Our Mission
Moon Phase Tonight was built with a single purpose: to be the clearest, most accurate, and most useful moon phase resource on the internet. Whether you arrive to check tonight's phase before a night walk, to time an astrological ritual, or simply out of curiosity about the sky above you, we want this site to give you an immediate, reliable answer.
We believe astronomical data should be free, fast, and beautifully presented. There are no paywalls, no account requirements, and no complicated interfaces — just the moon, and everything you need to understand it.
How We Calculate Moon Phases
All lunar calculations on Moon Phase Tonight are performed client-side in your browser using proven astronomical formulas. No data is sent to a server and no APIs are called — the calculations happen entirely on your own device the moment the page loads.
The Synodic Month
The foundation of our calculation is the synodic month — the time it takes the Moon to complete one full cycle of phases as seen from Earth. This is precisely 29.530588853 days. This is distinct from the sidereal month (27.32 days), which measures the Moon's orbit relative to the stars rather than relative to the Sun.
Reference New Moon
We anchor our calculations to a precisely known New Moon event: January 6, 2000 at 18:14 UTC. By calculating the elapsed time since this reference point and dividing by the synodic period, we determine the Moon's current age within its cycle.
Phase Determination
The Moon's age in days since the last New Moon determines its phase. The eight phases correspond to specific age ranges within the 29.53-day cycle. Illumination percentage is calculated using the cosine formula applied to the Moon's position in its cycle, ranging from 0% at New Moon to 100% at Full Moon.
Accuracy note: Our calculations are accurate to within a few hours for general phase identification. For precise astronomical timing of exact phase moments to the minute, we recommend cross-referencing with NASA's SKYCAL or the US Naval Observatory.
Moon Sign Calculation
The Moon moves through all twelve zodiac signs approximately every 27.3 days, spending roughly 2.5 days in each sign. Our moon sign calculation tracks the Moon's approximate position along the ecliptic and determines which of the twelve 30° zodiac segments it currently occupies.
Moon sign positions are significant in astrology for understanding emotional states, intuition, and the collective mood. We present this data as astrological context, clearly distinguished from the strictly astronomical phase data.
What Each Data Point Means
Current Phase: One of the eight named stages of the lunar cycle, from New Moon through Waning Crescent.
Illumination Percentage: The fraction of the Moon's Earth-facing surface currently lit by the Sun. At Full Moon this is 100%; at New Moon it is 0%.
Moon Age: The number of days elapsed since the most recent New Moon. Resets to zero with each new lunar cycle.
Moon Sign: The zodiac constellation the Moon is currently passing through, based on its ecliptic longitude.
Days to Full Moon: The estimated number of days until the next Full Moon, calculated from the current moon age.
Who We Are
Moon Phase Tonight is an independent website dedicated to making lunar data accessible to everyone. We are not affiliated with any government agency, observatory, or academic institution. Our content covers both the astronomy and the cultural and astrological traditions surrounding the Moon, and we strive to clearly distinguish between scientific fact and interpretive tradition.
We update our site regularly to ensure all information is current, accurate, and useful. If you spot an error or have a suggestion, please reach out via our contact page.
A Note on Astrological Content
Portions of this site discuss the astrological and spiritual traditions associated with the Moon — moon signs, void of course periods, manifestation practices, and so on. This content reflects longstanding cultural traditions and is presented for informational and entertainment purposes. It is not intended as medical, psychological, financial, or professional advice of any kind.
The astronomical data on this site — phase names, illumination percentages, moon age — is based on established celestial mechanics. The astrological interpretations are drawn from traditional astrology and should be understood as one lens through which many people find meaning in the lunar cycle.