Alan Stahler: Dark side of the moon, not
November 30, 2016
The moon has both a far side, and a dark side.
Just like Earth, the moon rotates under the sun in rotisserie-mode, so, like Earth, the moon has a dark side — AKA night — and a day side.
But the moon only shows one face to the Earth: The near side. We never see the far side of the moon.
Watching the moon from night to night, watching it wax from new to crescent to full, we see the dark side become day; then, as full moon wanes back to new, day gives way to night.
If you want to keep something cool, keep it out of the sun — put a roof over it, give it some shade.
Even better, paint the roof white; white reflects all the colors of the rainbow (black absorbs them all).
To describe how well an object reflects light (and keeps itself cool), planetologists refer to the object’s albedo — literally, its “whiteness.”
Snow and ice increase Earth’s albedo; so does light desert soil. More than anything else, though, Earth is whitened — and cooled — by clouds.
Viewing the moon tonight, and over the next few evenings, we see how the dark side contrasts with the sunlit crescent. But notice that the dark side is not completely dark.
As Earth turns in rotisserie-mode, daylight migrates from east to west. For some hours after sunset in California, the sun still shines down on Asia.
Shine a flashlight straight into a mirror, and it reflects straight back at you. But shine it on the mirror at an angle, and it bounces off at an angle.
Suppose the sun has set over California. It still shines bright over Asia. If the skies over Asia are clear, that sunlight hits the surface, and most of it is absorbed. But if it’s cloudy over Asia, the clouds reflect nearly all the sunlight back out into space. Some of that reflected sunlight hits the moon, making the dark side of the moon glow — with earthshine.
Clouds contribute more than anything else to Earth’s albedo. If there are more clouds, albedo increases — our planet reflects more light.
If albedo increases, earthshine increases — the dark side of the moon glows brighter.
Working backwards: If earthshine grows brighter, Earth’s albedo is increasing; Earth is getting cloudier; clouds are doing more to cool the Earth.
One of the biggest uncertainties in forecasting future climate is how clouds will evolve — how much will they cool us down?
For a dozen-or-so years, astronomers at Big Bear Solar Observatory, in the San Bernardino Mountains, have been monitoring earthshine, after sunset and before dawn (when sunlight reflects off clouds over Europe). In some years, earthshine has got brighter (high albedo, lots of clouds); in other years, earthshine has got dimmer (low albedo, fewer clouds). Similar earthshine observing programs are setting up in other parts of the world. More observations may uncover trends and correlations, perhaps some clues to the future of our great greenhouse gas experiment.
Friday and Saturday night, the crescent moon will share the western sky with planet Venus. Venus will grow brighter as she draws closer this winter. Covered completely with cloud, the albedo of Venus is twice that of Earth, five or six times that of the moon.
Al Stahler teaches nature classes for students of all ages, kid to adult. His science programs can be heard on KVMR-FM, and he may be reached at firstname.lastname@example.org.