About the Moon Phase Calculator
A free, instant tool that shows the Moon's phase name, illumination, age, and the next new and full moon dates for any date from 1700 to 2300.
What it is
The Moon Phase Calculator takes any calendar date you enter and tells you which of the eight named lunar phases applies to that date, how much of the Moon's disc is illuminated, how many days the Moon is into its current synodic cycle (moon age), and whether the Moon is waxing (growing brighter) or waning (growing darker). It also shows the dates of the next new moon and full moon from that date, and renders a simple SVG visual of the lit and dark portions of the Moon from either the Northern or Southern Hemisphere perspective.
Everything runs in your browser using a self-contained JavaScript algorithm — no network requests, no data storage, no accounts required.
What it does not do
Cannot show the live sky
This tool calculates the phase for a date you enter. It does not show a real-time sky view, star chart, or live Moon position. For live sky observation, use a dedicated planetarium app.
Not a precise ephemeris
The synodic-period method is accurate for everyday identification of phases but can be off by roughly one day for the exact timing of a phase peak. For precise astronomical timings, use the US Naval Observatory or JPL Horizons.
No storage or history
The tool does not save dates, track a lunar calendar over time, or remember any previous calculations. Each calculation is stateless and exists only while the page is open.
No real-time satellite data
The phases are computed entirely from the known synodic period, not from live satellite or telescope data. The result is a mathematical approximation, not a live observation.
The eight moon phases
🌑 New Moon
The Moon is between Earth and the Sun. The near side faces away from the Sun and appears dark from Earth. Illumination: ~0%.
🌒 Waxing Crescent
A thin, growing sliver of light appears on the right side of the Moon (Northern Hemisphere). Illumination: 1–49%.
🌓 First Quarter
The right half of the Moon (Northern Hemisphere) is fully lit. The Moon is one-quarter of the way through its cycle. Illumination: ~50%.
🌔 Waxing Gibbous
More than half the Moon is lit and still growing brighter. The lit portion bulges on the left side toward full. Illumination: 51–99%.
🌕 Full Moon
Earth is between the Moon and the Sun. The entire near side is illuminated. Illumination: ~100%.
🌖 Waning Gibbous
The Moon was full and is now shrinking. More than half is still lit but the dark side is growing on the right. Illumination: 51–99%.
🌗 Last Quarter
The left half of the Moon (Northern Hemisphere) is fully lit. The Moon is three-quarters of the way through its cycle. Illumination: ~50%.
🌘 Waning Crescent
A thin, shrinking sliver of light remains on the left side of the Moon (Northern Hemisphere) before the next new moon. Illumination: 1–49%.
How the calculation works
The calculator uses the Julian Day Number (JDN) method combined with the synodic month. The synodic month is the time it takes the Moon to return to the same phase as seen from Earth — approximately 29.53058770576 days. This period is very well measured and is stable enough for practical calculations across centuries.
Starting from a known reference new moon (January 6, 2000 at Julian Day 2451549.5), the tool converts your chosen date to a Julian Day Number, computes how many days have elapsed since that reference, and divides by the synodic period to find where in the current cycle the date falls. The result is a "moon age" in days, from which illumination and phase name are derived using a cosine model.
This approach is accurate for everyday use — the phase name and general illumination will be correct. However, because the Moon's orbit is slightly elliptical and perturbed by the Sun and other bodies, the exact moment of a phase transition (e.g., the precise hour of full moon) may differ by up to roughly one day compared to a full gravitational ephemeris. For casual reference, planning, and curiosity, this level of accuracy is more than sufficient.
The hemisphere view
The Moon's orientation differs between the Northern and Southern Hemispheres because the two hemispheres see the Moon from opposite orientations. From the Northern Hemisphere, a waxing crescent shows its lit side on the right; from the Southern Hemisphere, the same waxing crescent appears lit on the left. The Moon is the same; only the viewing angle differs.
The N/S toggle on the calculator flips the SVG visual to reflect this. The phase name, illumination, and age remain the same regardless of hemisphere — only the visual orientation changes. If you are in the equatorial region, the Moon's tilt can appear somewhere between the two extremes depending on local conditions.
Calculate the Moon's phase now
Free, no sign-up required. Works on any device.
Open the Moon Phase Calculator