The phases of the Moon have captured the imagination of astronomers and lay-people alike since the beginning of human existence. The Moon is the second brightest object in the sky, trailing only the Sun in total brightness, and is often thought of as the ruler of the night sky (although we shall learn that that night sky characterization is not entirely accurate when it comes to the Moon). The Moon is easily visible anywhere skies are at least somewhat clear, with no worries needed about light pollution or local conditions.
People since ancient times have looked up at this dominating night sky influencer and have noticed two astonishing facts that set it aside from stars and other celestial objects: 1) The Moon goes through phases. The amount of the Moon that appears lit up in the night sky changes over a period of 29 – 30 days, growing in light from a dark new Moon through a half lit quarter Moon all the way to a completely lit full Moon before it completes one cycle and starts the next one again. 2) More subtle, but every bit as important as the phases of the Moon, the Moon also dramatically changes its location in the sky from one day to the next. While stars rise 4 minutes earlier each day, the Moon can often rise over an hour later than it did the previous day. Therefore the position of the Moon in the celestial sphere, that imaginary star globe of the night sky, changes significantly from one evening to the next. Both these observational facts revealed to people from the earliest times that the Moon was different than other celestial objects.
The Moon continues to be an object of study both to professional astronomers and amateurs, as well as a topic of study in most introductory astronomy classes. If you are facing difficult questions about the Moon in your classes, don’t go it alone. Let our superb team of professional astronomers and astronomy educators help guide you through your most challenging questions. The Moon, particularly the Moon phases, is often difficult at first glance to understand, but don’t give up. The reward in coming to learn about the phases of the Moon is immense – it gives you a skill set you can use daily – every time you look up and see the Moon, you can put what you have learned to practice.
A Dark Side of the Moon? Many of us have heard the phrase “Dark Side of the Moon”, popularized by the very famous Pink Floyd album of the same name. It sounds mysterious and intriguing – what treasures does the Dark Side of the Moon hold in store for us? However, not so fast. Where does the phrase “Dark Side of the Moon” come from and is it actually true? Is there really a dark side of the Moon?
To answer these questions, let’s look at some of the basic details of the Moon. The Moon is the closest natural neighbor to Earth in the sky. Not only is it the Earth’s closest natural neighbor, it is also Earth’s closest (and only) satellite, orbiting (or going around the Earth) approximately once every 27.3 days. The Moon’s orbit around the Earth is somewhat eccentric, with a closest approach (perigee) of 362,600 km and a farthest distance (apogee) of 405,400 km. The semi-major axis (or average distance from the Earth to the Moon is 384,399 km. What does that distance mean to us in our everyday lives? Imagine that you decide to get in you car and make an imaginary drive to the Moon. Let’s say you drove a reasonable 60 miles an hour the whole time, not stopping for any breaks in your magic solar-powered car. It would take you 160 days, or just under 6 months or half a year, to drive to the Moon. Depending on your view of space, that is either a crazily large number – wow the Moon is far away! – or a fairly small number – it’s less than half a year after all.
One thing you may have noticed from looking at the Moon is that, ignoring the phases which cause different parts of the Moon to appear lit up, you are always seeing the same parts of the Moon every time you look at the Moon, no matter what phase it is! It is perhaps easiest to see this by looking at the darker spots on the Moon, called the maria. These darker spots are what make up the “Man in the Moon” or the “Rabbit in the Moon” or many other objects depending on your cultural background. These dark spots confused the first astronomers who believed that they were giant lakes or oceans. In fact the name maria means “Seas” in Latin. However, we now know that these are not seas of water (and in fact substantial amounts of surface water have never been detected on the Moon), but astronomers are slow to change, and even though no one believes they are oceans anymore they are stilled called Maria.
When you look at the Moon through its various phases, which we will discuss in detail shortly, you may notice that the locations of the dark spot do not change. Ones that are located more to the left side of the Moon as you see it from Earth are always on the left side. What is going on here? The Moon has become tidally locked to the Earth, which means that as the Moon orbits the Earth, it rotates at just the right rate so that the same side of the Moon always faces the Earth. This type of setup is called synchronous rotation. Synchronous rotation is when an object has a rotation period (the time for it to rotate once as observed from above or below) that is identical to its orbital period (the time to orbit or go around an object). So in the case here, the Moon’s rotation period is identical to the time it takes the Moon to orbit the Earth. Whenever we have synchronous rotation, it means by default that the same side of a satellite always faces the object that it orbits. The cause of synchronous rotation is a bit more than we have time to go into, but the short answer is that when the Moon was younger, it was still liquidy (molten rock or magma / lava) and the tidal forces from the Earth gradually slowed the Moon’s rotation down until it was synchronous.
