|MadSci Network: Astronomy|
If the Earth lacked its large Moon, the most obvious change would be that the tides would be nowhere near as marked. One reasonably obvious effect of this would be a more rapid rotation of the Earth. The Moon's gravity distorts the Earth (not only the oceans but also the solid part of the planet), raising bulges on our planet. The Earth spins on its axis, and the rotation of the Earth carries the bulges raised by the Moon away from the line joining Earth-Moon centers. The result is that Earth is being continuously flexed. Distorting rock heats it up, and that heat energy has to have a source; the energy source that is most easily tapped in this process is the rotation energy of the Earth. Over time, then, the tides cause the rotation of the Earth to slow down, in a process called tidal braking. This process has already run to completion on the Moon: it now rotates once per orbit (that is, once a month), so that the bulge raised on the Moon by the Earth's gravity doesn't move.
If the Earth had no big Moon, most of that tidal braking wouldn't have occurred. The Sun raises some tides on the Earth, but to a much smaller extent.
There would also have been profound effects on ecology: without the Moon raising tides, the tidal zones, the interface between the marine environment and the land environment, would be much more restricted. I have read some speculations that without the large tidal zones our Moon produces life would not have developed at all, but I think that at this point we don't know enough about the origins of life to take this as more than interesting speculation.
Another possibile difference comes not from the Moon itself but what we now think is the process that caused the formation of the Moon. The Giant Impact Hypothesis ) posits that a Mars-sized object collided off-center with the early Earth, with the Moon forming from some of the "splash" material from that impact. This impact, besides forming the Moon, leaves behind a somewhat larger, denser, more metal-rich Earth (though almost all of the extra iron ends up in the core).
There are also effects on the global circulation of the atmosphere (weather and climate) due to the Moon's tides, but disentangling those from the effects of the day/night cycle isn't easy.
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