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?

The eerie glow around a crescent moon is called earthshine.
The eerie glow around a crescent moon is called earthshine.

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.

Global warming and earthshine

Scientists are now using this reflected light or earthshine as a thermometer to monitor earth’s health.

New observations conducted at southern California’s Big Bear Solar Observatory suggest that the earth has dimmed by about 2.5 percent over the past five years. Which means it is not as bright as it was earlier. This decrease in the earth’s brightness, they point out, is connected to the increase in the global temperature.

In the last century, humans have burned large amounts of fossil fuels like petrol and diesel for their energy needs. These fossil fuels are mostly made up of an element called carbon. The burning of the fuels has released a lot of carbon into the earth’s atmosphere.

Carbon has a tendency to absorb heat and light. The extra amount of carbon in the earth’s atmosphere has increased the amount of heat and light it absorbs. As a result, the earth is getting warmer and is not reflecting as much light as it used to. Therefore, it is getting dimmer. Consequently, the earthshine is losing its brightness.

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.