Moon

For other moons, see natural satellite. The Moon is the second brightest object in the sky, after the Sun. It is the only natural satellite in orbit around the Earth, at a distance of 384,400 km. With a diameter of 3476 km and mass of 7.35×10^22 kg, the Moon is larger than any other satellite orbiting the other planets of the Solar System. The Romans called the Moon Luna and the adjective 'lunar' is still used to describe things related to the Moon.

The brightness of the Moon is from sunlight reflected off its surface. As the Moon orbits the Earth, different amounts of illumination of its surface are visible. This makes it appear as though it is changing shape throughout the month. When the Moon is almost directly between the Earth and the sun, little or no illuminated lunar surface is visible. This is known as a New Moon. As it moves round, an arc of illuminated surface becomes visible, known as a Crescent Moon, a 'waxing' (growing) crescent. The width of the crescent gradually increases until, when the Moon is a quarter of the way round its orbit, a full semi-circle is visible, a Half Moon. This gradually gets rounder. This is known as a waxing gibbous. When the Sun and Moon are on opposite sides of the Earth, the Moon appears as a complete disc: this is called the Full Moon. It is at this time that its light reaching Earth is at its greatest. The Moon continues on its orbit, gradually narrowing through a waning gibbous, and again to half, then to a waning crescent before completing the cycle of phases, and becoming a New Moon again.

Humans have often dreamed of traveling to the Moon. During the twentieth century, that dream became a reality. The Soviet Union was the first nation to send a space craft to the Moon. Luna 2 impacted the Moon's surface in 1959. The first photographs of the far side of the moon were taken by Luna 3, another Soviet craft, later that same year. The American Apollo program saw a series of manned missions to the Moon. The first men, Neil Armstrong and 'Buzz' Aldrin, landed on the Moon on July 20, 1969 as part of Apollo 11. Several other manned Apollo missions followed with the last, Apollo 17, reaching the moon on December 11, 1972. In total, 12 men have walked on the Moon.

The Moon's Orbit Around the Earth
The Moon does not orbit around the Earth’s Equator. It is offset, orbiting near the plane of the Ecliptic. The Moon’s orbit around the Earth has two different periods. The Moon’s sidereal period of about 27.3 days is its orbit around the Earth measured by comparison of the Moon’s motion to the fixed stars. The Moon’s synodic period, viewed from Earth as the phases of the Moon, is the Moon’s orbit around the Earth with respect to the Sun. The synodic, a period of approximately 29.5 days, is longer than the sidereal period because the Earth is moving while the Moon orbits around the Earth. In other words, a New Moon does not appear every 27.3 days, it appears every 29.5 days even though the Moon has already completed an orbit around the Earth more than two days earlier. In the period of one Earth sidereal year—Earth’s orbit around the Sun to the same point with respect to the stars which takes 365 days 6 hours 9 minutes 9.54 seconds—the Moon completes approximately 13.4 sidereal periods and 12.4 synodic periods. This is why it is possible to have more than one new moon a month on the modern calendar.

Gravitational effects the Earth and Moon exert on each other
The Moon’s orbit is getting larger, increasing by about 3.8 centimetres per year. The Moon’s rotation has slowed to the point that it presents only one face toward the Earth. The Earth’s rotational period is slowing down by about 2 milliseconds every 100 years. These are all due to the mutual gravitational effects between the Earth and the moon.

The Moon’s gravitational effect raises tides on Earth displacing the Earth’s water outward by meters and its solid surface by a few centimetres. This is called a “tidal bulge”:

In addition to this lunar tidal effect, the Earth’s rotation on its axis has a greater effect on this bulging configuration and the diameter of the Earth at the Equator, equatorial bulging, is about 23 kilometers, (about 0.4% of the Earth's radius) higher than it would be if the Earth did not rotate.

Since the centre of gravity for the Moon’s orbit around Earth, the barycenter, is not the centre of gravity for the Earth, as the Earth bulges outward, this barycenter or shared centre of gravity around which the Moon orbits actually extends further from the Earth’s centre of gravity.

The Earth’s rotation (approximately once every 24 hours) has the effect of exerting a force on the Moon’s orbital period around the Earth (once every 27.3 days). The tidal bulge created by the Earth’s rotation actually precedes the Moon by about 3 degrees, which exerts gravitational forces to pull the Moon forward in its orbit. This increases the Moon’s energy to resist Earth’s gravitational pull and allows the Moon to move away from the Earth and increase its orbit. As the Moon’s orbit increases its orbital period decreases and slows it down.

The Moon is also pulling back on the Earth, creating tidal friction on the Earth’s surface and slowing the Earth’s rotation at the rate of about 2 milliseconds every 100 years. This has a significant accumulative effect however and it is estimated that the terrestrial day about 900 million years ago was approximately 18 hours.

Tidal Locking
So the resultant ‘tidal friction’ of this mutual gravitational effect of the Moon and the Earth takes energy out of the Earth pulling it into the Moon’s orbit, and increases the Moon’s orbital diameter while slowing the moon’s orbit around the Earth. The slowing effect of the tidal friction would have taken place in the space of billions of years with the result that the Moon eventually slowed so much that it now keeps the same side toward the Earth today, a phenomenon called “tidal locking”.

Tidal locking will also happen on Earth. If the tidal friction continues unabated, the length of a day on Earth and the Moon’s orbital period will equalize at about 55 days. Then the Earth will always face the Moon on the same side just as we now only see one side of the Moon. The tidal bulge leading the moon today will point directly at the Moon and the Moon will no longer move away from Earth.

Tidal locking is a common occurrence between other bodies in the solar system. Pluto and Charon are already tidally locked.

Tidal Friction
Tidal friction is measured in several ways.

The length of the lunar month can be measured by measuring the thickness of tidal deposits, “tidal rhythmites”, preserved in the Earth’s rock layers over billions of years although the measurements currently cover only about 900 million years. The resultant studies have shown the tidal layers correspond to increases in the lunar month, hence a slowing of the lunar orbit. This rate of change has remained nearly unchanged.

The Earth’s rotational period has also been measured by bouncing laser beams off reflectors placed on the Moon’s surface by Apollo astronauts. These studies show increases in the Moon’s orbit.

Another way to study the phenomenon of tidal friction is to measure the change in the rotational period of the Earth. Very Long Baseline Interferometry studies utilising many radio telescopes on the Earth’s surface to study the positions of the quasars have shown that over time very accurate measures of the rotational period of the Earth show that the Earth’s rotation is in fact slowing down.

Disappearance of Solar Eclipses
Sometime within the next 500 million to 1 billion years the Moon will be so far away that total solar eclipses will no longer be possible. The Moon will have moved about 5% further away of the distance it is today and the lunar disc will be too small to hide the sun resulting only in partial solar eclipses

A Distant Orb or an Explosive Collision
There are two alternative scenarios. Eventfully the Moon’s orbit would stabilise at about 1.6 times the distance it is today. However this would take about 15 billion years.

On the other hand, as tidal locking is obtained, the Moon would lose momentum and the orbit then degrade and eventually collide with the Earth.