Geoscience Reference
In-Depth Information
Family Members
Isaac Newton showed that the Earth and the Moon are bound by an invisible yet
irresistible force from which neither will ever escape: gravity. One might say that
the relationship resembles that of the members of a family, which include spouses,
siblings, and sons and daughters, whose bonds not even death can part. No wonder
then that scientists came to nickname the three prominent explanations of the
Moon's origin the daughter, sister, and spouse theories. One of them we met in
part
2
.
Although we see tides ebbing and flowing in the oceans, Newtonian mechanics
requires that the solid Earth and Moon also pull tides in each other. The effect
slows the Earth's rotation while causing the Moon to recede. As we learned, Ge-
orge Darwin worked backward to calculate that 56 million years ago the Earth and
the Moon would have been so close together as to be one. According to the preval-
ent nebular hypothesis of his day, this must have been the time when the molten,
rotating Earth spun off the infant Moon, her lovely “daughter.”
The nebular hypothesis eventually fell out of favor, replaced by the planetesimal
hypothesis of Thomas C. Chamberlin and Forest Ray Moulton. In their model,
gravity from astar passing close tothe primordial Sunhad pulled outlong,curving
stringers of solar material, like the spiral arms that astronomers had begun to pho-
tograph in distant “nebulae.” The nucleus of a spiral was thought to be a sun in
the process of formation, while the spiral arms themselves would coagulate into
planets and the much smaller planetesimals. Further accretion would lead to the
present population of planets and moons, while leaving over unaccreted debris that
became the comets and asteroids.
The nebular analogy came a cropper when in the mid-1920s Edwin Hubble
showed that the nebulae are not primitive solar systems but galaxies like our
Milky Way galaxy, only much larger and farther away. Cosmologists dropped the
Chamberlin-Moulton hypothesis, though they did retain the idea that the Earth and
the Moon might have been born together from a primordial cloud of cold particles.
In that case, the two are “sisters.”
The third of the three theories came from one of the most redoubtable characters
in American science: Thomas Jefferson Jackson See, or T. J. J. See (1868-1962).
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