Why Does the Moon Glow?

Quarter Moon by Karl Perera

The moon is a beautiful thing. I know that since I got my telescope just over a year ago, it was one of my main targets for photography and still is. It is a familiar yet mysterious object and when it is out in its glory, man, is it bright?

Check out this page to find out more about the telescope I bought and how much you’ll need to spend on a telescope that will enable you to observe and photograph the Moon.

One thing that man has marveled at for millennia is the stunning glow that comes from the moon. Why is it so bright and what makes the moon glow so much? Oh, and the answers are nothing to do with cheese đŸ™‚

An answer to the question “why does the moon glow?” is that it reflects the sun. The surface of the moon is not highly reflective. The reason why the moon is bright is because of the scattered sunlight that hits it directly and then reflects back to Earth depending on its position in orbit.

There are many other theories as to why the Moon glows and none of them have been proven. The most popular theory is that it is because of the solar wind that hits the surface of the moon and then bounces back.

The light bouncing off of Earth lights up the Moon and these reflected light waves scatter in all directions. These light waves reflect off Earth’s atmosphere and bounce back to Earth, where they can be seen as moonlight. Light waves coming directly from the Sun also reflect off the moon’s surface.

The moon is not a light source and does not emit light. The reflection of this light is what we see as the illuminated face of the moon.

How Bright is the Moon?

The moon reaches its maximum brightness when the half that faces the earth is lit, ie. the full moon phase. The minimum brightness is called the New Moon and at this point in its cycle, we cannot see it in the night sky. The reason for this is that the side of the moon facing away from us is then lit and the side of the moon facing us is dark.

Also when the moon is full the sunlight hits the moon straight on and this increases the brightness we see. The brightness of a full moon is -13 magnitude, which means the Moon glows very brightly indeed! When the moon is only a quarter illuminated, however, it is about six times less bright than a full moon because of the angle at which the sunlight hits the face of the moon.

Personally, I have looked at the moon through a telescope and found it dazzling, so much so that I had to place a moon glow filter in the eyepiece of my telescope to make it easier to observe the moon. This is particularly the case on the full moon.

The fact that the moon is so bright makes it relatively easy to photograph. You just need very short exposures in the order of milliseconds. Other targets are not so easy. If you are interested in other kinds of astrophotography, here are my thoughts on whether astrophotography is easy or not.

I have photographed the glowing moon with my DSLR and also with a dedicated planetary camera. To get a good photo of the moon does not require an expensive camera. Find out more about the best cheap cameras for astrophotography. Also, your camera does not need a high resolution of many megapixels. For other targets such as deep sky or galaxies, how many megapixels your camera has may be important.

To learn more about the exact measurements of the moon’s brightness, read this information from a recent study published on Skyandtelescope.org.

moon
Full Moon Photo by Karl Perera

How Reflective is the Surface of the Moon?

According to Livescience.com, the moon only reflects between 3 and 12 percent of the light that hits it. This means it is not as reflective as many would have us believe. The different phases of the moon are due to the orientation of the moon with respect to the position of the sun and we, therefore, view the moon with different amounts of it lit by the sun. From our perspective here on Earth, the angle of the sun as it hits the moon also changes and so the brightness varies.

There are different areas on the moon that may reflect light differently. The brightest areas are known as the Lunar Highlands and other areas are darker and reflect less light. These darker areas are the “mare” or sea areas. The reason for this name goes back a long time when it was thought that these dark areas could be seas like on Earth. Now we understand that these areas were actually produced by lava flow and the result is a darker volcanic rock.

On top of the rock of the moon is a fine cover of what is called regolith. This is finely ground particles of rock that are like dust or fine soil and cover the surface of the moon up to 5-10 meters deep in places. There are also some volcanic glass particles. As has been said this surface material is not actually very reflective but is light in color and the lightness may exaggerate the brightness of the Moon’s surface.

To compare the Moon’s reflectivity with other planets in the Solar System is interesting. For example, I have noticed that Jupiter is extremely bright when I view it through my telescope eyepiece. Photographing Jupiter requires exposure of a fraction of a second, but the same goes for the Moon depending on what percentage of the Moon is lit up. However, to quantify this:

ObjectAlbedoReflected light
Moon0.1212%
Jupiter0.5252%
Earth0.3030%
Venus0.7575%
How reflective is the Moon compared to other planets?

The color of most of the surface is whitish-grey and this also helps the color that we see to appear bright white.

The fact that the moon has no atmosphere increases the amount of light that we see reflecting from the moon. The atmosphere, if it existed, would obscure some of this light and the moon would appear to glow less.

When we see the Full Moon, for example, it is hugely contrasted against the black background of the dark sky and this makes it seem to us that it is much brighter than it really is. The fact that it is nighttime also means that our eyes are accustomed to dark light conditions and so if we quickly look at the Moon it dazzles us. The glow of the Moon, therefore, is a visual effect that our minds perceive and not really a glow at all!

Conclusion

The moon does not really glow but just appears that way. As explained above, this effect is mostly due to the reflected sunlight from the sun. Despite shining very brightly the surface of the Moon absorbs a lot of light and also reflects some off into Space. There is also some light that is projected onto the moon in the form of reflected sunlight from the Earth and especially the oceans. This has only a small effect but contributes to the Moon’s brightness.