On any clear night, it’s always apparent if the Moon is out. Its unmistakable glow permeates the night and helps light our way. Nights without the Moon are, without a doubt, much darker. What is it, though, that makes our Moon so bright?
A Reflection of Sunlight
The Moon is made entirely of rock and has no way of shining on its own. If left to its own devices, we wouldn’t even be able to see the Moon at its roughly 238,900 miles (384,470 kilometers) distance from Earth.
Instead, like all rocky celestial objects in the Solar System, the Moon relies on the light from the Sun to provide illumination. The light we see coming from the Moon is actually a reflection of the Sun’s rays that reach the lunar surface.
Although the Moon appears bright white through the Sun’s reflection, the Moon is actually made up of dull gray anorthosite and even darker gray basalt. Because of this, the Moon is quite dark compared to other rocky celestial bodies.
In addition to a dull, dark color, the Moon is also anything but flat. Without much of an atmosphere, the Moon was hit by countless meteors over its long history. The craters that formed, along with the mountains, valley, and trenches covering the lunar surface make for a very poor mirror.
In fact, the Moon only reflects about 12% of the light that reaches its surface. Most of that light is absorbed by those dark rocks that the Moon is comprised of.
Factors That Contribute to the Moon’s Brightness
Even though the Moon shouldn’t be very bright in the sky, it does shine brightly down on us. There are a few good reasons for this:
It’s Distance From Earth
Let’s face the facts – the Moon is by far the closest natural object to us in the entire Solar System. The Moon sits in space 238,900 miles (384,470 kilometers) on average from Earth. In contrast, our next closest neighbor Venus is over 66 million miles (106 million kilometers) from Earth when it is at its closest.
This distance changes slightly as the Moon revolves around the Earth. The Moon at its closest to Earth, known as perigee, is 225,623 miles (363,104 kilometers). At its farthest point from Earth, its apogee, the Moon sits 238,855 miles (384,400 km) away.
While imperceptible most of the time, a Moon that’s closer to the Earth does shine a little bit brighter.
A Supermoon occurs when the Moon is at perigee and it is in its full phase. A Supermoon can appear up to 16% brighter than a Full Moon at its average distance from Earth.
It’s Distance From the Sun
Just as the Moon’s distance from Earth changes during orbit, so does Earth’s distance from the Sun as we orbit our star.
When we are at our closest to the Sun (closer to 91 million miles instead of the usual 93), the Moon ends up closer to the Sun as well. As a result, the Sun’s rays on the lunar surface are more intense, causing an increase in brightness of about 4%. It’s hard for the human eye to notice the difference, though.
Our Moon is actually the fifth-largest natural satellite in the entire Solar System. It’s nearly the same size as the planet Mercury and roughly 25% the size of the Earth.
An object of the Moon’s size has more surface area to reflect light off of, and more light gets passed down to Earth. It goes without saying that a smaller Moon would be a lot less bright.
Moon’s Altitude in the Sky Means Less Atmosphere To Fade Colors
Any object shines more brightly when pointed directly at something else. A flashlight focused on a person’s face will reveal much more than a flashlight that glances off that same face from a lesser angle.
This holds true for celestial objects as well. When the Moon is higher in the sky, the intensity of its light is more direct on the Earth and shines brighter. When the Moon settles in near the horizon, it is noticeably dimmer because its light is spread out more. A Moon that sits lower in the sky also has more atmosphere to travel through.
Speaking of atmosphere, Earth’s atmospheric conditions play an important role in how brightly we see the Moon. It should come as no surprise that a Moon on a clear night will shine more brightly than an evening with haze, dust, or smog in the air.
The Brightness of a Full Moon
Looking up at the night sky, we are sometimes taken aback by just how bright a Full Moon appears in the sky. It may surprise you to hear that the Moon is already at 50% of its brightness just 2.5 days after the Moon is full. At the Quarter phase, it is only 10% as bright.
When the Moon is full, the Sun’s rays directly hit the face of the Moon that we can see. These rays are reflected directly back to the Earth, creating an incredible luminous glow.
Once the Moon moves even slightly out of the path of the Sun’s light as seen from Earth, a lot of it misses the Earth and is lost to space.
The Full Moon does have the added benefit of rising right as the Sun sets. This means it has no Sun to share the sky with and easily drowns out planets and stars.
Moon’s Brightness Compared to Other Celestial Objects
Even though the Moon is incredibly bright to us, it is one of the dimmest objects in the entire Solar System. The Moon reflects only 12% of the rays that reach its surface.
In contrast, the Earth’s ice and deserts help us reflect 31% of the light that reaches us. Venus’s cloud cover allows it to reflect 75% of the light that reaches it. Neptune’s icy moon Triton is able to reflect 85% of all light. If Triton were in place of our Moon, it would shine around seven times brighter!
Although the Moon gets its glow from the Sun, many other factors play into the Moon’s intensity in the sky. We’re lucky to have a neighbor so close by that we can admire and enjoy when its shine helps illuminate an otherwise dark night.