tides return, excellent fishing expected

Captain Torres markets his premium charters for the May 1-3 and May 16-18 windows, coinciding with the new moon and full moon spring tide periods. He prices these dates at a premium since experienced anglers specifically request lunar-aligned trips. The three-day window around each major phase typically shows the strongest tidal current effects that concentrate baitfish and attract larger predatory species.

Example 4: Yuki's Wedding Date Selection

Yuki is planning an outdoor evening wedding in Tokyo and wants a visible moon as part of the romantic atmosphere. She is considering dates in October 2026 and uses the moon phase calculator to find the best option for a beautiful moon in the evening sky:

• October 3: First Quarter (50%) — moon visible in the south during early evening
• October 10: Waxing Gibbous (88%) — bright moon rising in the east during dinner
• October 17: Full Moon (100%) — dramatic full moon rising at sunset, peak illumination
• October 24: Last Quarter (50%) — moon does not rise until midnight, not visible during the event

Yuki selects October 10 for the wedding, choosing the waxing gibbous phase. The 88% illuminated moon rises in the east during the dinner reception, providing a bright and beautiful backdrop without the high-tide concerns that might affect beachside venues during a full moon. The first quarter (October 3) would also work but provides only half the illumination. For tracking exact dates, our day of year calculator helps with countdown planning.

Moon Phase Reference Table

Cycle days are approximate and based on the 29.53-day synodic month. Rise/set patterns are generalized for mid-latitudes.

Tips and Complete Guide

Understanding the Lunar Cycle

The 29.53-day synodic month creates a rhythm that has influenced human cultures for millennia. The Islamic calendar is entirely lunar, with months beginning at the sighting of the waxing crescent. The Chinese calendar combines lunar and solar elements, with the Chinese New Year falling on the second new moon after the winter solstice. Easter's date is determined by the first full moon after the spring equinox. Understanding the lunar cycle connects you to these cultural traditions and helps predict events tied to moon phases. The Moon's orbit is not perfectly regular: individual synodic months range from about 29.18 to 29.93 days due to gravitational perturbations.

Moon Phase and Night Sky Observation

The moon phase is the single most important factor for astronomical observation planning. A full moon washes out all but the brightest stars and planets, reducing the number of visible stars from thousands to mere hundreds. For deep-sky observation (galaxies, nebulae, star clusters), the new moon period provides optimal conditions. The general rule is that the best astronomical observing occurs within 5 days of new moon in either direction. Lunar observers, however, prefer the quarter phases when shadows along the terminator (the line between light and dark) create dramatic contrast on craters, mountains, and valleys.

Photographing the Moon

Photographing the Moon effectively requires understanding both the phase and the rising/setting times. The most photogenic lunar images typically occur when the Moon is near the horizon, where atmospheric distortion can add color and size appears exaggerated due to the Moon illusion. For detailed surface photography, the quarter phases reveal the most surface texture along the terminator. A common exposure starting point for a well-lit Moon is f/11, ISO 200, and 1/250th second (the "Looney 11 rule"). During the crescent phase, try longer exposures to capture Earthshine — the faint illumination of the dark portion caused by sunlight reflected off Earth. Our sunrise calculator and sunset calculator help time moonrise photos.

The Moon and Outdoor Activities

The moon phase affects many outdoor activities beyond astronomy. Night hikers and campers benefit from planning around the full moon for maximum natural illumination on trails. Conversely, wildlife photographers and nature watchers often prefer darker conditions for spotting bioluminescence or nocturnal animals that are more active during new moon periods. Surfers and coastal fishermen track moon phases because of their influence on tidal patterns — spring tides around new and full moons create the largest waves and strongest currents. Scuba divers favor the days around full moon for enhanced underwater visibility from increased ambient light penetration.

Common Mistakes to Avoid

• Assuming the full moon is always visible all night: The full moon rises approximately at sunset and sets approximately at sunrise. However, atmospheric conditions, building obstructions, and the precise timing of moonrise relative to sunset mean the Moon may not be visible immediately after dark. Check moonrise times for your specific location.
• Expecting instant darkness after the full moon: The waning gibbous phase immediately following the full moon is still extremely bright (90%+ illuminated for several days). If you need dark skies, wait at least 5-7 days after full moon for the illumination to drop below 50%.
• Confusing the Moon's orbital period with the phase cycle: The sidereal month (27.32 days) is different from the synodic month (29.53 days). Moon phases follow the synodic period. Using the wrong cycle length will produce incorrect phase predictions.
• Ignoring moonrise and moonset times: Knowing the phase is not enough — you also need to know when the Moon is above your horizon. A beautiful waxing gibbous Moon is useless for evening photography if it does not rise until after midnight at your latitude.
• Relying on the visual indicator alone: The Moon's appearance is hemisphere-dependent. The visual representation in our calculator is generalized. In the Southern Hemisphere, the illumination direction is reversed. Focus on the phase name and illumination percentage rather than the visual alone.

Frequently Asked Questions