This was an interesting little collection of what-if scenarios, exploring several Earths that never were along with a few Earths that I hope won’t be.
The first chapter explores that very question, i.e. what if the moon didn’t exist? It then goes on to explain the moon’s origins and how the earth would be different without it. This ran from the lower tides to the shorter day and other surprising tidbits.
Later chapters explore Earth with a closer moon, Earth with a 90-degree axis tilt, and an Earth with a larger sun. We also get some lovely disaster scenarios of nearby supernovas, another star passing through our solar system, and my personal favorite, the effect on Earth of the impact of a black hole, both big and small.
I confess there were a few bits that dragged for me, mostly on evolutionary impacts, but the astronomical and geological explanations were great. He’s apparently written a follow-on book to explore some other scenarios, titled What if the Earth Had Two Moons? I’ve already sent the sample to my Kindle, so we’ll see if it grabs me.