Q: What would the Earth be like...

Q: What would the Earth be like if it had no moon?

A: It would have much smaller tides, but that is probably the least important of the moon’s effects, said astronomer John E. Mosley of the Griffith Observatory.

The acquisition of the moon appears to be a random and catastrophic accident in which a small planet crashed into the Earth and was captured by its gravitational field. The aftereffects of that collision may well be what has made Earth habitable by humans, Mosley said. In the first place, the pre-moon Earth had a very thick atmosphere much like that of Venus. Much of that atmosphere was stripped away in the collision, giving us the thinner air we have today. Second, the collision helped give the Earth’s axis its present tilt and stabilized it in that configuration. Without that stable tilt, the Earth’s axis would oscillate wildly, like that of Mars, giving “really hot summers and really cold winters.” The Earth’s spin on its axis would also be much slower, producing days much longer than 24 hours.

The planetarium at Griffith Observatory has a new show called “No Moon” that explores some of these possibilities. The show runs through fall.