We take the moon for granted, but Earth would be a very different place had our nearest neighbor only achieved half of its present mass when it formed some 4.5 billion years ago in a titanic collision. In fact, we might not even be here to appreciate it at all.
Let's start with eclipses. In one of those bizarre cosmic coincidences, our moon today is positioned at just the right distance between Earth and the sun for its diameter to completely block out the sun during a total solar eclipse—the next of which will occur on Friday, August 1. [see ScientificAmerican.com's special report on the eclipse].
But what would happen if the moon had only grew to half its present mass? Assuming our half-size moon was composed of rock as dense as that of the actual moon, it would still be 80 percent as large across as the full-size version (based on the relationship between a sphere's volume and radius that you learned in grade school).
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