Moonlight Brightness Calculator
Enter any date, location, and cloud cover to see how many lux of natural moonlight will actually reach the ground — based on lunar phase, altitude, distance, and atmospheric extinction.
Moonlight Brightness Calculator
Latitude & Seasonal Effects
Two full moons at the same date can produce noticeably different lux readings depending on where — and when — you're standing.
Latitude & Moon Altitude
The moon's maximum height above the horizon depends on your latitude. Near the equator, it can pass near the zenith, minimizing atmospheric extinction. At high latitudes, it stays lower on the horizon even at its peak, so light travels through more air mass and arrives dimmer.
Winter Full Moons Ride High
The full moon sits opposite the sun. In winter, when the sun is low, the full moon rides high overhead — the same reason winter full moons often look strikingly bright. In summer, the pattern reverses.
Elevation
Observing from higher elevation means less atmosphere overhead altogether, reducing scattering and extinction regardless of the moon's angle — a small but measurable boost for mountain observing sites.
Regional Haze & Humidity
Humid or hazy air scatters more moonlight before it reaches the ground than clear, dry air, even at identical altitude and phase. This is why the same phase can feel dramatically brighter in a dry climate than a humid one.
Local Weather
📍 Check Conditions
Enter a city above to see current conditions.
Lux Reference Scale
Illuminance by Moon Phase
Approximate lux at the ground under clear, dark-sky conditions with the moon high overhead. Actual readings vary with altitude, atmosphere, and cloud cover — use the calculator above for a specific date and location.
| Phase | Illumination | Approx. Lux | Notes |
|---|---|---|---|
| 🌑 New Moon | 0% | ~0.0001 | Starlight and airglow only |
| 🌒 Waxing Crescent | ~25% | ~0.003 | Faint, low in the west after sunset |
| 🌓 First Quarter | 50% | ~0.01 | Roughly 25x dimmer than Full, not half |
| 🌔 Waxing Gibbous | ~75% | ~0.07 | Rising brightness curve steepens near Full |
| 🌕 Full Moon | 100% | ~0.25–0.30 | Opposition surge spike, peak of the month |
| 🌖 Waning Gibbous | ~75% | ~0.05 | Slightly dimmer than waxing at same fraction |
| 🌗 Last Quarter | 50% | ~0.008 | Rises near midnight, visible into daylight |
| 🌘 Waning Crescent | ~25% | ~0.002 | Thin arc low in the east before sunrise |
The Science of Moonlight
How do we calculate the brightness of a rock 238,000 miles away? This tool utilizes the Krisciunas-Schaefer Photometry Model to simulate lunar luminance based on four critical physical variables.
Phase Angle
As the moon wanes, shadows lengthen inside craters, hiding bright soil. This causes brightness to drop non-linearly. A 50% lit moon is only 10% as bright as a full moon.
Atmospheric Extinction
Light scatters as it travels through air. When the moon is low on the horizon, its light passes through a thicker Air Mass, significantly reducing Lux intensity.
Opposition Surge
The "Retro-Reflection" effect. When the Sun is directly behind Earth (Full Moon), the moon's porous surface reflects light straight back, creating a 40% spike in brightness.
Orbital Distance
The moon's orbit is elliptical. During Perigee (Supermoon), it is ~30,000 miles closer than Apogee, making the moonlight up to 30% more intense.
Moonlight for Outdoor Activities
How much lunar light you actually need depends on what you're doing under it.
Night Hiking
Above roughly 0.05 lux (waxing/waning gibbous or brighter), most hikers can follow an established trail without a headlamp once their eyes adapt. Near new moon, a light is effectively mandatory off-trail.
Wildlife Viewing
Many nocturnal species reduce activity under bright moonlight to avoid predators — a pattern researchers call lunar phobia. Crescent and new-moon nights tend to produce more sightings of small mammals and owls.
Astrophotography
For Milky Way or deep-sky imaging, moonlight above ~0.01 lux (roughly first/last quarter and brighter) washes out faint detail. Crescent-to-new windows are the standard target for wide-field night sky shots.
Camping & Night Navigation
A gibbous-to-full moon (roughly 0.05–0.3 lux) is usually enough to move around camp, read a map, or set up a tent without artificial light, especially with 15–20 minutes for eyes to reach full night adaptation.
Frequently Asked Questions
Why does it say 0 Lux during the day?
While the moon technically reflects light during the day, the Sun is roughly 400,000 times brighter. The calculator filters out sunlight to show purely lunar contribution. Additionally, during a Full Moon, the moon is exactly opposite the sun — meaning if the sun is up, the Full Moon is physically below the horizon.
Why is the Lux number so low (0.25)?
The human eye is incredibly adaptive. Our night vision (scotopic vision) is logarithmic, meaning 0.25 Lux feels relatively bright to us in total darkness. In terms of raw electromagnetic energy, however, moonlight is very dim. For context, a typical living room is 50 Lux, while direct sunlight is up to 100,000 Lux.
Does this account for light pollution?
No. This calculator estimates natural lunar illuminance. In a city, "Skyglow" (streetlights reflecting off clouds) can easily reach 0.1 to 0.5 Lux, overpowering the moon. In a city, clouds make the sky brighter; in nature, clouds make the sky darker. This tool assumes a natural, dark-sky environment.
Why does altitude matter?
When the moon is low on the horizon, its light must travel through a thicker layer of Earth's atmosphere ("High Air Mass"). This atmosphere scatters the blue light and absorbs intensity, which is why a rising moon looks orange and is significantly dimmer than a moon directly overhead.
Can I read a book by moonlight?
Technically, yes, but it is difficult. The threshold for reading large print is roughly 0.20 Lux. A "Supermoon" (Perigee Full Moon) can reach ~0.30 Lux, which allows for reading headlines, but fine print requires color vision (photopic), which generally activates above 3.0 Lux.
What is the "Opposition Surge"?
The moon's surface is covered in porous, dusty soil (regolith). When lit from a sharp angle, these particles cast tiny shadows on each other, darkening the moon. When the sun is directly behind you (a Full Moon), those shadows disappear, causing a disproportionate spike in brightness known as the Opposition Surge.