In search of Theia

These emails normally arrive on Tuesdays and Fridays, but we’re changing things up this week because I wanted to reach you in time for tonight’s full moon. This story was published in Emergence Magazine, and it completely changed the way that I think about the Moon, and the impact it still has on the world we live in. So wherever you are, I hope you’ll have chance to catch a glimpse of tonight’s Snow Moon. And as pictures emerge from NASA’s Artemis mission in the coming weeks, I hope you’ll remember Theia, the hypothesized planet that started it all…

I take a turn through the low Appalachian ridges of central Pennsylvania and the Moon drifts back to the left side of the car. It is low on the southern horizon and exactly high enough for my three-year-old daughter to see from her rear-facing car seat. Most things are not. I wait with anticipation for it to enter her view, a possible highlight on a long monotonous drive. A few seconds later she shouts, “I can see the Moon!” It’s hard to miss, nearly full and with few clouds to hide it. It shines alone above black hills, the sole source of light in a stretch of highway distant from any city. And a little later, my daughter asks, more quietly, “Is the Moon following us to Pittsburgh?” And so it appears to be. The Moon as Earth’s acolyte, the Moon as timeless companion. “Yes, it is following us,” I say, knowing the feeling.

But as a scientist, my second thought is, well, no. That’s not how the Moon works – the distance to the Moon being so vast, and our six hours of traveling, though an eternity to a three-year-old, being in fact quite short – the Moon’s companionship along the dark highways is a simple trick of perspective. Among those tricks, the sense of following chiefly arises from the phenomenon of motion parallax, where objects closer to us appear to move faster than objects farther away. This gives the Moon an oddly serene sense of floating along. It appears effortless for the Moon to keep up with our travel. Yet despite this appearance, I think, the Moon doesn’t follow us any more than a distant mountain or tower.