Moon Phase Calculator
Calculate the current moon phase and upcoming full and new moon dates. Enter values for instant results with step-by-step formulas.
Formula
Illumination = (1 - cos(2π × Moon Age / 29.53)) / 2 × 100%
Moon age is the number of days since the most recent new moon, computed from a known reference new moon (Jan 6, 2000). The synodic month averages 29.53059 days. Illumination follows a cosine curve from 0% (new) to 100% (full) and back.
Worked Examples
Example 1: Moon Phase on a Specific Date
Problem: What is the moon phase on March 20, 2026?
Solution: Days since reference new moon (Jan 6, 2000) = 9,570 days\n9,570 mod 29.53 ≈ days into current cycle\nIllumination = (1 - cos(2π × age/29.53)) / 2 × 100%
Result: Calculate using the tool above for the exact phase, illumination, and upcoming full/new moon dates.
Example 2: Planning a Stargazing Night
Problem: Find dates with minimal moonlight for deep-sky observation.
Solution: Best stargazing occurs within ±5 days of new moon when illumination is below 25%. Use the calculator to find the next new moon date and plan your observation window accordingly.
Result: Target dates around the new moon for darkest skies.
Frequently Asked Questions
How are moon phases calculated?
Moon phases are determined by the Moon's position relative to the Sun as seen from Earth. The synodic month (new moon to new moon) averages 29.53 days. By counting days since a known reference new moon and dividing by the synodic period, we determine where the Moon is in its cycle. The illumination percentage follows a cosine curve: 0% at new moon, 50% at quarters, and 100% at full moon.
What are the 8 phases of the Moon?
The 8 principal phases are: (1) New Moon — 0% illuminated, between Earth and Sun. (2) Waxing Crescent — 1-49% illuminated, growing. (3) First Quarter — 50% illuminated, right half lit. (4) Waxing Gibbous — 51-99% illuminated, growing toward full. (5) Full Moon — 100% illuminated, opposite the Sun. (6) Waning Gibbous — 99-51% illuminated, shrinking. (7) Last Quarter — 50% illuminated, left half lit. (8) Waning Crescent — 49-1% illuminated, shrinking toward new.
Why does the Moon appear to change shape?
The Moon always has half its surface illuminated by the Sun — what changes is how much of that illuminated half we can see from Earth. As the Moon orbits Earth (taking 29.53 days), the angle between the Sun, Moon, and Earth changes, altering our view of the sunlit portion. The Moon does not produce its own light; it reflects sunlight.
What formula does Moon Phase Calculator use?
The formula used is described in the Formula section on this page. It is based on widely accepted standards in the relevant field. If you need a specific reference or citation, the References section provides links to authoritative sources.
Is Moon Phase Calculator free to use?
Yes, completely free with no sign-up required. All calculators on NovaCalculator are free to use without registration, subscription, or payment.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.