The same side of the Moon always faces the Earth. Think about this for a second. This means that for as long as we and our ancestors have been looking up at the Moon, our closest neighbor in space, we’ve only seen one side of it. The other side has been permanently hidden from our view. It was not until 1959 that we got our first ever pictures of the other side of the Moon, in this case coming from the Luna 3 Soviet probe, and it wasn’t until 1968 and the Apollo 8 mission that human beings actually saw the other side of the Moon with their eyes.
The picture here shows the near side of the Moon on the left and the far side of the Moon on the right. Notice, especially on the near side of the Moon the darker, round areas – these are the maria discussed above.
This mysterious other side of the Moon is sometimes called the “Dark Side of the Moon”, but as we will
note this picture is very much NOT to scale. To be at scale, the Sun would need to be about 75 times bigger than it is in the picture and located hundreds of times farther to the left. Right now, the left side of the Moon is facing the Sun. That means that the left side of the Moon is going through daytime and getting lit up by the Sun. Wait, though. This left side of the Moon is the part that we never see from Earth – the Far Side of the Moon. So, the dark side of the Moon is not really dark all the time at all --- in fact every bit of the Moon receives the same amount of sunlight each orbit, approximately 14.5 - 15 Earth days of sunlight in one big chunk followed by 14.5 – 15 days of darkness. So, sorry fans. There is really no dark side of the Moon. A far side, yes, but not a dark side.
Moon Phases Let’s now turn our attention to probably the most famous aspect of the Moon, its phases. These phases have been observed since literally the first time our ancestors gazed up at the Moon. The 29 – 30 day cycle of the phases even gives us the concept of the Month, and many calendar systems, including the Islamic Calendar, the Hebrew Calendar, and the Asian Lunar Calendar. One important fact to consider first is this: whenever you are seeing the Moon lit up in the sky, that light is NOT being produced on the Moon itself. There are not Moon aliens up there all holding up flashlights at us. No, instead what you are seeing is reflected sunlight. Light from the Sun reflects off of the Moon. That is in fact the reason we see every object in our solar system (except for the Sun itself) – light from the Sun reflected off an object.
The phases of the Moon can be summarized as follows: Phases occur because we see different amounts of the lit side of the Moon as it orbits the Earth. At any given time, barring a lunar eclipse, half the Moon is in daylight, while the other half is in darkness. However, depending on the exact positions of the Earth, Moon, and Sun, we as observers here on Earth, see only some of the lit side of the Moon, giving us phases. As the Moon changes its location in its orbit around the Earth, the amount of the lit side that we can see changes, causing the appearance of the Moon to move through its phases.
In the figure here you see the Moon in its orbit around the Earth and the eight different phases that it goes through. Refer back to this picture frequently in our discussion of Moon phases.
Let’s start with the Moon phase that, ironically, is one of the more challenging ones to see, and one that, as we shall see later, is never actually visible in the sky at night: the new Moon. The set up of the new Moon is the same one you saw in our discussion above of whether there is a dark side of the Moon. For the New Moon, the Moon is located in between the Earth and the Sun. The part of the Moon that is lit up by the Sun is, in this case, the far side of the Moon. The part of the Moon that we see in the sky is the non-lit side. So, for us here on Earth, the New Moon looks completely dark. We can say that the New Moon is when none of the lit side of the Moon is visible. An interesting fact we will explore in greater detail below is that a New Moon is actually in the sky during the day time and not the night time here on Earth. Have you ever seen what looks like a very faintly lit Moon in the sky during the middle of the day? What you are actually seeing there is the new Moon. “But wait,” you say. “I thought none of the lit side of the Moon was visible during a new Moon, so why can I see the Moon in the sky, even faintly lit”. What you are seeing in this instance is called earthshine. Some light from the Sun reflects off of the Earth, back to the Moon, and then back to us. All these extra reflections explain why the Moon looks so faint when you are able to see it in the sky.
