Explore how moonlight influences human health and its connection with natural light cycles for better well-being.
The Moon shines because its surface reflects sunlight. Even when it looks very bright, the Moon reflects only 3–12 % of the sunlight that strikes it.
How bright the Moon appears from Earth depends on its position in orbit. The Moon circles Earth every 29.5 days, and during this journey the Sun lights it from changing angles.
This motion—together with Earth’s own orbit around the Sun—creates the lunar phases (full, quarter, etc.). At any moment, only the half of the Moon facing the Sun is illuminated; the other half is in shadow.
I took my first moonlit photograph in 1998 with my father’s old Kodak camera. The frame was almost blank, proof that I had no idea what I was doing. Yet I knew I wanted to keep the feeling of standing under the stars, and I had to learn how.
I set moonlight photography aside for years. Later I read that moonlight brightness varies by many photographic stops and is hard to predict in advance. In fact, under clear skies it is as predictable as daylight.
Several factors drive this variation. The most obvious is phase: between quarter Moon and full Moon the illuminance changes by a factor of about 10, equal to roughly 3.5 stops. Orbital distance is another factor. Earth-to-Sun distance ranges from 0.9833 AU at perihelion to 1.0167 AU at aphelion, while Earth-to-Moon distance varies from 356 400 km at extreme perigee to 407 000 km at extreme apogee. Because illuminance follows an inverse-square law, these shifts can be noticeable; lunar illumination changes 6.9 % for Sun-distance variation and 30 % for Moon-distance variation, together adding about one-third stop—enough to alter the mood of a slide-film image.
A third influence is the opposition effect. The lunar surface is sprinkled with tiny glassy particles that act as retro-reflectors. When the observer lies almost on the line between Sun and Moon, the returned light can exceed a simple diffuse reflection by a factor reported anywhere from 1.35 to 20. Even at the low end, this adds at least one-third stop—significant when shooting slide film.
The final source of variation in moonlight brightness is atmospheric attenuation—extinction, in astronomer’s jargon. This measures how much light is absorbed or scattered as moonlight traverses Earth’s atmosphere. Two quantities matter: the extinction coefficient (loss per unit of air mass) and the air mass itself (the path length). Molecular absorption, Rayleigh scattering, and aerosol scattering set the coefficient. Dry, clear air yields a small value; moist, dusty air can raise it sharply. Air mass ranges from one “air mass” at the zenith to roughly forty when the Moon sits on the horizon.
Brightest moonlight coincides with the Moon at perigee and Earth near perihelion at full phase. A perfectly opposite geometric alignment is impossible—Earth’s shadow would create a lunar eclipse—but, ignoring that, a phase angle of zero, extinction coefficient 0.11 mag per air mass, and the Moon overhead give about 0.046 foot-candles (LV –2.0) before the opposition surge. Including the surge, brightness may rise 35 % (literature consensus) to as much as 20-fold; the conservative value yields LV –1.7. A winter field measurement on the Kelso Dunes registered LV –2.2, consistent with these estimates.
The “Looney 11 Rule” treats the full Moon as 250 000 times dimmer than the Sun, implying a shutter speed 18 stops slower than the Sunny 16 guideline—about 44 min at f/16 with ISO 100 film, neglecting reciprocity failure. The actual luminance ratio is closer to 402 000 (18.6 stops), so the rule typically underexposes by at least two-thirds of a stop, often desirable to preserve a nocturnal look.
Although lunar reflectance favors longer wavelengths, moonlight is slightly warmer than sunlight. Under a high, clear full Moon, landscapes nevertheless look bluish because the Purkinje shift reduces red sensitivity as vision transitions from cone- to rod-mediated detection.
That’s why sunlight looks “warm” (more yellow), while moonlight looks “cool” (bluer—because the low light level shifts our color perception), even though the actual colors are almost identical.
This can become a practical issue in long-exposure night photography: the image may resemble daylight, losing the intended sense of mystery. Objectively, the photo is accurate, but it doesn’t match human vision; adding a slight blue tint often restores the nocturnal feel.
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