Moon Phase Calculator
Find the Moon phase and illumination percentage for any past or future date. See a realistic illustration of the lit Moon, its age in days, how far it is through the lunar cycle, and the exact dates of the next New Moon, First Quarter, Full Moon, and Last Quarter. Phases are computed with Jean Meeus' astronomical algorithm and can be mirrored for the Southern Hemisphere, with a step-by-step breakdown.
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About Moon Phase Calculator
The Moon Phase Calculator tells you the phase of the Moon and its illumination percentage for any past or future date. It draws a realistic picture of the lit Moon, reports the Moon's age in days, shows how far it has travelled through the current lunar cycle, and lists the exact dates of the next New Moon, First Quarter, Full Moon, and Last Quarter. Phase times are computed with Jean Meeus' astronomical algorithm and can be mirrored to match the view from the Southern Hemisphere.
What Are the Phases of the Moon?
The Moon does not produce its own light; we only see the half lit by the Sun. As the Moon orbits Earth over roughly 29.5 days (one synodic month), the angle between the Sun, Earth, and Moon changes, so the sunlit portion we can see grows (waxes) and shrinks (wanes) through eight recognizable phases.
| Phase | Illumination | Approx. Age | Appearance |
|---|---|---|---|
| 🌑 New Moon | 0% | 0 days | Invisible — the lit side faces away from Earth |
| 🌒 Waxing Crescent | 0–50% | 1–6 days | A thin sliver growing each night |
| 🌓 First Quarter | 50% | ~7.4 days | Right half lit (Northern Hemisphere) |
| 🌔 Waxing Gibbous | 50–100% | 8–14 days | More than half, still growing |
| 🌕 Full Moon | 100% | ~14.8 days | Fully lit and round |
| 🌖 Waning Gibbous | 100–50% | 16–21 days | More than half, now shrinking |
| 🌗 Last Quarter | 50% | ~22 days | Left half lit (Northern Hemisphere) |
| 🌘 Waning Crescent | 50–0% | 23–29 days | A shrinking sliver before the next New Moon |
Moon Phase Formula
This calculator first finds the most recent New Moon before your chosen date to get the Moon's age, then derives the illuminated fraction from the phase angle.
The cycle fraction runs from 0 at New Moon to 1 at the following New Moon. The cosine term is what makes illumination change slowly near New and Full Moon and quickly around the quarters.
How the Phase Dates Are Computed
The dates and times of the New Moon, First Quarter, Full Moon, and Last Quarter come from the periodic series in Chapter 49 of Jean Meeus' Astronomical Algorithms. Each phase is indexed by a number k (whole numbers for New Moons, plus 0.25, 0.5, and 0.75 for the quarters and Full Moon), and dozens of periodic and planetary correction terms refine the time to within a couple of minutes for modern dates. Event times are then shifted into the UTC offset you select.
Why the Moon Looks Mirrored in the Southern Hemisphere
An observer in Sydney is, in effect, standing "upside down" compared with an observer in London. Because of that, the same Moon appears flipped left-to-right. A waxing crescent lit on the right in the Northern Hemisphere is lit on the left in the Southern Hemisphere. Choosing your hemisphere flips the illustration so it matches the Moon you actually see in the sky.
How to Use This Calculator
- Choose a date: Use the date picker to select any past or future date. The tool starts on today's date.
- Set your hemisphere and time zone: Pick Northern or Southern Hemisphere and your UTC offset so the picture and phase times match your location.
- Click Calculate: The tool computes the Moon phase and illumination for that date instantly.
- Review the results: See the lit Moon, the phase name, illumination percentage, Moon age, the lunar-cycle progress, and the upcoming principal phases.
Frequently Asked Questions
What is a moon phase?
A moon phase is the shape of the sunlit part of the Moon that we can see from Earth. As the Moon orbits Earth over about 29.5 days, the angle between the Sun, Earth, and Moon changes, so the visible lit portion grows and shrinks through eight phases from New Moon to Full Moon and back.
How is moon illumination calculated?
Illumination is the percentage of the Moon's visible disc that is lit by the Sun. It is approximately (1 − cos of the phase angle) ÷ 2. It equals 0% at New Moon, 50% at the First and Last Quarter, and 100% at Full Moon.
What is the moon's age?
The Moon's age is the number of days that have passed since the last New Moon. It ranges from 0 at New Moon to about 29.5 days at the end of the cycle. First Quarter falls near day 7, Full Moon near day 14.8, and Last Quarter near day 22.
Why does the moon look different in the Southern Hemisphere?
Observers in the Southern Hemisphere are effectively upside down relative to the Northern Hemisphere, so the Moon appears mirrored. A waxing crescent lit on the right side in the north is lit on the left side in the south. This tool can flip the illustration to match your hemisphere.
How accurate are the phase dates?
The dates and times of the New Moon, quarters, and Full Moon are computed using Jean Meeus' astronomical algorithm, which is accurate to within a couple of minutes for modern dates. Times are shown in the UTC offset you select.
Can I check the moon phase for a past or future date?
Yes. You can enter any date from the year 1700 to 2400 and the calculator will show the Moon phase, illumination, age, and the surrounding principal phases for that date.
Additional Resources
Reference this content, page, or tool as:
"Moon Phase Calculator" at https://MiniWebtool.com/moon-phase-calculator/ from MiniWebtool, https://MiniWebtool.com/
by miniwebtool team. Updated: June 29, 2026