Moon Illumination

Moon illumination is the percentage of the moon's visible disc that appears illuminated from Earth, providing a precise numerical measure of lunar brightness from 0% (new moon) to 100% (full moon). While moon phase gives you a categorical description (new, crescent, quarter, gibbous, full), moon illumination provides exact percentage values that enable more nuanced planning for night photography. A 95% illuminated moon provides almost as much light as a 100% full moon but occurs a day or two before or after the actual full moon, potentially giving you extra shooting opportunities. An 80% illuminated moon (late first quarter or early waning gibbous) still provides substantial illumination for moonlit landscapes while being dimmer than full moon.

Moon illumination determines the fundamental tradeoff in night photography: light for landscapes versus darkness for astrophotography. High illumination (80-100%) provides enough moonlight to shoot landscapes handheld or with short exposures, illuminating foreground details and creating visible shadows—perfect for moonlit seascapes, mountain landscapes, and architectural night shots. A full moon (100% illumination) at high altitude can provide enough light for exposures of just 1-4 seconds at ISO 1600-3200, allowing you to capture flowing water motion or freeze some subject movement. However, high moon illumination is the enemy of astrophotography—it washes out the Milky Way, dims all but the brightest stars, and creates severe sky glow that makes capturing faint deep-sky objects nearly impossible. For serious Milky Way photography, you need moon illumination below 20-30%, and ideally below 10% for the best results. A new moon (0% illumination) provides the darkest skies but offers no moonlight for foreground illumination—astrophotographers planning pure star or Milky Way shots prioritize new moon periods, while those wanting both stars and moonlit foreground must carefully balance illumination levels or plan separate exposures. Intermediate illumination levels (30-70%) offer compromises: a 50% illuminated quarter moon provides enough light to faintly illuminate landscapes during long exposures while still allowing brighter Milky Way features to be visible, though not with the drama of new moon conditions.

Moon illumination varies continuously throughout the month in a predictable sine wave pattern: starting at 0% during new moon, it increases daily by roughly 3-4% through waxing crescent and first quarter, reaches 100% at full moon (day 14-15), then decreases through waning gibbous and last quarter back to 0%. The exact illumination percentage for a given date repeats monthly, though the moon's position in the sky (altitude and azimuth) varies seasonally. PhotoWeather provides real-time moon illumination percentages, allowing you to set precise thresholds for your photography goals. Planning moonlit landscapes? Set a minimum of 75-100% illumination and combine with high moon altitude for maximum light. Pursuing dark-sky Milky Way shots? Set a maximum of 15-20% illumination to ensure minimal sky glow. Want to shoot star trails with some foreground detail? Look for 25-40% illumination from a crescent moon that sets mid-session, providing early light before dark skies. By integrating moon illumination with moon altitude, rise/set times, and weather forecasts, PhotoWeather helps you plan night photography sessions that match your specific lighting needs—whether that's bright moonlight, complete darkness, or the nuanced balance between the two.