The Earth Is Getting dimmer

If you have viewed a crescent moon on a clear night, you would have observed an eerie glow around the moon. This is called earthshine. Do you know why the moon glows?

Our earth and the moon act as giant mirrors that reflect sunlight. Actually most objects reflect light. A mirror reflects almost all the light that falls on it. And an object that does not reflect light is called opaque.

The sun’s rays contain heat and light, which keep the earth warm. However, our earth reflects as much as 30 percent of the heat and light that falls on it. This is how our earth manages to keep its temperature under control.

A lot of the reflected light reaches the moon, which, in turn, reflects it back to earth. The light that is reflected back from the moon makes it glow.

Watching Earth’s brightness

Scientists use earthshine as a kind of mirror to keep an eye on our planet. By measuring how brightly the Moon glows with reflected earthshine, they can tell how much sunlight the Earth is bouncing back into space.

That amount goes up and down as the Earth changes — mainly with its clouds, and with the haze of dust and pollution in the air. More cloud and haze, and the Earth reflects more light; less of them, and it reflects less. Watching these changes helps scientists understand the Earth’s climate and how it is shifting.

Steven Koonin, a physicist at the California Institute of Technology, points out that earthshine could reveal how hot our planet is becoming.

“It’s really amazing, if you think about it, that you can look at this ghostly reflection on the Moon and measure what Earth’s climate is doing”, says Koonin in an article in the National Geographic web site – www.nationalgeographic.com.

Editor’s note (July 2026): The middle of this article once explained Earth’s changing brightness incorrectly, mixing up the greenhouse effect with how much sunlight the Earth reflects. We have rewritten that part.