his book is a follow up to the book Comins wrote back in 1995. His earlier book
was called What if the Moon Didn't Exist? In that book, which I also read and enjoyed, he described ten
different scenarios for an alternate Earth, starting with a description of our planet but without its
signature Moon. In this new volume, Comins follows the same format - he presents ten scenarios for alternate
earths, and then describes possible effects on the inhabitants of the alternate Earth.
The first chapter is titled: What if the Earth Had Two Moons? Dimaan, Lluna, and Kuu. Comins
likes to give his alternate Earths each a unique name. In this chapter, Comin has renamed the Earth as Dimaan, and Lluna and
Kuu are the two moons that orbit it. Comins goes into great detail to describe how it might be possible for
a planet like ours to have two large moons - it takes a bit of rigging, but eventually everything lines up in
a stable system (its not permanently stable, but the two orbiting moons would not collide until a distant future).
What would be different for the inhabitants of Dimaan? The tides would certainly be different, as would eclipses
and the night skies (the amount of moonlight on Dimaan would be 4x greater than what we Earthlings experience.)
The second chapter is titled: What if the Earth were a Moon? Mynoa and Tyran. This was one of
my favorite chapters. It describes an Earth (named Mynoa) just like our planet, except it orbits around a Neptune size planet - the
alternate Earth is itself a satellite of a huge gas giant. This has some dramatic effects on the planet. The major
effect is that Mynoa becomes synchronously locked to the rotation of Tyran - Mynoa will always present
the same face toward the large planet. Inhabitants on one side of the Mynoa will see the giant planet Tyran overhead all the time
(I liked Comins description of the amount of light falling upon the inhabitants, as Mynoa goes through
frequent eclipses as it circles Tyran), while inhabitants of the far side would see gloriously dark night skies and would
be unaware of Tyran's existence until they journeyed half way around the planet.
Chapter three is titled: What if the Moon orbited backwards? Anillo. The moon of Anillo is called
Noom (Moon spelled backwards, of course!) - it orbits Anillo in the direction opposite from the direction of its rotation. Unfortunately, this has bad
results. On our Earth, gravitational attraction between the tides and our moon have the net effect of slowing the rotation of our planet, but also transferring
the Moon in slightly greater orbit (our moon drifts further away from us by one and a half inches every year). But on
Anillo, Noom will come closer and closer each year until a final catastrophic breakup when Anillo's gravity
rips it apart. If one falling rock could wipe out the dinosaurs, imagine the prospects for any lifeforms on Anillo
when an entire Noom disintegrates overhead.
Chapter four is titled: What if Earth's Crust Were Thicker? Dichron. This is another fascinating
chapter, describing how different the planet would be if the crust were much thicker. This would interrupt the
plate tectonics. Unlike Earth, where the continents move about, on Dichron everything will be fixed in place.
But because the continents do not move, the heat of Dichron's molten core is trapped in place. On Earth, there are 15 to 20 active volcanoes a year, which
is how our planet radiates heat from the core. On Dichron, this heat will be trapped by the thicker crust, so it will
keep building up until a "hot spot" develops and an entire section of the crust will liquify and melt through! There
wouldn't be any permanent landmasses, after millions of years, every part of the crust would be melted and reformed.
This would have some hazardous effects on any beings living on Dichron!
The remaining chapters are (I don't have space to give a synopsis of each chapter, but I read them all):
Chapter 5: What if the Earth formed Fifteen Billion Years from Now?
Chapter 6: What if There Were a Counter Earth, a Planet in Earth's Orbit on the Other Side of the Sun?
Chapter 7: What if the Earth had Formed Elsewhere in the Galaxy?
Chapter 8: What if the Sun Were Less Massive?
Chapter 9: What if the Earth Had Two Suns?
Chapter 10: What if Another Galaxy Collided With The Milky Way? This last chapter is a bit of
a cheat, because the Andromeda Galaxy is already on a collision course with the Milky Way - our distant descendants
will live through this collision.
I like this book because it is fun to think of alternate lifeforms and societies that might arise
under these alternate Earth environments. Maybe a SF author will be inspired by these ideas and construct an elaborate
alternate history. Or perhaps a director will create a film for a planet like Mynoa, orbiting around giant Tyran - that
would create some memorable visuals!