By now everyone interested in astronomy should know that it was 400 years ago that Galileo looked at the Moon with a telescope and changed the way we look at every object in the sky forever. So it is sometimes hard for me to understand why there isn't more interest in observing the Moon among amateur astronomers. However, it astonished me that until the middle of the nineteenth century that astronomers were more interested in what the Moon was made of instead of how Earth got such a big Moon.
Our Moon is the fifth largest in the solar system. Ganymede, belonging to Jupiter is the biggest. Next Titan, belonging to Saturn, then Callisto and Io belonging to Jupiter. Then the Moon, with a capitol M. What the first four have in common is, as big as they are, they are all less than 1/1000 the size of their parent, but our Earth's Moon is almost 1/4 the size of Earth and only 2% smaller than Io. Yet, no one seemed interested in how Earth wound up with a Jupiter size Moon. In 1755, Immanuel Kant wrote a theory that the solar system evolved from a cloud of gas that began to rotate, and the Sun and planets, including the Moon were made as the solar system developed. This theory came about because George Louis Le Clerc had noted in 1749 that all of the six planets, all that were known at that time, and their moons all moved in the same direction.
George Howard Darwin, wasn't trying to create a theory on the origin of the Moon. He was working on his theory of tidal friction and like his father, Charles, came up with a controversial theory. The theory of tidal friction? The Earth is a sphere with two bulges created by the oceans tides. One is closer to and a little ahead of the Moon, the second farther from and behind the Moon. The Moon's gravity tends to pull the nearer bulge backward, because it is behind that bulge. The more distant bulge is pulled forward. The gravitational force on the nearer bulge is stronger. The overall pull is backward against the the direction of the Earth's rotation. The Moon is gradually slowing the Earth's rotation, this makes the day, in time, get longer. This as George Darwin discovered, would also make the Moon gradually move outward away from Earth. As soon as he figured that out the obvious facts are there. Back in time the Moon was much closer to the Earth, the day was much shorter, and the Moon orbited the Earth much faster. Don't forget, in 1878 all the calculations were being done on paper so Darwin's theory is quite an achievement, even if his final theory was flawed. He managed to "run the movie backward", we might say, to a time when the Moon would have circled the Earth every 5 hours 36 minutes and was at a distance he estimated to be only 5,000 miles away, and a day on Earth to have been between 3 and 5 hours long.
Darwin came to the conclusion that the Earth had been pear shaped, because of its speed of rotation it had thrown off the bump which became the Moon. Although Darwin never endorsed their idea, two scientist proclaimed the scar from that event was the Pacific Basin.
This theory would stand unrefuted until 1969.
The next theory was the Moon was an object orbiting in the Solar System, it fell into Earth's gravity and became our Moon. This would require the Moon to have approached Earth, most likely from the rear at a greater speed to over take Earth, ( which is orbiting the Sun at an average of 66,000 miles an hour) hit the brakes and make a ninety degree turn in front of Earth and then go into orbit.
The next theory was: there was some stuff left over from the formation of the Earth and it became the Moon. We now know after a certain size the biggest body gets all the stuff period.
The answers would have to wait until men landed on the Moon
Next month, dating the features and craters of the Moon, and a new theory of the Moon's origin.
Sept 04 Full Moon.
06 Mercury, stationary at right ascension, begins retrograde motion.
09 Perseid meteor shower.
10 The Moon is in the Plieades.
12 The Moon at last quarter.
16 The Moon 3 deg from Venus in the morning sky.
18 New Moon.
20 Mercury at inferior conjunction with the Sun.
22 Autumn equinox
26 The Moon is at first Quarter.