From Earth, our planet’s only natural satellite, the Moon, appears as the largest and brightest celestial body in the night sky, making it a perfect target for amateur astronomers. Large craters can be distinguished on the Moon’s suface even with the unaided eye, as well as the great dark plains known as maria, from the Latin word for seas (singular: mare – pronounced mar-ray).
Since the Moon orbits the Earth with the same side always towards us, these darker maria, alongside the lighter, cratered “highlands” and mountain ranges, give the Moon its distinctive “face”, familiar the world over.
The Far, or “Dark”, Side of the Moon
The side of the Moon facing away from the Earth is often known metaphorically as “the dark side of the Moon”, although, the proportion of this far side of the Moon that is illuminated by the Sun varies in the same way as the side facing Earth.
The fact that the same side of the Moon always faces the Earth is not merely a coincidence. When the Moon was first formed, it would have been spinning at a different rate. However, the speed of the Moon’s rotation on its own axis has gradually synchronised with its orbit around the Earth due to the effect of tidal gravitational forces between the Earth and the Moon. The Moon is said to be tidally locked in its orbit around the Earth.
This far side of the Moon is much more cratered than the near side, which has more protection from meteorite bombardment due to the presence of the Earth.
The Earth’s Tides
Because of the Moon’s proximity and mass, the Moon exerts a strong tidal influence on the Earth. This causes the surface of the Earth’s oceans to bulge outwards on the side facing towards the Moon, due to the increased gravitational pull of the Moon on the side of the Earth nearest to it.
Although the Moon can be considered to orbit the Earth, the two bodies actually orbit their common centre of gravity, or barycentre. This lies at a point approximately three quarters of the distance from the centre of the Earth to the Earth’s surface. Because of the weaker gravitational pull of the Moon on the far side of the Earth – i.e. a weaker centripetal force from the rotation of the Earth around the Earth-Moon barycentre – the oceans also bulge outwards from the Earth of the side facing away from the Moon.
As the Earth spins on its axis, once a day, the tidal bulges of the oceans remain pointed towards and away from the Moon, resulting in two high and two low ocean tides a day on Earth.
The Sun also exerts a tidal influence on the Earth, however, this is weaker than the Moon’s tidal influence, since the Sun is much further away from the Earth. This results in particularly high and low tides when the Sun and Moon are aligned in opposition or conjunction, i.e. at times of full and new moons. These are known as “Spring tides” although they occur twice every month. When the Sun is aligned at roughly 90 degrees to the Moon in the sky, i.e.at the first and third quarter lunar phases, particularly weak tides, known as neap tides, occur, since the Sun and Moon’s tidal forces are pulling at a right angle to each other.
Exploration of the Moon
The Moon is still the only celestial body, apart from the Earth, upon which human beings have set foot.
For information on the Apollo mission moon landings, see Project Apollo.
As the Moon orbits the Earth, roughly every 28 days, the proportion of the Moon’s disk illuminated by the Sun, as viewed from Earth, changes, night by night. This lunar cycle is divided into eight main stages, known as the phases of the Moon.
The months of the year are roughly based on the length of the full lunar cycle, hence the name “month”.
The fact that one week is seven days long is most likely because there are roughly seven days from a full or new moon to a point when half the Moon’s disc is illuminated (i.e the first of third quarter).
Note that the exact timings of the Moon’s phases will vary, very slightly, according to the position of the observer on Earth, due to the effect of lunar parallax. However, this effect is very small, so that, at any particular moment, the Moon’s phase will appear the same at every point on the Earth’s surface where the Moon is visible in the sky.
Note, also, that the entire hemisphere of the Moon that is facing the Sun, at any particular moment, is illuminated – not just the part of the Moon visible from Earth. This illuminated hemisphere travels around the Moon as it orbits the Earth, much like the illuminated hemisphere of the Earth changes as it spins on its axis throughout the day. It is the varying angle at which we view the illuminated side of the Moon from Earth that gives the appearance of phases.
The dividing line between the illuminated hemisphere and the hemisphere that is in darkness is known as the “terminator”.
The main lunar phases are listed below.
Since the Moon’s orbit is inclined at an angle to the orbital plane of the Earth around the Sun (the ecliptic plane), the Moon does not cover the Sun’s disk at every new moon (i.e. the Moon has a different declination to the Sun). However, when a solar eclipse does occur, it is always at the new moon.
