ASTRONOMY 101: THE MOON

Our Moon is far more important to all life on Earth than most people realise.  Simply illuminating the night sky or being responsible for ocean tides is of relatively little significance when one realises its true effect on the Earth and us.  It has been intrinsic to the very evolution of life itself, and by definition the appearance of homo sapiens!  And it's not hard to see why it is so important. 

The Moon with labelled 'seas' and craters.

To start with it is the only Moon of a major planet in the solar system that is so large relative to its parent planet, indeed many astronomers consider the Earth/Moon system to be a double or binary planetary system.  At 385,000 kilometres distant it is the closest celestial body to the Earth.

The Moon was created from the Earth itself, confirmed by the geological experiments undertaken during the NASA Apollo program.  Unlike the Earth, there is very little iron on its surface and it consists of terrestrial mantle material.

About 4.6 billion years ago, an object the size of Mars struck the Earth. This early planet, which has been named Theia, was partially absorbed into the Earth, but a large amount of debris was also sprayed out into space. Gravity pulled the debris into orbit around our planet and, as the numerous fragments collided, they began to clump together. The Moon was formed as these clumps grew larger and larger.

The Moon was created by a Mars-sized impactor striking the young Earth about four billion years ago.  The Earth was nearly completely destroyed by the collision, and the ejecta from the Earth's mantle and the material from the impactor itself eventually came together to form the Moon.

At that point the Moon orbited the young Earth at only a fraction of its current distance, and would have looked enormous in the sky.  The Earth's day was about eight hours in duration, but over four billion years the friction of the lunar gravity has slowed this down to twenty four hours.  Eventually, the velocity of the Moon orbiting the Earth matched that of its rotational speed so that now the Moon is tidally locked to the Earth: the same side of the Moon is visible to the Earth at all times and until the unmanned Soviet and NASA probes of the early 1960s, no human being had ever seen the far side.  Notice it is called the 'far side' and not the 'dark side', a serious misnomer as it receives the same attention from the Sun as the Earth-facing side.

Most importantly, the Moon has prevented the Earth wobbling uncontrollably on its axis and hence over billions of years has provided the climatic stability essential for the evolution of complex life forms.

Apollo 11 Mission Highlights. It's July 16, 1969 and with their Saturn V booster on the launchpad at Cape Kennedy, Neil Armstrong and Buzz Aldrin are about to embark on the most momentous journey in the history of mankind. Ultimately they will successfully land their lunar module Eagle on the Moon and be the first humans to set foot on another world.

Precision experiments left on the lunar surface by the Apollo astronauts confirm that the Moon is receding from the Earth by a couple of centimetres per year.

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