With the Sun settled below the horizon, the Moon is the brightest object in our nighttime sky. The larger it is, the more intently the Moon beams down on our planet, its glow piercing the otherwise dark night. In this article, we take a look at what exactly makes our Moon shine the way it does.
Does the Moon Shine On Its Own?
The Moon is covered mainly with gray rocky anorthosite with a bit of dark, hardened basalt lava thrown in here and there. If these names don’t mean much to you, it’s because these types of rock have no special characteristics of their own – especially when it comes to light.
Just like most objects on Earth, lunar rocks do not emit any form of light at all. There is nothing on the Moon at all that attributes to its brightness in the sky. Without some help from another source, our celestial neighbor would be just another dull, lifeless orb in the Solar System.
Where Does the Light We See From the Moon Come From?
Every tiny bit of light that comes to us from our rocky satellite does not originate there. Just as our Earth receives light from the Sun, so too does the Moon. This light is reflected off the Moon’s surface before cascading down to Earth.
In this way, the Moon functions like a mirror that bounces the Sun’s light down to Earth. The Moon, however, is a really bad mirror.
How Does the Moon Reflect Light if It Is a Rock?
Any known object in the Solar System either emits light (like the Sun) or reflects it. We’re able to view objects on Earth because they reflect light into our eyes so we can see them. This holds true for the Moon as well.
While the Moon appears bright white from Earth, this color is an illusion created by the Sun’s rays. When astronauts reached the Moon for the first time, they discovered the surface was quite dark.
The Moon’s lack of a discernible atmosphere leaves the surface fully susceptible to the full impact of foreign objects. This has left the lunar surface far from smooth, with countless craters, mountains, trenches, and volcanic remains.
If you can picture a faded, bumpy mirror, you’ll have a somewhat accurate picture of how the Moon reflects sunlight.
Astronomers characterize how much of the Sun’s light an object in space reflects using the albedo scale. This scale ranges from 0, meaning an object is pitch black, to a 1, meaning the object is incredibly bright and reflective.
Our Moon sits at a 0.12 on the scale, meaning it only reflects 12% of the sunlight that shines upon it. Compared to Saturn’s Moon Enceladus, which has an albedo of 0.99, the Moon is not very reflective at all.
As you’ve probably discovered, the Moon’s brightness varies depending on which phase it is in.
The Moon’s Brightness at Different Phases
The Moon obviously shines the brightest during a Full Moon, when the entire front face of the Moon is visible to us. As the Moon gets smaller with each subsequent day, you’ll be surprised to discover just how much darker it becomes.
Just 2.4 days after (or before) a Full Moon, the Moon is only half as bright as when it is completely full. Although still 95% lit, the slight shift in the Moon’s location relative to the Sun makes a huge difference.
Furthermore, by the time the Moon reaches the Last Quarter phase, it is only roughly 10% as bright as it was when it was full. Due to the landscapes on each side of the Moon, a First Quarter is actually just a tiny bit more reflective.
How Bright Is the Moon Compared to the Sun?
While reflectivity is measured on the albedo scale, celestial brightness as seen from Earth is evaluated using magnitude. The smaller a number is on this scale, the brighter the object is in the sky.
The Sun is the brightest object that we can see by far. It’s so bright during the day that it completely drowns out all other heavenly objects, with the exception of the Moon. At -26.7, the Sun is 14 magnitudes more luminous than the Moon at -12.7.
Believe it or not, this rating puts the Sun 398,110 times brighter than our Moon. In other words, you would need to put that many Moons in our sky to make up the same brightness as the Sun.
Why Does the Moon Shine During the Day?
Since the Moon’s shine comes solely from the Sun, it carries the same brightness whether it’s daytime or nighttime on Earth. The Moon, due to its closeness to both the Sun and the Earth, ends up shining bright enough to be visible.
Everything else in the sky, including stars and planets, are much less bright and get lost to the Sun’s rays during the day.
The only time we don’t see the Moon during the day is when its orbit around the Earth puts the Sun and the Moon in different parts of the sky.
Does the Earth Shine Like the Moon?
As we mentioned earlier, all objects in the Solar System have some ability to reflect light. The Earth is no exception. Being almost four times bigger than the Moon, the Earth has the ability to reflect a lot more light.
The Earth’s water, however, is a significant absorber of light and steals more than anything else on our planet. Even so, the Earth has an albedo rating of about 0.37. This means it reflects close to 37% of the light that reaches it.
If you were one of the lucky few to have stood on the Moon, you’d be able to tell just what a difference this is.
In fact, a full Earth from the Moon’s surface would be 43 times brighter than a full Moon as seen from Earth. When sunlight is exposed to the most ice and desert, the Earth can get up to 55 times brighter than the Moon.
While the Moon has no glow of its own, it still gives us incredible moonlit nights. Even so, we owe even the moonlight to the Sun’s powerful rays. After all, without them, we may not even know that our celestial neighbor is floating up there around us.