2. "The Dark Side of the Moon"
( Hint - it doesn't have one! )
“The dark side of the Moon” is a phrase very commonly used by people with no knowledge of astronomy. I’ve even heard it used in a TV programme, which was part of the BBC’s most supposedly “reputable” science series. But it’s utterly nonsensical – there is no such place!
2.1. The two-faced Moon
As is well known – though probably not as well known today as it used to be, before people walked around with their gaze permanently towards their mobile phones! – the Moon always keeps the same face towards the Earth. Whenever we see it in the sky, we see the same pattern of bright and dark features; that’s because we are always looking at the same half of its surface. This has been known for four centuries, ever since people first observed it through telescopes. Its opposite hemisphere had never been seen until 1959, when the Soviet space probe Luna 3 flew past the Moon and returned the first photographs of it. It’s that unseen side which is commonly, but incorrectly, referred to as the “dark side”.
( What was revealed by Luna 3 was a major surprise. Scientists had always assumed that the far side would be similar to the side we see – but it isn’t! It consists almost entirely of rugged “highland” regions, with very few of the smooth plains, known as maria, which dominate the Earth-facing side. The reason for this dichotomy is still not understood. )
Why we only see one side of the Moon is very simple – though an alarming proportion of people apparently don’t understand it. I once came across a gentleman who was convinced that the Moon doesn’t rotate, and I’ve seen it stated, in a children’s book on astronomy, that it’s because the Earth and Moon rotate at the same speed. Both of those are completely wrong.
The real reason is because the Moon rotates on its axis in exactly the same time which it takes to orbit the Earth, 27.5 days, and in the same direction. This is shown in Fig. 1.
In the diagram, we are looking down from above the plane of the Moon’s orbit, and from above its north pole, and we see the Moon at four different positions in its orbit. The line bisecting the Moon represents the division between its visible and unseen hemispheres; the small tickmark labelled P represents the position of the crater Ptolemaeus, which is almost in the centre of the Earth-facing side. You can see that as the Moon travels around its orbit, it also makes a single rotation on its own axis.
You can also easily demonstrate the principle for yourself. Get a friend to stand in the middle of a room, to represent the Earth. Walk around them in a circle, while at the same time, turning yourself around at a random rate – but no so fast as to make yourself dizzy! Your friend will alternately see your face and the back of your head. Now do it again, but rotate around your own axis in the same time that you take to walk around the circle. ( You will effectively be walking sideways! ) Now your face is always towards your friend, and they never see the back of your head. This is exactly what happens with the Moon!
In fact, this synchronous rotation, as it’s called, is a natural consequence of gravitational interactions; there are several other planetary satellites in the Solar System which also keep the same face towards their parent planets. All orbiting bodies evolve towards this state over a very long period of time, measured in billions of years; the smaller body reaches it more quickly. Over an extremely long timescale, the Earth-Moon system would eventually reach a state where both kept the same faces towards each other – but this won’t actually happen before the Sun becomes a red giant and destroys them!
Actually, it isn’t quite true that we only see half of the Moon’s surface; there are two effects which allow us to see a little more. Because its orbit is elliptical, rather than circular – though it’s so close to a circle that if it was drawn to scale, you would have to look closely to tell the difference – its speed of motion varies slightly. This means its rotation on its axis alternately gets slightly ahead and behind. From our point of view, it appears to rotate slightly back and forth “sideways” ( taking the direction of its north pole as “up” ), so that we alternately see a little further round to its west side, then to its east. This is called libration. Due to some complex gravitational effects, it also performs a slight up and down “nodding” motion, called nutation, which causes its north and south poles to alternately nod slightly towards and away from us – so we also alternately see a small region around each pole. The combination of these two effects means that 59% of the Moon’s surface is visible to us at some time or other. The remaining 41% is indeed forever hidden from us, and has only been revealed to us by spacecraft. ( Only 24 humans - the Apollo astronauts - have ever seen the far side with their own eyes! )
2.2. Unseen doesn’t mean dark!
You will have noticed that I refer to the side which we don’t see as the “far side”, not the “dark side”! No-one really knows how the erroneous term “dark side” originated; perhaps it comes from an old-fashioned mystical use of the word “dark” to mean unknown or mysterious. But the fact that we can’t see the far side doesn’t mean that it’s literally dark!
