The Moon is not a light source; it is a passive reflector of the Sun’s light, which fundamentally changes how its illumination must be measured. Unlike a light bulb that generates its own light, the Moon’s output cannot be rated in a single, fixed value like lumens. Therefore, to quantify moonlight, one must instead consider the intensity of light that actually reaches the Earth’s surface. This necessity leads to the use of a different photometric unit that accounts for distance, distribution, and the receiving area.
Understanding Light Measurement: Lux Versus Lumens
A lumen is a unit of luminous flux, which quantifies the total amount of visible light emitted by a source in all directions. It measures the light source’s total output, regardless of where that light travels or how it spreads out. Using lumens to describe the Moon is inaccurate because the Moon is simply reflecting a small portion of the sun’s flux, not emitting light itself. Furthermore, the immense and variable distance between the Moon and Earth makes the total output of the original source irrelevant to the illumination received on Earth.
The correct unit for measuring the light from the Moon is the lux, which is a unit of illuminance. Lux measures the density of light, specifically the amount of luminous flux falling onto a specific surface area. One lux is defined as one lumen distributed over one square meter. The lux value decreases as the distance from the light source increases because the light spreads out over a larger area. This unit is the practical measurement for determining how brightly a surface, such as the Earth’s ground, is illuminated by a distant source like the Moon.
Quantifying Moonlight: Illuminance Received on Earth
The illuminance provided by the Moon on a clear night is significantly lower than many people might assume. Under optimal conditions, the maximum illuminance from a full Moon at its highest point in the sky is typically around 0.2 to 0.3 lux. This maximum value is a measurement taken at ground level and assumes a clear atmosphere with the Moon directly overhead.
Modern scientific studies place the absolute maximum closer to 0.32 lux, achieved only when the Moon is perfectly full and near its closest point to Earth. For most observers at mid-latitudes, the illuminance is more commonly in the range of 0.05 to 0.2 lux. The Moon’s surface is dark, with an average visual albedo, or reflectivity, of only about 12%, explaining its relatively low lux value despite its proximity.
In astronomy, the Moon’s brightness is also described using Apparent Magnitude, a logarithmic scale where lower numbers indicate brighter objects. The full Moon has an average apparent magnitude of about -12.7. While this scale is useful for astronomers, the lux value remains the standard unit for quantifying the practical illumination level on the ground.
Variables Influencing Lunar Brightness
The specific lux measurement of moonlight is not a fixed number, as it is constantly affected by a combination of astronomical and atmospheric factors. The most significant of these is the lunar phase, which dictates the percentage of the Moon’s surface illuminated from the Earth’s perspective. The light drops off dramatically as the Moon moves away from its full phase; a half-moon provides only about 7% to 10% of the light of a full Moon.
The Earth-Moon distance also varies because the Moon’s orbit is elliptical. When the Moon is at perigee, its closest approach to Earth, it appears slightly larger and brighter than when it is at apogee, its farthest point. This difference can alter the received illuminance by up to 20%, sometimes creating an exceptionally bright “supermoon.”
Atmospheric conditions also play a major role in scattering and absorbing the faint light before it reaches the ground. Clouds, haze, and microscopic particles like dust or pollution reduce the amount of light that penetrates the atmosphere, a process known as atmospheric extinction. Even on a seemingly clear night, the light is slightly diminished, and a thin layer of cloud cover can easily cut the received lux value in half.
Contextualizing Moonlight: Comparisons to Artificial Sources
To understand the practical dimness of moonlight, it helps to compare the maximum 0.3 lux value to common artificial and natural light sources. A typical well-lit office environment, for instance, is illuminated to a level of approximately 300 to 500 lux. Therefore, the brightest full Moon provides less than one-thousandth of the light needed for comfortable reading or working indoors.
The contrast is even clearer when looking at street lighting, where a typical street lamp can generate an illuminance of 10 to 20 lux on the pavement directly below it. Even a single candle flame, when placed at a distance of about one meter, can produce an illuminance of roughly 1 lux. Compared to the Sun, the difference is staggering, as direct midday sunlight can reach 100,000 lux or more. The faintness of moonlight explains why the human eye must rely on specialized scotopic (rod) vision to navigate under its light.