Moon Age Calculator
Calculate the moon's age in days since the last new moon for any date.
How It Works
Enter any date. The calculator converts it to a Julian Day Number, applies the synodic month formula to determine the phase fraction, then multiplies by 29.53059 to express the result in days since the last new moon. A moon age near 0 means a new moon just occurred; near 14.77 means you are at or near a full moon; near 29.53 means the next new moon is imminent.
Formula
Moon Age Reference Table
- Age 0–1 day: New Moon — dark, not visible
- Age 1–7 days: Waxing Crescent — thin sliver, visible after sunset in the west
- Age ~7 days: First Quarter — right half illuminated
- Age 7–14 days: Waxing Gibbous — more than half lit, growing
- Age ~14.77 days: Full Moon — 100% illuminated, rises at sunset
- Age 15–22 days: Waning Gibbous — still bright but shrinking
- Age ~22 days: Last Quarter — left half illuminated
- Age 22–29 days: Waning Crescent — thin sliver in morning sky
- Age ~29.53 days: New Moon again — cycle resets
Historical Uses of Moon Age
Before printed calendars, sailors and farmers tracked moon age to predict tides, plan fishing trips, and time planting. Ancient Babylonians could predict lunar phases to within hours using this same basic cycle concept thousands of years before modern astronomy. Many traditional calendars (Hebrew, Islamic, Hindu lunar calendar) define the month by one complete lunation — one full 29.53-day moon age cycle.
Moon Age vs. Moon Phase vs. Moon Illumination
These three measurements describe the same orbital position in different ways. Moon age gives the elapsed time in days. Moon phase expresses the same information as a fraction from 0 to 1 (or as a named stage like "Waxing Crescent"). Moon illumination gives the percentage of the disc that appears lit (0% at new moon, 100% at full moon), calculated from the phase angle using a cosine formula. All three are computed from the same underlying synodic month calculation.
Frequently Asked Questions
What is moon age?
Moon age is the number of days that have elapsed since the last new moon. It ranges from 0 at new moon to approximately 29.53 days when the next new moon begins. It is the most intuitive way to express where the Moon is in its cycle.
What moon age corresponds to a full moon?
A full moon occurs at approximately moon age 14.77 days — exactly halfway through the 29.53-day synodic month. In practice, the exact moment the Moon appears "full" may vary by a few hours depending on your time zone.
Can I calculate moon age for historical dates?
Yes — the synodic month formula works for any date within several centuries of the reference epoch (January 6, 2000). For ancient historical dates more than a few centuries before or after 2000, small accumulated errors may affect accuracy by a few hours.
What is the synodic month?
The synodic month (29.53059 days) is the time between two successive new moons — one complete moon age cycle. It differs from the sidereal month (27.32 days, the time for the Moon to orbit Earth relative to distant stars) because Earth moves along its orbit around the Sun during that time, requiring the Moon to travel slightly farther to "catch up" and align with the Sun again.