Astrojelly guide · Updated October 11, 2026 · Calculation methods and limits
What does this Moon phase calculator show?
The tool calculates the Moon's phase and illuminated fraction for your selected instant. Use the date, local time and city to specify that moment. These are astronomical results, separate from any traditional meanings attached to a new or full Moon.
For today's phase, choose today's local date and a time that interests you. A phase label can change within a calendar day, so a timestamp is more informative than a date alone. For exact new and full Moon events in a month, open the monthly lunar calendar.
How to find the Moon phase on your birthday
- Choose your birth date and known local time.
- Select your birthplace so the timezone matches the recorded date.
- Generate the Moon phase result.
- Read the illumination and phase alongside its timestamp.
If your time is unknown, a noon reference is only an approximation for that day. Near a phase-label boundary, explore more than one time before giving your birthday one fixed label.
Waxing, waning and illumination explained
Waxing describes the portion of the cycle in which the visible sunlit fraction grows toward a full Moon. Waning describes the portion in which it decreases toward a new Moon. A crescent shows less than half of the disk illuminated; a gibbous Moon shows more than half. At a quarter phase, roughly half the visible disk is sunlit.
Illumination is a geometric fraction. A result near 50% does not by itself tell you whether the Moon is waxing or waning, because that fraction occurs on both sides of the cycle. Read the phase label as well.
Moon phase is different from Moon sign
A Moon phase concerns the Sun-Earth-Moon geometry and our view of the sunlit side. A Moon sign is the zodiac segment containing the Moon's longitude. A full Moon is not a zodiac sign, and two full Moons need not occur in the same sign.
Use the Moon sign calculator for a natal placement, the phase calculator for the phase at an instant, and the lunar calendar for upcoming exact events. Each answers a different question.
Why might a calendar show a different local date?
A new or full Moon event happens at one instant worldwide. That instant can fall on different calendar dates in different timezones. A calendar set to UTC may therefore show a different date from one set to New York or Los Angeles.
A phase result also does not guarantee that the Moon is above your horizon or visible through the weather. Rise and set times, clouds, horizon obstructions and observing conditions are separate questions.
Learn more about the Moon
NASA's Moon phase guide explains the changing view of the sunlit Moon. Astrojelly uses Astronomy Engine for its astronomical calculations. If you use lunar themes as journal prompts, treat that as your own reflective practice rather than a scientifically established prediction.
Explore this month's lunar calendar ↗