Before we move on to the other Moon phases, it is interesting to note something about a new Moon. Although it may appear that there are two to three days in each Moon phase cycle where the Moon is not visible in the sky and none of it is lit up by the Sun, technically, a New Moon is an exact moment. It is defined as the moment when, as seen from Earth, the Moon is located in the same two-dimensional direction as the Sun. The two-dimensional phase is necessary here because as we will learn in our accompanying article about Eclipses, the Moon orbits the Earth with a tilt or inclination to its orbit and so does not always lie in an exact straight line with the Sun as seen from Earth. However, when the Moon’s direction on a plane is the same as the Sun’s direction on a plane, both measured from Earth, that is the instant that is considered a New Moon. If you search for Moon phases online and look for a calendar of new Moon dates, you will see that each new Moon is listed as both a date and time. This means that the whole world experiences a new Moon at the exact same moment, no matter where on the globe a person is located. Since time zones cause different places on the Earth to have different times on the clock, it means that a New Moon will occur at a different time on the clock depending on where on the Earth a person is located. For instance, a new Moon that occurs on March 2nd at 10pm in New York City would occur at 7pm on March 2nd in Los Angeles, since 7pm in Los Angeles is the same time as 10pm in New York City, and 3am on March 3rd in London and 11am on March 3rd in China. Realistically it will seem like a new Moon exists for about 3 days each lunar cycle, but keep in mind that technically it is an exact moment, and an exact moment only, for the new Moon.
What happens after a new Moon? The Moon is now beginning to make its 29-30 day journey around the Earth and no longer is the side lit by the Sun 100% hidden on the far side of the Moon. Instead, we will at first see a very small fraction (a few percent) of the lit side of the Moon. This is called a crescent Moon, which is defined as when we see some but less than half of the lit side of the Moon. A crescent can take many different appearances, from a slender arc in the sky on the days immediately after a new Moon all the way to a Moon that is nearly half lit. Every day as the Moon continues in its orbit around the Earth, more and more of the Moon is lit up. The crescent becomes bigger. The crescent Moon is probably the most iconic Moon phase. If you ask a child to draw the Moon, they will either draw a circle or will likely draw a banana shaped crescent Moon. Next time you watch a movie from DreamWorks Studio watch the short animation that announces the DreamWorks Studio and you will see a person fishing while sitting on the edge of a crescent Moon.
If you look carefully at the crescent Moon phases, you will notice that there are times when it is the right side of the Moon that has the lit crescent. At other times, you may notice that the left side of the Moon has the lit crescent. The “banana” can be facing either direction. How do you know which one you will see, the left side lit banana or the right-side lit banana? It is not random, but rather follows a very straightforward pattern. If you have just had a new Moon and are now getting a more and more lit up crescent Moon each day, you will always see the right side of the Moon lit in the crescent shape. We will see coming up shortly what conditions will cause the left side of the Moon to appear lit up in a crescent shape.
Just to recap where we are in our Moon phase story, we currently have a crescent Moon that is getting more and more lit up each day – the “banana” is growing. When exactly half the Moon appears lit, the Moon phase is called a quarter Moon. In our example here, we can more specifically call it a First Quarter Moon. The whole right side of the Moon appears lit to us in the sky. What is happening with the locations of the Sun, Earth, and Moon to create a quarter Moon? Quarter Moons occur when the Sun and Moon are in perpendicular directions from each other as seen from Earth. The location of the Moon and the location of Sun, both as recorded from the Earth, are at a right angle to each other, as seen in the diagram below.
In the diagram, the left side of the Moon is lit up by the Sun, but we see the top half of the Moon from Earth, so for us, we will see half of that lit side, which gives us a quarter Moon. Just like the New Moon, the quarter Moon is an actual moment in time when the Moon is at that exact perpendicular location, but realistically people see a quarter Moon in the sky for approximately 2 – 3 days, just like a New Moon. Before we move on to the next Moon phase, let’s answer a quick puzzling question. Why in the world is it called a Quarter Moon when half the Moon appears lit? Students will sometimes get very creative with fractions to try to answer this question, arguing that we are seeing half of the lit half, so half of a half is a quarter. While this is true, the name quarter Moon comes from a much more straightforward place. If you watch the orbit of the Moon around the Earth and assume the new Moon as a starting point, when we get to the First Quarter Moon, the Moon has made the first quarter of its orbit around the Earth. That’s where the Quarter terminology comes from.
After the First Quarter, the Moon continues in its orbit around the Earth and every day more and more of the lit side of the Moon is visible from here on Earth. The phase of the Moon that comes next is called a gibbous Moon. This is probably the least well-known of the Moon phases, but it is just as common as a crescent Moon. The gibbous Moon is defined as a Moon phase where we can see more than half but less than all of the lit side of the Moon. In other words it lies right in-between a quarter Moon (half appears lit) and a full Moon (all appears lit). Each day, observers from Earth will see more and more of the lit side of the Moon and the gibbous will appear fuller and fuller. The gibbous Moon is the harder Moon phase to try to draw a sketch of – it does not have the straightforward definition of a new, quarter, or full Moon and does not seem to fly out of the pen the way that the banana-shaped crescent Moon does. To draw any Moon phase, you always start with drawing a half circle. Now to finish the shape or phase we need to add an additional line to connect the two ends of the half circle. For a quarter Moon, that line is straight, but for a crescent Moon it curves back in toward the original half circle and for a gibbous Moon it curves outward, away from the original half circle.
