How Bright is the Moon?

By: Mike Luciuk

How bright is the Moon? That depends on a number of factors. The phase of the Moon is the primary one. Other obvious factors are the Earth-Moon and Sun-Moon distance as well as atmospheric transparency and extinction. Albedo and its variation is the final major factor affecting lunar brightness.

Visual albedo is defined by the “reflectivity of the surface of a planet, moon, asteroid, or other celestial body that does not shine by its own light. Albedo is measured as the fraction of incident light that the surface reflects back in all directions. A perfect reflector by definition has an albedo of unity, i.e., all the incident light is reflected; a body that reflects no light at all would have an albedo of zero.” Actually, real surfaces never have albedos of exactly zero or one, but something in between.

Astronomers have determined the visual albedos of our planets. From NASA’s planetary sites, the brightest is Venus with an albedo of 0.65. That means 65% of incoming sunlight is reflected from the cloud-covered planet. The remaining 35% contributes to the heat energy of Venus. Mercury, at 0.11, has the lowest planetary albedo. Earth’s albedo is 0.37; Mars is 0.15; Jupiter, 0.52; Saturn, 0.47; Uranus, 0.51; Neptune 0.41. Pluto’s albedo varies from 0.5 to 0.7.

It should be pointed out that these planetary albedos are averages. Taking Earth as an example, clouds vary from 0.4 to 0.8, snow varies from 0.4 to 0.85, forests vary from 0.04 to 0.1, grass is about 0.15, and water varies from 0.02 with the Sun directly overhead to 0.8 at low levels of incidence. So the Earth’s albedo varies, and depends on the extent of cloudiness, snowfall, and the Sun’s angle of incidence on the oceans. With an average albedo of 0.37, 63% of incoming solar energy contributes to the warmth of our planet. It’s obvious that if cloud cover were to decrease significantly, the Earth’s surface temperature would increase, contributing to other factors of global warming such as the amounts of greenhouse gasses.

Our Moon’s average visual albedo is 0.12. The brightness of the Moon changes dramatically as its phase changes. During first and third quarters, the visible Moon is 50% illuminated by the Sun, but its brightness is only about 8% of full Moon — an increase of 2.7 magnitudes. The Moon’s visual albedo on its illuminated segment gets progressively smaller as the angle between the Earth and Sun on the Moon (phase angle) increases. A major reason for this decrease of visual albedo with increasing phase angle is the greater creation of shadows on the irregular lunar surface, thereby reducing reflected light back to Earth.

A graphical illustration of phase angle versus lunar brightness

On Earth, we never see a perfectly full Moon, since the true phase angle we see is in the order of 5 degrees. With a zero degree phase angle the Moon would be in Earth’s shadow, and we would experience a total lunar eclipse. Apollo astronauts reported that a true full Moon is about 30% (0.2 magnitudes) brighter than what we see here on Earth.

So if the full Moon as seen on Earth has a visual magnitude of –12.7, its brightness at first quarter (phase angle 90 degrees) would be magnitude –10.0, a brightness reduction of 12x. Since we see the Moon half illuminated by the Sun at first quarter, a 6x brightness reduction implies an effective lunar albedo reduction from .12 to .02.

It should be pointed out that as we reach new Moon, earthshine becomes a factor. Someone on the Moon sees a “full Earth” when we see a new Moon. As seen from the Moon, our Earth would look about 100x brighter than our full Moon. This is because of the Earth’s larger size and higher albedo. Imagine being on the Moon and seeing a full earthrise at magnitude –17.7, with earthlight dimly illuminating and casting shadows on the lunar scenery.

Sperry Observatory is open for observing from 7:30 pm - 10:30 pm every Friday night with the exception of the 1st Friday of each month from September to May. A presentation on an astronomy related topic is given at 8:30 pm. This is free and open to the public.