Why Does the Moon Glow? Find out Now

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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 has been 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? But why is the moon so bright? We’ll be looking at these questions here, so keep reading.

One thing that man has marvelled at for millennia is the stunning glow that comes from the moon. Why is the moon so bright, and what causes this? The answer has nothing to do with cheese 🙂

The explanation why the moon appears to glow is called moonshine science by many.

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Why does the moon glow?

An answer to the question “Why does the moon glow?” is that it reflects the sun. The surface of the moon is not highly reflective1. The moon glows so brightly because of the scattered sunlight that hits it directly and then reflects to Earth, depending on its position in orbit.

Lunar glow from the Solar Wind

There are many other theories to answer the question “Why does the Moon glow?” 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 back2.

The solar wind is made up of protons and electrons which travel at high speed from the sun. These particles cannot be seen but many hit the surface of the moon and interact with it. The particles can cause lunar glow as they react with moon dust. We see this glow on the night side of the moon.

The light bouncing off the earth lights up the moon, and these reflected light waves scatter in all directions. These light waves reflect off of 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 surface3.

How does the moon reflect light if it is a rock?

The moon is not a light source and does not emit light. If it did, it would need an internal energy source like a star or other astronomical object. The moon is made of rock. The reflection of this light is what we see as the illuminated face of the moon.

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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, sunlight hits the moon straight on, which increases the brightness we see. The brightness of moonlight increases at the full Moon and decreases at other points in the lunar cycle4. 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.

I have looked at the moon through a telescope and found it so dazzling, 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.

How Reflective is the Surface of the Moon?

The moon only reflects between 3 and 12 percent of the light that hits it5. 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.

Why is the moon so bright?

We’ve already seen that it is not because it has a highly reflective surface. Neither is there any light source coming from the moon. So why does the moon glow so much?

There are several reasons why the moon is so bright, and these include the following:

Different areas on the moon may reflect light more. 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. We now know that the darker volcanic rock in these areas is actually the result of the lava flow.

These brighter areas that catch our attention when we view the moon affect its apparent brightness. In my case, I find the full moon almost too bright to look at for more than a few seconds, and certainly not without a moon glow filter.

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The Supermoon effect

The full moon may appear to be much larger than it is when it is in a certain position in the sky. Therefore, this is another reason why the moon is so bright when it is in this state. The moon is then called a supermoon.

The Distance of the Moon

You might ask “Why does the moon glow much more than other objects in the sky such as the planets”?

The moon is much closer than other astronomical objects, which means it appears much brighter than other planets and bodies. For example, let’s compare the moon’s brightness to other planets in the night sky.

The moon is only about 239,000 miles from Earth and other planets are millions of miles away. This means that when the moon is illuminated any more than about 70% it begins to cast shadows on the ground. I’ve observed that in a full moon there is no longer real dark during the night. My rural location of Bortle 5 becomes like a city location due to the brightness of the moon.

The contrast of a bright moon

One further reason why the moon is so bright is that we view it in very dark conditions, and it contrasts with the dark skyline around it. The dark night sky makes our eyes more sensitive to bright light, and our pupils are made smaller, meaning that the sudden sight of the moon makes the brightness have an immediate, strong impact.

Why does the moon glow? How Does it reflect light if it is a rock?

On the surface of the moon is a fine cover of what is called regolith6. These finely ground particles of rock are like dust or fine soil and cover the surface of the moon up to 5-10 meters deep in places. Some volcanic glass particles reflect a small amount of light. As has been said this surface material is not very reflective but is light in color and the lightness may exaggerate the brightness of the Moon’s surface.

The moon reflects light from the sun true or false?

Does the moon reflect light from the sun? Yes, it does. It is true, but let’s look at how much it reflects.

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 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 interpret and is not completely true to reality!

Conclusion

Why does the moon glow? It doesn’t glow but appears to. It can all be explained by moonlight science! 

As explained above, this effect is mostly due to the reflected sunlight from the sun. Does the moon absorb light from the sun? Despite shining brightly, the surface of the Moon absorbs a lot of light and reflects some off into Space. Some light is projected onto the moon as reflected sunlight from the Earth, especially the oceans. This has only a small effect but contributes to the Moon’s brightness.

References

  1. Flynn, B., Vallerga, J., Gladstone, G., & Edelstein, J. (1998). Lunar reflectivity from Extreme Ultraviolet Explorer imaging and spectroscopy of the full moon. Geophysical Research Letters, 25. https://doi.org/10.1029/98GL02483.
  2. Saul, L., Wurz, P., Vorburger, A., D.F.Rodr’iguez, M., Fuselier, S., McComas, D., Mobius, E., Barabash, S., Funsten, H., & Janzen, P. (2013). Solar wind reflection from the lunar surface: The view from far and near. Planetary and Space Science, 84, 1-4. https://doi.org/10.1016/j.pss.2013.02.004.
  3. Douglas, A. (1937). The Corona by Reflection from the Moon. Nature, 140, 156-157. https://doi.org/10.1038/140156b0.
  4. Rackham, T. (1965). A note on lunar ray systems. Icarus, 4, 544-546. https://doi.org/10.1016/0019-1035(65)90030-8.
  5. Zhang, J., Ling, Z., Liu, J., Wu, Z., Li, B., & Ni, Y. (2015). Lunar absolute reflectance as observed by Chang’E-1 Imaging Interferometer. Science China Physics, Mechanics & Astronomy, 58, 1-6. https://doi.org/10.1007/S11433-015-5680-9.
  6. Colwell, J., Batiste, S., Horányi, M., Robertson, S., & Sture, S. (2007). Lunar surface: Dust dynamics and regolith mechanics. Reviews of Geophysics, 45. https://doi.org/10.1029/2005RG000184.