While it keeps the same face towards the Earth, it does not keep the same face towards the Sun! As stated earlier, the Moon rotates on its axis every 27.5 days – so every part of it faces the Sun at some time. Every point on its surface experiences a cycle of day and night, just as every point on the surface of the Earth does – but its periods of daylight and darkness each last a fortnight. So there is no part of the Moon which is always dark!
Think about the phases of the Moon, which change from “New” to “Full” Moon and back every month; that is of course where the word “month” comes from. The phases are caused by the changing position of the Moon in the sky, relative to the Sun, as shown in Fig. 2.
In the diagram, the direction of the Sun is to the right. At each position in the Moon’s orbit, the white half represents the half of the Moon which is sunlit, and the black half the half which is in darkness. Obviously, at any given moment, half of its surface is lit by the Sun – but apart from at Full Moon, not all of that sunlit half is turned towards the Earth. At each position, the corresponding picture in the outer circle shows the appearance of the Moon as seen from Earth.
At New Moon, when the Moon is in line with the Sun, its sunlit half is entirely facing away from us, so we can’t see it at all. At Full Moon, when it’s directly opposite the Sun in the sky, its sunlit half is entirely facing towards us. At First Quarter and Last Quarter, when it’s 90 degrees away from the Sun, half of its sunlit side is facing towards us, so we see it as a semicircle or “half moon” shape. And every other phase in between.
Note that at most New Moons, the Moon isn’t exactly in line with the Sun, but passes either above or below it from our point of view. That’s because the plane of its orbit is tilted by 5 degrees to that of the Earth’s orbit. Similarly, at most Full Moons, it doesn’t pass through the Earth’s shadow, but passes above or below it. That’s why we don’t see an eclipse of the Sun at every New Moon, or an eclipse of the Moon at every Full Moon.
Note also that while the Moon orbits the Earth in 27.5 days, the cycle of its phases, between successive New or Full Moons, takes 29.5 days. That’s because, as it orbits the Earth once per 27.5 days, the Earth is also orbiting the Sun once per year. So when the Moon returns to the same position in its orbit, relative to the Earth, the direction of the Sun has changed by about 27 degrees, so the Moon has to move that much further round its orbit, before we see it at the same phase again.
Note also that three or four days either side of New Moon, when its phase is a thin crescent, we can often see the rest of its disc – that is, the part of its night side which is facing us – illuminated by a faint blue-green glow. This is earthshine – sunlight which is reflected from the Earth towards the Moon, then reflected back from the Moon to our eyes. At all other phases except Full Moon, the earthshine is still there, but we can’t see it, as it’s overwhelmed by the bright glare of the sunlit part of the Moon.
So think about it. At New Moon, when the Moon is between us and the Sun, it follows that the side of it which we can’t see is being illuminated by the Sun. And at First or Last Quarter, when only half of the side facing us is sunlit, it follows that half of the other side must also be, doesn’t it?
So while the Moon does indeed have a “far” side, which we never see, it doesn’t have a “dark” side!
2.3. It doesn’t “only come out at night!”
While I’m on the subject of the Moon, I’ll address another misconception; I find it quite horrifying to think that I actually need to write this!
A couple of years ago, a friend of mine told me a story which defied belief, about something which he heard on the local radio station in our home city. During a phone-in programme, someone called in, expressing amazement that he had just noticed that he could see the Moon in daylight. The radio host was equally baffled, and asked if anyone could call in, who could explain this puzzling phenomenon.
Er – what????? This was surprising – why, exactly? So it appears that a significant proportion of people actually believe the ridiculous, and frankly childish, notion that the Moon “only comes out at night”! I might expect that from a five-year-old, but that any adult could believe it is something I find quite staggering!
