Earthlight

Sunlight hits our big Earth.

Then the light bounces off the clouds. It travels to the Moon. This makes the dark Moon glow.

This glow is very soft. You can see it at night. Look for a thin Moon. The rest of the Moon will show too.

Long ago, a man named Da Vinci saw this. He thought it came from oceans. Now we know clouds help more. It is a pretty sight.

Have you ever seen a thin crescent Moon at night?

You might see the rest of the Moon glowing too. This soft light is called earthshine. It happens when sunlight hits the Earth. The light bounces off our clouds and oceans. This light then travels to the Moon. It lights up the dark parts of the Moon.

Long ago, Leonardo da Vinci saw this. He thought the light came from the oceans. Now we know clouds help more. Some people call this the "Da Vinci Glow."

Earthlight is very bright for people in space. In 1969, an astronaut named Michael Collins saw it. He said the light was bright enough to read a book. On the Moon, earthshine is much brighter than a full moon on Earth. It is 43 to 55 times brighter!

Scientists use earthlight to study our planet. They look at the light to learn about Earth's environment. This light helps us see how our world works from far away.

Have you ever looked at a thin crescent Moon? You might see the rest of the dark Moon glowing too. This soft, ashen light is called earthshine. It is a type of planetshine. Earthlight is the light from the Sun that bounces off Earth. It reflects from our surface and our clouds. This light then travels through space to the Moon. It makes the unlit parts of the Moon visible to us.

This light works in a very specific way. First, sunlight hits the Earth. Then, the light reflects off the clouds and the oceans. This reflected light moves toward the Moon. It provides a dim illumination to the dark side. On the Moon, this light is quite strong. It has a maximum brightness of -17.7. However, it is only 0.01% as bright as direct sunlight.

People have noticed this for a long time. In the 16th century, Leonardo da Vinci saw it. He thought the light came from Earth's oceans. We now know that clouds reflect much more light. Because of him, some call it the "Da Vinci Glow." This glow is even mentioned in an old Scottish poem. The poem calls it the "auld muin in her airm."

Seeing this light can be a big deal in space. During the Apollo 11 mission in 1969, Michael Collins saw it. He was 19,000 kilometres from the Moon on July 19. He said the light was bright enough to read a book. On the Moon, earthshine is very bright. It can be 43 to 55 times brighter than a full moon on Earth.

Earthlight is useful for more than just looking pretty. Scientists study it to learn about Earth's environment. They look at how much light Earth produces. It also helps light up parts of the Moon that the Sun never hits. Astronauts like Dr. Sian Proctor have even written about it. She wrote a poem and a book about the human perspective of earthlight.

Earthlight is a specific type of light known as planetshine. It occurs when sunlight reflects off the Earth and travels to the Moon. This reflected light illuminates the dark, unlit portions of the lunar surface. This phenomenon is often called earthshine when viewed from our planet. It allows us to see the entire lunar disk even during a thin crescent phase. This happens because the Moon is receiving both direct sunlight and indirect light from Earth.

The mechanism behind earthlight is a multi-step process of reflection. First, sunlight hits the Earth's surface and its atmosphere. The light then reflects off the Earth's clouds and its oceans. While Leonardo da Vinci once believed the oceans were the main source, we now know clouds provide much more reflected intensity. This reflected light travels through space toward the Moon. Once it reaches the Moon, it provides a dim, ashen illumination to the shadows. This makes the night side of the Moon visible to observers on Earth.

There are different ways to describe this light based on the lunar phase. During a waxing crescent, the light is seen in the western sky after dusk. During a waning crescent, it is seen in the eastern sky before dawn. Observers sometimes use poetic names for these stages. A waxing crescent is called "the old Moon in the new Moon's arms." A waning crescent is called "the new Moon in the old Moon's arms." High contrast photography can also capture this light during a solar eclipse.

History shows that humans have studied this glow for centuries. In the 16th century, the artist and scientist Leonardo da Vinci remarked upon it. Because of his early observations, the phenomenon is sometimes called the "Da Vinci Glow." The light is even captured in historical literature. An 18th-century Scottish poem, "The Ballad of Sir Patrick Spens," mentions the "auld muin in her airm." More recently, the light has inspired modern space explorers. Dr. Sian Proctor, a mission pilot for Inspiration4, wrote a poem and a book about the human perspective of earthlight.

The intensity of earthlight varies greatly depending on where you are. From Earth, earthshine has a maximum apparent magnitude of -3.69. On the Moon, the light is much more powerful, with a maximum apparent magnitude of -17.7. However, this light is still only 0.01% as bright as direct sunlight. For an astronaut on the Moon, the light is incredibly bright. During the Apollo 11 mission in 1969, Michael Collins reported that the light was bright enough to read a book. He observed this while approaching lunar orbit at a distance of 19,000 kilometres. Earthshine during a lunar night can be 43 to 55 times brighter than a full moon on Earth.

Astronauts have experienced the extremes of lunar light and darkness. During the Apollo 8 mission on December 24, 1968, the crew flew into the shadow behind the Moon. Bill Anders described the unlit side as a "big black void" that was as black as he had ever seen. This darkness was so profound that the Moon was invisible to the crew. They could only see a black silhouette against a tapestry of stars. This experience was different from the bright earthlight seen in orbit. The far side of the Moon and lunar eclipses are the only times the night becomes that dark.

Earthlight serves several important scientific and operational purposes. It allows for the remote studying of both the Moon and the Earth. Scientists can study Earth's environment by measuring the amount of earthlight it produces. This light also provides illumination to certain areas on the lunar near side that the Sun never reaches. Beyond visible light, the Moon also reflects radio frequency transmissions. This is used in Earth–Moon–Earth communication systems. Earthlight remains a vital connection between our planet and our celestial neighbor.