Silent Night (Science Advent 23)

(Every day until Christmas, I’ll be posting a science-related image.)

Day 23: Christmas Eve

The Black Marble: three views of Earth at night. Left: the Americas; center: Europe, Africa, the Middle East; right: Asia and Australia. [Credit: NASA Earth Observatory]

The Black Marble: three views of Earth at night. Left: the Americas; center: Europe, Africa, the Middle East; right: Asia and Australia. [Credit: NASA Earth Observatory]

Planets produce very little light of their own: we see them primarily through reflected sunlight, and the small amount of emitted infrared light from their internal heat sources. However, our planet emits a little visible light, and we are the cause. It’s not a lot of light, taken globally: the planet is still dark at night. Nevertheless, it’s a change, since without us Earth and Mars would be equally black on the side unlit by the Sun.

Life has shaped our planet. In the early years of Earth, the atmosphere was very different: far more carbon dioxide and methane, no appreciable oxygen. Life made the atmosphere the planet has today: cyanobacteria (also known as blue-green algae) gradually altered the chemical balance over billions of years. Today, a single species possesses the greatest power to change the planet: the human species. We have an unfortunate tendency in many cultures to think of ourselves as separate from nature and the planet, as though we’re something introduced from the outside rather than part of the world. However, that’s not true, and the photos above show in a small way how we’ve influenced our planet.

When I look at the images above, I’m struck by several contradictory emotions. As a lover of astronomy, I wish we didn’t shine so many lights upward, where they do us no good, waste energy, and make it much harder to see the night sky. As a lover of beauty, I appreciate the patterns of civilization on an aesthetic level. As an imaginative person, I wonder if an alien astronomer (if they are not terribly far away) might be able to identify our planet as inhabited by the lights on the night side of Earth, and if we could learn to do the same for exoplanets. And as a lover of humanity, I wonder as always if we can achieve peace among ourselves, and work together to resolve the pressing problems of climate change that threaten to destroy so much. Looking at our world from above like this, I believe the effort is worthwhile.

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