The Moon will not be visible from Earth at the exact time that a new moon occurs because it will be too close to the Sun in the sky. However, shortly after the new moon, the Moon will appear as a narrow crescent shape, close to the setting Sun. This is also commonly refereed to as a new moon, although technically this is a waxing crescent moon.
At the start of the lunar cycle, just after the new moon, the illuminated crescent will be on the right-hand side of the Moon to observers in the Northern Hemisphere and on the left-hand side in the Southern Hemisphere. This is because people in the Southern Hemisphere are the opposite way up, with respect to people in the Northern Hemisphere of the globe.
The new crescent moon will be visible in the evening sky and, at the end of the lunar cycle (an old moon), the crescent moon will rise before the Sun in the morning sky, with the illuminated crescent on the opposite side.
Waxing Crescent phase
This phase occurs between the new moon and the “first quarter” half moon. At three to four days from the new moon, the illuminated portion of the Moon appears as a wide crescent, about a quarter of the way across the Moon’s disc.
When the illuminated proportion of the Moon, visible from Earth, is increasing, night by night, i.e. between the times of new moon and full moon, the Moon is said to be “waxing”. After the full moon, the Moon is said to be “waning”, as the illuminated proportion of its disk, as seen from Earth, is decreasing.
During the crescent moon phase, the side of the Moon that is not directly illuminated by the Sun is often visible, due to reflected light from the Earth, known as “earthshine”. This effect is sometimes known as “the old moon in the new moon’s arms”.
First Quarter (Half Moon) – Waxing
At the point in the Moon’s orbit where the angle between the positions of the Sun, the Moon and the Earth is 90 degrees, the Moon will appear to be half full to an observer on Earth, with the terminator line falling straight down the middle of the Moon’s disk.
This occurs around 7 days from the time of the new moon, which is why it is called the first quarter, since it is one quarter of the full 28 day lunar cycle, even though half the Moon’s disk is illuminated.
When this occurs during the waxing stage, the right-hand half of the Moon is illuminated as viewed from the Northern Hemisphere and the left-hand half is illuminated in the Southern Hemisphere (note that it is still the same side of the Moon that is illuminated by the Sun, however).
When around three quarters of the Moon’s disk is illuminated, as seen from Earth, the Moon is said to be in its “gibbous” phase. When the Moon is waxing, the portion of the Moon in darkness will be on the left-hand side to observers in the Northern Hemisphere and on the right-hand side in the Southern Hemisphere.
The terminator will be roughly three quarters of the way across the Moon’s disk two to three days before the full Moon and 10 to 11 days from the start of the lunar cycle.
Full Moon occurs at the precise point when the Moon is on the opposite side of the Earth to the Sun – i.e. in opposition – with the full face of the Moon illuminated, as seen from Earth. Because of lunar parallax, this will occur at slightly different times, depending on the observer’s position on Earth.
Lunar eclipses will always occur at a full moon, however they do not occur during every lunar cycle because the plane of the Moon’s rotation around the Earth is offset at an angle to the ecliptic plane.
There are around 14 days – roughly two weeks – between a new moon and a full moon.
This phase occurs between the full moon and the “third quarter” half moon.
The terminator will again be roughly three quarters of the way across the Moon’s disk two to three days after the full moon and 17 to 18 days from the start of the lunar cycle.
The waning gibbous phase can be distinguished from the waxing gibbous phase because the left-hand side of the Moon will be the side illuminated by the Sun during a waning phase, to observers in the Northern Hemisphere, and the illuminated side will appear to be on the right to observers in the Southern Hemisphere – the reverse of when the Moon is waxing.
Last Quarter (Half Moon) – Waning
At this point, the terminator will again fall exactly down the centre of the Moon’s face, as in the waxing “first-quarter” half-moon phase. However this time, the left-hand half of the Moon’s face will be illuminated to Northern Hemisphere observers, with the right-hand side illuminated in the Southern Hemisphere.
This phase occurs roughly 21 days after the last new moon, roughly a week after the full moon, and a week before the next new moon.
Crescent – Waning
The waning crescent phase occurs between the last quarter half moon and the new moon that will start the next lunar cycle. At three to four days before the new moon (24 to 25 days from the last new moon), the illuminated portion of the Moon will be a wide crescent, about a quarter of the way across the Moon’s disc.
During the waning crescent phase, the left-hand edge of the Moon is illuminated in the Earth’s Northern Hemisphere, with the right-hand edge illuminated in the Southern Hemisphere – the reverse of the waxing crescent phase.
This waning crescent moon will be visible in the morning sky, as opposed to the waxing crescent moon, which is visible in the evening sky.