Eventually (and eventually in this case means approximately 14 to 15 days after the New Moon), the Moon reaches the point where it is on the exact opposite side of the Earth from the Sun. Now, the part of the Moon that is lit up by the Sun is exactly the same part of the Moon that we are seeing from Earth. To us, the Moon appears completely lit, and we call this phase a full Moon. Full Moons, because the entire Moon face is lit, seem to be the brightest Moon phase in the sky, by far. In fact, if you can go out to a dark place for astronomical observing, you will find that you can see many fewer stars if the full Moon is in the sky compared with other phases. The full Moon is a type of natural light pollution (just like street lights and building lights are of the artificial variety) and limit the number of stars that can be seen. Professional astronomers, when submitting an application to use a telescope for observations, often request that they be assigned observing nights that are not full Moons in order to be able to better see their often faint targets in the sky.
Just like new Moon and first quarter, full Moon technically is an exact time – the exact moment when the Moon is on the opposite side of the Earth from the Sun. Realistically there will likely be a two to three day window when the Moon will appear to be full in the sky.
What happens after a full Moon? The Moon continues on in its orbit around the Earth and the consequence of this is that no longer do we see the entire lit side of the Moon. At first, an observer from Earth will still see most of the lit side, but a small region along the right hand size will be dark. What phase is this? Well, since more than half of the lit side of the Moon is visible from the Earth then we are back to a gibbous phase. So, the full Moon is immediately followed by another gibbous phase. Now, as the Moon continues its orbit, less and less of the lit side of the Moon is visible each day, which means that the gibbous Moon phase will be a little less than it was the previous day. Eventually, only half of the Moon will be visible and we will be back to a quarter Moon. The pattern continues and now even less that half of the lit side of the Moon will be visible, giving us a crescent Moon. The crescent will start out well lit but each day less and less of the crescent Moon will be lit and visible until eventually none of the lit side is visible and we are back to a new Moon. The cycle is complete.
If you examine what has happened, the Moon essentially spent half its orbit showing more and more of its lit side to Earth and then the second half of its orbit showing less and less until it was back to where it started, ready to start again. To summarize, the Moon phase order looks like: new Moon, crescent Moon, quarter Moon, gibbous Moon, full Moon, gibbous Moon, quarter Moon, crescent Moon, and new Moon. The whole process takes 29½ days to go from new to full and back to new again. This is just over 4 weeks in length, so one way to roughly space out the phases of the Moon is to think of them in terms of weeks. It takes approximately one week to go between each of the major stages of the Moon (new, full, and quarter). So, if it is a new Moon today, in one week it will be a quarter Moon, then in another week, a full Moon, in another week back to a quarter Moon, and finally in one more week back to new Moon.
Waxing versus Waning. Our discussion so far has left out an important part of Moon phases. If you look up Moon phases online you will see that the phase, for instance, is not listed as simply a crescent Moon, it is listed as a waxing crescent Moon or a waning crescent Moon. The same is true for the gibbous phase and for the quarter phase (although we call the Moon a first quarter Moon instead of a waxing quarter Moon and a third quarter Moon instead of a waning quarter Moon). Only new Moon and full Moon are exempt from the waxing or waning designation. What do waxing and waning mean? They refer to how the lit portion of the Moon is changing from one day to the next. If the Moon is waxing, that means that more and more of the Moon is appearing lit each day. If the Moon is waning, then it means that less and less of the Moon is appearing lit each day. Waning (and waxing to a smaller extent) are words used in the English language to generically describe increase and decrease. You could say, for instance, that after a long, hard day at the office, your interest in work is waning or decreasing. So, by attaching the term waxing or waning to the Moon phase you can immediately tell if the Moon is on its path between new and full Moon (when it would be called waxing since more is appearing lit each day) or on its path between full and new Moon (when it would be called waning since less is appearing lit each day).