( Conversely, an astronomer friend told me that his daughter realised for herself, with no prompting at all, that she could see the Moon in daylight, at the age of five. )
It doesn’t help, when some adults apparently deliberately teach that silly idea to children. Another friend, now deceased, who was a retired teacher, built his own portable planetarium, which he used to take to local schools. On one occasion, when he was giving a show at a junior school to a class of eight-year-olds, the teacher said something to the kids about the Moon “only coming out at night”. My friend took her to a window, pointed to the sky, and asked her, “What’s that?”
“The Moon”, she replied.
“So why did you just tell the kids that it ‘only comes out at night’?”
“Because that’s what they like to hear at this age.”
Again – what????? Since when is it a teacher’s job to tell children “what they like to hear”, instead of telling them the truth?
Think about it. The Moon is not always visible at night, is it? Hence the saying, “There’s no Moon tonight”, to describe a night which is especially dark, due to the absence of moonlight. So if it isn’t visible at night, where do people think it goes?
The Earth orbits the Sun once per year; at the same time, the Moon orbits the Earth once per month. So it follows that there must be times when the Moon is in roughly the same direction in the sky as the Sun, doesn’t it? And at those times, it follows that it must be above the horizon at the same time as the Sun, doesn’t it?
As the Earth rotates, the Moon rises and sets each day, just as the Sun does. Each day, it spends roughly 12 hours above the horizon and 12 below it, hidden from our view. ( This actually varies, due to the aforementioned tilt of its orbit. Sometimes it’s above the horizon for a bit longer than 12 hours in the Northern Hemisphere, and a bit less in the Southern; at other times, it’s the other way around. )
Of course, the times at which the Sun rises and sets, and the length of daylight, vary by several hours at different times of the year. The reason for this will be discussed in a later essay on the cause of the seasons. For simplicity, in the following explanation, let’s consider a time close to the Vernal Equinox ( 21 March ), when the lengths of daylight and night are equal everywhere on Earth, and the Sun rises at 6 a.m. and sets at 6 p.m. local time, no matter where you are.
Look again at Fig. 2. At New Moon, the Moon is almost in line with the Sun, so it’s above the horizon all day, at the same time as the Sun. Obviously, we can’t see it, as its sunlit side is facing away from us. At Full Moon, it’s opposite the Sun in the sky, so it’s above the horizon all night; it rises when the Sun sets, and sets when the Sun rises. At all other phases, the situation is somewhere in between.
For a couple of days either side of New Moon, the Moon is a thin crescent, and is still quite close to the Sun in the sky – so we still can’t see it, as it’s overwhelmed by the glare of the Sun. But as it moves further from the Sun, and we see more of it illuminated, it becomes visible in daylight. When it’s 45 degrees from the Sun, it rises and sets three hours after the Sun, i.e. at 9 a.m. and 9 p.m. respectively – so it’s in the sky for three quarters of the daytime. You might not notice it during full daylight, but it becomes very easily visible in twilight, just after sunset.
( A thin crescent Moon, when it first becomes visible in twilight, is often incorrectly called “New Moon”. This has great importance in some cultures; in the Islamic world, important dates, such as the start of Ramadan, are determined by the first sighting of the crescent Moon, rather than the actual time of New Moon. )
At First Quarter, the Moon is 90 degrees from the Sun, so it rises and sets six hours after the Sun; it rises at noon and sets at midnight. During the afternoon, it’s very clearly visible in full daylight, even on a bright sunny day, if you bother to look.
Similarly, at Last Quarter, it rises and sets six hours before the Sun, so it’s visible in daylight during the morning. And so on for any other phase in between.
So there you have it. Despite the bafflement of that radio caller, seeing the Moon in daylight is perfectly normal. Try looking up now and again, and see for yourself!
A puzzling phenomenon... er, NOT!
Photo courtesy of Mary McIntyre
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