The moon fascinates us. There it is, hanging in the sky, the same side always facing us. Yet what we see changes daily depending on the relative positions of the sun, Earth and moon. Its cycle of changing from a mostly invisible new moon to a completely illuminated and bright full moon and then back to a new moon takes about 28 days. On March 8 it will be in its first quarter and it will be full on March 16. These lunar cycles are the basis for the Hebrew, or Jewish calendar, which is still used to determine the dates for many religious holidays even though most people now use the Gregorian calendar.
My fascination with the moon started when I was four years old. My mother told me that I would wake her up in the middle of the night to talk with her, and one of the things I frequently said was something like “There is the moon, isn’t it pretty?” And I would continue to learn about the moon and the planets for a number of years until I was 10 and birds stole my attention.
I probably assumed that the moon had always been there. I am sure it never crossed my mind to ask how it came to be there, but I just learned its story as I was studying to teach Island geology for ACE MV (beginning on March 13).
A supernova probably created our solar system about 4.567 million years ago, when a star exploded and blasted most of its mass into outer space to create a mostly flat molten disc. Gradually gravity coalesced these hot materials into eight planets (alas, poor Pluto is no longer considered a planet). The molten Earth did not have a moon.
Fifty million years later, the gravity from a still molten proto-Earth had coalesced most of the supernova’s debris that was near its orbit around the sun. There were still smaller protoplanets that collided with proto-Earth; most of them were swallowed whole and became part of our planet.
Computer modeling suggests that a protoplanet about one third the Earth’s size collided into the molten Earth slightly off-center, deforming proto-Earth so it was no longer round. The rapid rotation of this irregular shape caused a chunk of Earth’s molten surface and part of the protoplanet to seperate from Earth, but not far enough away to escape from its gravity. Some of this molten material returned to proto-Earth, but the rest of it coalesced into the moon. According to computers and the laws of physics, a moon created in this manner would always have one side that faced the Earth. And such a collision may explain the Earth’s 23 degrees tilt away from the vertical, a reason for why we experience the seasons as we do.
Analysis of the 840 pounds of moon rocks brought back to Earth by the Apollo astronauts supports this theory; the moon’s chemical composition is very similar the composition of the Earth’s surface. If they were composed of very different materials the theory would not have been supported and scientists would have had to devise another theory about the moon’s creation. Scientists also believe that the moon was only 15,000 miles from the Earth’s surface when it was “born.” Now it is about 239,000 miles away, and that distance increases by about 1.5 inches per year.
After the moon was created, the Earth’s day might have been only five hours long and there were 1,750 days a year. Vastly different from today’s 24 hours per day and 365 days in a year. The best analogy, often cited by physicists, is that of a spinning figure skater who spins more rapidly when her arms are tucked close to her sides and slower when she spreads and extends her arms out. Other evidence changing day length are from 900 million-year-old sediments in Utah showing 18.9 hours in a day (464 days in a year), and from coral growth patterns from about 400 million years ago showing a day length of 21.9 hours or 400 days in a year.
The proximity of the moon would have also affected the tides as well, perhaps creating tides of magma bulging outward about a mile toward the moon. Assuming that even then there were two tidal cycles in a five hour day, imagine how fast this molten bulge would have raced around the earth! Anything that tried to cool down and form rocks would have melted and been swept away.
Of course there is no direct evidence for this theory, called the Big Thwack or Big Splash, as no one was there to see it. Since water, oxygen and our atmosphere did not exist, humans could not have survived. Too bad, as it would have been fascinating to see all this happen.
Think about this when you next see the moon.
Robert Culbert leads guided birding tours and is an ecological consultant living in Vineyard Haven