Let’s imagine for a moment that you were tasked with identifying a Moon phase. Figuring out whether it is crescent, quarter, or gibbous might not be too difficult, but what about deciding if the Moon is waxing or waning? You could wait until the next day and see if more of the Moon or less of the Moon appears lit, but this is not only a non-instantaneous solution but also a challenging one because from one day to the next the amount of the Moon that is lit might change by only a small amount, making It difficult to determine conclusively with your eyes. Fortunately there is an easier way. If you examine the Moon phases you will notice that there are times when the right side of the Moon appears lit and other times when the left side of the Moon appears lit. This is true for all the phases (except, of course, new and full Moon). Which side of the Moon is lit? It is not random but instead follows an easy and very useful rule. If the right side of the Moon is lit, then it is a waxing Moon. If the left side of the Moon is lit, then it is a waning Moon. So you now have an instantaneous way of determining a Moon phase. Let’s say for instance that you looked at the Moon and it was the right side of the Moon that was lit and in total some but less than half of the Moon appeared lit. Well, the right side being lit tells you that the Moon is waxing and the less than half (but some) of the Moon appearing lit tells us it is a crescent Moon, meaning that the entire Moon phase is called a waxing crescent.
When is the Moon in the Sky? Here’s an additional fact that often surprises individuals about Moon phases. The Moon is not always up in the sky at the same time each day. We think of the Sun as being the marker of the day and the Moon as being the marker of the night, but this is not strictly true. The Moon, it turns out, spends just as much time in the sky during daylight hours as it does during nighttime hours. Consider for instance the new Moon. If you look back at the diagram of the location of the Earth, Moon, and Sun during a new Moon, you will notice that an observer standing on Earth has to look in the same direction to see the new Moon in the sky as they do to see the Sun. If they are looking in the same direction as the Sun, can it be nighttime? No, the Sun is in the sky so it must be daytime. Indeed, the New Moon follows the Sun perfectly – it rises right at sunrise and sets right at sunset. The new Moon is simply not in the sky at night. This should match your own personal observations as well.
Remember earlier in this article when we discussed Earthshine and how sometimes you can see the new Moon in the sky because it is dimly lit by the reflection of sunlight off the earth. If you have ever experienced this, you’ll recall that it happens during daytime – the new Moon is up in the sky the same time as the Sun. On the opposite extreme is the full Moon. Now the Moon is in the exact opposite direction in space from the Sun as seen from the Earth. As a result, an observer who sees the full Moon in the sky will not be able to see the Sun because the Earth and the ground will be in the way. The full Moon, as many of us have likely experienced, is the true king of the night. It rises right at sunset and sets right at sunrise, lighting up the Earth for the entire night. If you are ever planning to get lost in the wilderness at night, choose a full Moon because it will light your way all night long. More practically, however, if you are wanting to see a whole bunch of stars, especially faint ones, or catch faint sky objects like our Milky Way galaxy, then you do NOT want to observe during a full Moon --- the natural light pollution from the Moon will greatly limit how many stars you can see and how faint an object is visible.
How and why does the time the Moon is in the sky change each day? The why it changes has to do with the fact that the Moon is constantly in orbit about the Earth. As the Moon moves, say, from new Moon to full Moon, it no longer lines up directly with the Sun and there will be times during a day when the Sun is up in the sky, but the Moon is not. The change is not the same each day – it varies depending on exactly how far the Moon is from the earth and more specific details about its orbit around the Earth. On average, though, the Moon rises about 45 minutes later each day. 45 minutes later each day works out to 12 hours over the 15 day time from new Moon to full Moon, which makes sense because the Moon is going from rising at sunrise to rising at sunset, a difference of about 12 hours. One final note connected to the different rising and setting times of the Moon also connects with the idea of waxing and waning Moons. A Moon that is waxing will always be visible in the sky at sunset. A Moon that is waning will not be visible in the sky at sunset. This means that you have to be careful if you are asked to go outside and observe a Moon phase. The time that you observe the phase makes a big difference, as the Moon is not always up in the sky at any one given time. Don’t make the mistake of going outside, say, at sunset, saying you can’t see the Moon and therefore it must be a new Moon. While that could be true, the more likely explanation is that the Moon is in some sort of waning phase and won’t be visible in the sky until later in the evening or early morning.
Conclusion There is so much to learn about our own Moon. It may seem like just a nearby object in the sky, but in reality there are so many important impacts that the Moon has on our everyday life. Perhaps the most significant impact is the phases of the Moon. The Moon’s phases is where our concept of the month comes from and is a constant beacon of how our sky is changing despite the permanence that often seems so prevailing in astronomy. 29.5 days of constant change – new growing to full; full shrinking back to new. There are other significant impacts that the Moon has on us here on Earth as well, including creating tides and creating that awe-inspiring show in the sky we know as an eclipse. These topics will be the subject of a future article.
As your studies take you metaphorically to the Moon, let us help you. We have skilled astronomers and educators who know space and know the Moon like the back of their hands. Don’t go it alone --- let us help you learn about the Moon, its phases, and all that is has to offer.