The familiar near side of the Moon, forever turned towards us. Image Credit: NASA
When we talk about the Moon we often describe it as having a ‘familiar face’, a nod to the distinctive pattern of bright highlands and dark maria that has been turned towards us for millennia, visible to every human who has ever stood on Earth.
But why do we only see this one side of the Moon?
We know that Earth spins about its axis, so why don’t we get to see the full lunar surface as our Moon does the same?
If you could have a bird’s-eye view of the Moon orbiting Earth, you would see that the Moon rotates once on its axis every 27.3 days, which also happens to be the same amount of time it takes to complete one orbit of our planet.
The result is that from our perspective on terra firma we see the same lunar hemisphere at all times; if the Moon were to spin faster or slower than once per orbit we would see all of it.
In the proper astronomical parlance, we say that the Moon is ‘tidally locked’ to Earth.
You may also come across the expression ‘synchronous rotation’, which means the same thing.
It wasn’t always this way. When the Moon formed some 4.5 billion years ago, it was spinning much more rapidly than it is today.
Earth’s gravity causes a rocky tidal bulge in our companion, which means it is lemon-shaped rather than a neat sphere, with a pinched end facing our planet.
Back in the Moon’s fast-spinning early history, the location of that bulge kept changing, shifting across the surface much in the same manner as our ocean tides.
This effectively acted as a brake, gradually slowing our companion’s spin speed until it fell into equilibrium with its orbital period.
At this point the hemisphere facing us became locked in place.
But our view still changes
Though the Moon always keeps that same side towards us, even a cursory glance will show you that it is not consistently illuminated from one night to the next.
What you are seeing here is the changing phase of the Moon.
By phase, we simply mean the proportion of sunlit Moon visible from Earth.
The essential point to remember is that although only a fraction of the Moon may be lit from our vantage point, a full 50 per cent of the Moon is lit at any one time. We just can’t always see it.
The cycle of lunar phases (also known as a lunation) runs from new Moon to full Moon and back again, and takes 29.5 days to complete.
Use our illustration below to help you imagine that you are looking down on Earth, the Moon and the Sun, and that they are in a line with the Moon in the middle.
This is the point of new Moon, where no sunlight falls on the lunar hemisphere facing us.
At full Moon the reverse is true: Earth sits in the middle of a line with the Moon and the Sun, and the near side of the Moon is fully lit.
There are two more points in the lunar cycle with precise names, first quarter and last quarter.
First quarter occurs between new and full Moon, and marks the point when the Moon is 50 per cent illuminated to us on Earth, appearing as a near-perfect semicircle.
Last quarter occurs between full and new Moon, and in this case the opposite half of the lunar near side is illuminated.
In both cases, these phases occur when the Earth sits at the right angle of a triangle with the Sun and the Moon.
That both of these phases are called quarters is something of a misnomer – the quarter being referenced is the proportion of the lunar cycle the Moon has progressed through, not the illumination we see from Earth.
The rest of the lunar phases are described as being waxing or waning (growing or shrinking) and crescent or gibbous (less than or more than 50 per cent illuminated).
After new Moon, the terminator – the line separating lunar day and night – appears to creep from east to west, giving rise to the waxing crescent phases.
After first quarter, the Moon moves through its waxing gibbous phases until it reaches full Moon.
From here the phases play out in reverse order, from waning gibbous to last quarter then waning crescent back to new, where the cycle starts all over again.
The Moon’s phase changes based on its relative position to Earth and the Sun. Credit: Steve Marsh
Although it takes the Moon 29.5 days to complete a lunar cycle (a period known as the synodic month), it only takes 27.3 days to complete one orbit of our planet (a sidereal month).
This discrepancy arises from one lunar cycle being defined as the time it takes for the Moon to return to the same phase as seen by an observer on Earth – because Earth itself is moving, hurtling through space on its own orbit around the Sun, it takes the Moon that little bit longer to catch up than complete an orbit of its own.
You may also wonder why, given the Moon sits in the middle of a line with Earth and the Sun at the point of new Moon, solar eclipses are such rare events.
And likewise, why we don’t experience guaranteed lunar eclipses at the time of full Moon.
It’s because the Moon’s orbit around Earth is tilted by around 5° with respect to Earth’s orbit around the Sun.
What happens, in most instances, is a near miss.
Kev Lochun is a science journalist and production editor on History Revealed Magazine