Earth's natural satellite.


Natural satellite is an astronomical object, that orbits a planet. The more familiar expression for a natural satellite is moon. Earth's moon is named the Moon with capital M. It is common among planets in our Solar System to have moons. In fact, only Mercury and Venus have no moons. The rest of the planets except for the Earth, have more than one.


Moon´s average distance to the Earth is 384.400 km or about one light second. If we knew how to travel with the speed of light, it would take us one second to get there. It took Apollo 11 Mission 102 hours and 45 minutes from Earth launch to the Moon landing.


The Moon was according to the widely accepted giant-impact hypothesis formed 4.5 billion years ago when Earth collided with Mars-sized planet. During that time, heavy elements (such as iron and nickel) have already sunk to the Earth’s core. The collision was only powerful enough to eject lighter elements from the Earth's crust and mantle out in space, from which later the Moon has formed. This explains the lower density of the Moon and the absence of iron in the rocks.


The Moon does not have an atmosphere or magnetic field. Therefore, it is directly exposed to the meteorite bombardment, solar wind, and cosmic radiation. Consequently, liquid water cannot exist on the surface. In 2009 we found water ice in permanently shaded craters near the lunar south pole. This was an important discovery since it facilitates establishing a permanent manned base on the surface of the Moon.


The Moon is tidally locked to the Earth. This happens because the Moon rotates around its axis in exactly the same time (approximately 27 days) as it takes to orbit the Earth.This means that it is always showing the same face towards Earth. Before 1959, when the Soviet spacecraft Luna 3 passed the far side, no one knew how it looked like. And interestingly, it is quite different. It lacks lunar seas, called maria. Until present time scientists still disagree on the most probable cause of this phenomena. One of the hypotheses proposes that due to the tidal locking, the Moon`s crust on the earthward side is thinner. Meaning, when the Moon was bombarded by large asteroids, the lava on the earthward side protruded from the mantle more easily, filling large basins, today known as lunar seas or maria.


Another effect of tidal locking is that Moon is drifting away from the Earth with the average rate of 3.8 cm per year. The Moon's gravitational force is producing ocean tides. At the same time, it is also stretching Earth's crust for a few centimeters. The effect is slowing Earth's rotation, increasing the length of the day on Earth for 17 microseconds each year.


The Earth and the Moon orbit each other and are therefore regularly involved in eclipses. A lunar eclipse occurs when the Moon passes directly behind the Earth in its shadow, and solar eclipse occurs when the Moon gets between the Earth and the Sun. If the orbiting planes of the Earth and the Moon were parallel, we would, from the Earth´s perspective, observe total solar eclipse every new moon and a lunar eclipse every full moon, which means every month. But since the lunar orbit is inclined 5°, the eclipses occur only on rare occasions when the Moon is crossing Earth's ecliptic at exactly the right time.


The Moon has played a special role in human history. Races to the Moon demonstrated power and the first step on the Moon was long seen as one of the greatest achievements in human history. Today, we see the Moon as the springboard on our way further into deep space. Space exploration shifted from competing to collaborating. Moving to outer space allows us not only to obtain new data about the physical Universe around us, but also gives us the opportunity to explore ourselves as human beings.

Writing: Ana Skerjanc and Bojan Ambrozic

©2021 by Stellar Milieux