Moon Facts

There is something providential in the purely accidental phenomenon of Earth’s possessing a satellite so large and situated so close, as the Moon. It must be accidental. Consider that Venus, the planet closest to Earth and almost identical in size, has no satellite at all. Consider, too, that all other satellites in the Solar system, excluding Earth’s, are but small fractions in size compared to their primaries. If you were to use the average proportion of these satellites relative to their primaries, and create a satellite for the Earth relative to its mass, you would get a satellite that would no more than 200 kilometers across, at most, if it existed at all.

But we do have a Moon. It is in fact 3,475 kilometers in diameter, a little bigger than one-fourth the size of Earth. In relation to its primary, you would consider the Moon to be the largest satellite in the whole Solar system. Because the Moon is so big and so close, she has been so intimately interwoven into the fabric of Man’s way of thinking.For instance, you can reasonably argue that without the visible face of the Moon in the sky, mankind might never have had the impulse to study the sky in detail at all. Man would have gazed upwards in wonder, of course, but he might merely have admired it. Other than the occasional comet, only the Sun and the Moon appear larger than a point of light in our sky. Comets appear only occasionally, however, and their effect on early men was limited to nothing more than to fill their hearts with superstitious dread; you might still retain some of those subliminal fears.

You cannot look at the Sun for more than a moment – it is much too bright. Well, you can look longer when it is obscured by mist, but even then it appears merely as a disc of light, without any features that you can distinguish.The Moon is vastly different. She is much softer in light and you can gaze at her for as long as you want. And men did, from the earliest times. Their study paid off at once. They could see the Moon changing in shape every night, entering through a regular cycle of phases that is so visible to the naked eye.

Nowadays, you can simulate their study by using Google Earth, Part Of The Free Google Pack Download as well as similar software. You can look very closely at the phases of the Moon. You will realize that because the Moon moves about the Earth in a slightly elliptical orbit, she always looks the same size and the regularity of its cycle of phases is not complicated by other factors in celestial mechanics. This factor actually made the phase-change a highly informative field of study in the days when astronomy was in its unsophisticated, formative periods.

If you observe the Moon visually, or track her phases in Google Earth’s feature, you’ll notice that the cycle of phase changes is completed in a little over 29 days, which is a particularly convenient length of time. For the prehistoric farmer and hunter, the cycle of seasons (the year) was particularly important but it was difficult to note that the seasons repeated themselves over 365 and a fraction days. The number was too big to track easily. But to count 29 or 30 days from one new Moon to the next, and then to count twelve or thirteen new Moons to each year, was much simpler and much more practical. And once the regular change of phase of the Moon was imprinted on mankind’s psyche, the marking of a calendar for keeping track of the seasons of the year in terms of the phases of the Moon was the next step.

Then civilization was born. Moon tracking was very interesting for the ancients. You can relive their fascination with the phases of the Moon with the use of Google Earth. 

The Moon is extremely old. The planet Earth’s only natural satellite has existed as an independent body for over 4.6 billion years – as old as the Earth itself.One of the most enduring mysteries in science is the question of how the Earth happened to have such a huge satellite. In comparison to its primary, the Moon is a little over one-fourth of the size of the Earth, making it the largest satellite in the Solar system.

Well, almost. In 1978, the Moon lost that distinction when scientists discovered Charon, satellite of Pluto, which measured about one-half of Pluto’s diameter; in 2006, the Moon regained the distinction, since Pluto itself has been reclassified as not a true planet. Some satellites of Saturn and Jupiter are physically larger than the Moon, but they themselves are dwarves compared to their giant primaries.

You may have wondered about the origins of the Moon. There currently are four major scientific theories about that:

Fission: In brief, this theory states that the Moon is a chunk of material spun off from a rapidly spinning Earth around 4.6 billions years ago. The theory first developed from the fact that the Moon is moving away from the Earth, currently at a rate of 4cm per year. The Moon’s gravitational pull on Earth, which you can see in the regular cycle of the tides, is gradually slowing down the Earth’s spin, and this in turn results in the Moon gradually spiraling out of Earth’s influence. Today, the Moon orbits the Earth from 381,000km away. A few billion years ago, she orbited just tens of thousands of kilometers away. Before that, the theory holds the Earth and Moon were physically joined. Because of the planetary physics involved, astronomers now consider this theory highly implausible.

Co-Accretion: According to this theory, the Moon condensed from the cloud of gas, dust and debris that orbited the Earth. It has been suggested that the Earth accreted first out of denser material, and the Moon used up the leftover lighter materials. You might find support there because they have identical oxygen isotopes in their rocks, but then you would also ask: how could one of the driest worlds in the Solar system have formed alongside one of the most watery worlds?

Capture: The tiny moons of Mars and the small satellites orbiting the four giants are really captured asteroids. Would you think that the Moon once have been an independent body captured by the Earth’s gravity? In this scenario, the Moon came within 64,000km of Earth; alternatively, the Moon originally orbited the Sun near Mercury, but upon a close encounter with Mercury, the Moon moved into Earth’s gravitational embrace. There are attractive features about the theory but it also fails to explain the identical oxygen isotopes in Earth and Moon rocks, which imply the two bodies were formed in the same vicinity of the early Solar system.

Collision: This “Big Whack” hypothesis throws a Mars-size planet into ancient Earth, causing a violent collision and throwing a vast cloud of debris into space. Much of this material eventually coalesced to form the Moon. The Big Whack theory is currently the most favored theory of the Moon’s origin because it solves more of the problems and issues than any other theory.Collisions and cosmic impacts are gaining recognition as a major force in the evolution of the Solar System. Gone are the times when our own little corner of the Universe was viewed as a place of eternal tranquility, save for the occasional visiting comet. Mercury, Venus and Uranus all bear signs of past collisions. If you look at the surface of the Moon, through your telescope or the Google Earth software, you will see the evidence of countless impact scars. So, the idea that a much smaller planetary collision gave birth to the Moon has now gained credence among planetary scientists.