# Moon Phase Calculator

> View current and future moon phases with illumination percentage and lunar calendar

**Category:** Utility
**Keywords:** calculator, tool
**URL:** https://complete.tools/moon-phase-calculator

## The eight moon phases

The moon's phases are defined by how much of its sunlit half is visible from Earth:

**New Moon (🌑)**: The moon is between Earth and the sun. Its sunlit side faces away from us — the moon is not visible.

**Waxing Crescent (🌒)**: A small sliver of light appears on the right side (in the northern hemisphere). The illuminated area is growing.

**First Quarter (🌓)**: The right half of the moon is illuminated. This is roughly 7 days after new moon.

**Waxing Gibbous (🌔)**: More than half the moon is lit, still growing toward full.

**Full Moon (🌕)**: The entire face of the moon is illuminated. Occurs roughly 14.77 days after new moon.

**Waning Gibbous (🌖)**: More than half lit, but now shrinking after full moon.

**Last Quarter (🌗)**: The left half is illuminated. About 22 days after new moon.

**Waning Crescent (🌘)**: A thinning sliver on the left side. The cycle is nearly complete.

## How moon phases are calculated

The calculation starts from a known reference new moon and uses the synodic period to determine phase:

**Synodic month**: 29.530588 days — the time for the moon to return to the same phase relative to the sun.

**Moon age** = (days elapsed since reference new moon) mod 29.530588

**Illumination** = (1 - cos(2pi x moon_age / 29.53)) / 2

This formula produces a smooth curve from 0% (new moon) to 100% (full moon) and back to 0%. The reference new moon used is January 6, 2000 at 18:14 UTC, a well-documented astronomical event.

## Why the moon has phases

The moon does not produce its own light — it reflects sunlight. As the moon orbits Earth (taking about 27.3 days for one orbit), the angle between the sun, moon, and Earth changes. This changing geometry determines how much of the illuminated lunar surface faces Earth.

When the moon is between Earth and the sun (new moon), we see only the dark side. Two weeks later, Earth is between the moon and sun (full moon), and we see the fully lit face.

The moon's orbital period (27.3 days) differs from the synodic period (29.53 days) because Earth itself is moving around the sun — the moon must travel a bit extra to "catch up" and return to the same phase.

## Moonrise and moonset

The moon's rise and set times follow a predictable pattern based on phase:

- **New moon**: Rises and sets near the sun — rises at dawn, sets at dusk. Not visible.
- **First quarter**: Rises around noon, sets around midnight.
- **Full moon**: Rises at sunset, sets at sunrise. Visible all night.
- **Last quarter**: Rises around midnight, sets around noon.

Exact moonrise and moonset times depend on your latitude and the date, similar to sunrise/sunset calculations.

## Moon phases and their effects

Moon phases have real and measurable effects on Earth:

**Tides**: The moon's gravity pulls on Earth's oceans. During new and full moons, the sun and moon align, creating stronger "spring tides." During quarter moons, they pull at right angles, producing weaker "neap tides."

**Fishing and hunting**: Many anglers and hunters follow lunar calendars, as animal feeding activity is believed to increase around full and new moons.

**Gardening**: Biodynamic farming traditions plant crops by lunar phase — root vegetables during waning moon, leafy plants during waxing.

**Photography**: Full moons provide natural light for nightscape photography. New moons create ideal conditions for astrophotography and Milky Way shots.

## How to use

1. The calculator shows today's moon phase automatically when you open it
2. To check a specific date, enter it in the date field
3. Click "Calculate Moon Phase" to update the results
4. View the visual moon diagram, phase name, and illumination percentage
5. The progress bar shows where you are in the 29.5-day lunar cycle
6. The lunar calendar shows moon phases for the entire selected month
7. Click "Today" to return to the current date

## FAQs

**Q:** How accurate is this moon phase calculator?


**A:** The calculator is accurate to within a few hours for most dates. It uses the synodic month constant and a known reference new moon. For scientific or nautical applications, use a dedicated ephemeris or NASA's JPL Horizons system.


**Q:** What is the difference between waxing and waning?


**A:** "Waxing" means the illuminated area is growing (moving toward full moon). "Waning" means it is shrinking (moving toward new moon). From the northern hemisphere, the lit portion moves from right to left as the cycle progresses.


**Q:** When is the best time to see the moon?


**A:** Full moons rise at sunset and are visible all night. Quarter moons are best viewed in the early evening (first quarter) or early morning (last quarter). New moons are not visible.


**Q:** Why does the full moon look different every month?


**A:** The moon's orbit is elliptical. When it is closest to Earth (perigee), it appears larger — a "supermoon." When farthest (apogee), it looks smaller. The difference in apparent size is about 14%.


**Q:** What is a blue moon?


**A:** The most common modern definition is the second full moon in a calendar month. This happens roughly every 2.5 years because 12 lunar cycles (354 days) fall short of the solar year (365 days), creating a "catch-up" full moon occasionally.


**Q:** Does the moon look the same everywhere on Earth?


**A:** The phase is the same worldwide — everyone on the night side of Earth sees the same percentage illuminated. However, the orientation may differ: in the southern hemisphere the lit side appears on the left during waxing phases, opposite to the northern hemisphere.

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*Generated from [complete.tools/moon-phase-calculator](https://complete.tools/moon-phase-calculator)*