moon is gradually moving away from the earth (because the time taken by moon to make one revolution around the earth goes on increasing), logically it can be concluded that this is the continuation of an outward momentum initiated by the fission. Hence it is concluded that once moon might have been “closer to the earth than it is now” (Schrunk, 1).
The reason for the split is also well-explained in fission theory. It is speculated that some kind of gravitational imbalance within the earth caused by its previous shape might have made it spit out a piece which became moon. The fission theory is supported by the fact that the core matter of moon is “not as massive as the earth’s” and also that same nonradioactive stable oxygen isotopes are found on the “terrestrial rocks” of both earth and moon (Gergo, 4,5). Thus this theory speculates that it was from the less dense mantle of the earth that moon was broken away as a separate entity.
The weakness of fission theory is that there is a scientifically proven minimum distance near to the earth below which moon cannot exist in solid form and can exist only as “a ring of debris” (Schrunk, 1). Hence moon could never be close to earth within this minimum distance and continue to exist as a solid cosmic body. After proving this in 1873, Edouard Roche contested fission hypothesis and put forth the “co-accretion” theory (Schrunk, 1). Co-accretion theory said that earth and moon most probably might have been formed “at the same time, in the same neighborhood of the solar system” (Schrunk, 2). This is an assumption based on the similarities in structure between earth and moon like the presence of oxygen isotopes.
A third theory on the origin of moon was proposed by Thomas.J.J.See (qtd in Schrunk) who theorized that “moon was … a captured satellite” (Schrunk, 2). This theory was supported by one fact which came to light in that period. It was proven that at least some of the satellites of Saturn