Moon Phases :: Earth, Sun, and Moon Geometry

The Moon is not a light-generating body, meaning that she doesn’t actually produce any of her own light. All of the light that the Moon appears to illuminate our sky with, which we call moonlight, isn’t actually moonlight at all, but sunlight reflected off the lunar surface that is 450,000 times less intense than direct sunlight, due to this reflection. So then where do the Moon phases come from? They come from the the Sun, from the Moon’s orbit around the Earth, and from the specific perspective that we are observing the Moon from.

Moon Phases :: Earth, Sun, and Moon Geometry

As you can see in the diagram below, the Moon orbits the earth in a counterclockwise direction (looking down upon the system from above the North Pole) taking 29.53 days to complete 1 revolution around the Earth, which we call a lunar month. At the same time as the Moon is orbiting the Earth, the Earth is orbiting the Sun also in a counterclockwise direction, which as you know takes about 365.25 days.

Over the course of one lunar month the position of the Moon in relation to the Earth and the Sun changes. Once a month the Moon will be behind the Earth being pulled along by the Earth in its orbit (which is to the right in the diagram below), locked in place by our mutual gravitational attraction. Then about 7 days and 9 hours later the Moon will be on the far side of the Earth opposite the Sun. About 7 days after that the Moon will move into a position “in front” of the Earth in the direction of our motion around the Sun, and 7 days later the Moon will come directly between the Earth and the Sun, at which point the lunar cycle will repeat itself.

These four positions are the 4 major alignments of the Sun-Earth-Moon system. There are about 7 days 9 hours between them, which adds up to a total of ~29.53 days (as there are 4 of these periods in 1 orbit of the Moon) which is where we derive the length of the lunar month. This can all be seen if you take a moment to study the diagram below (Fig. 1).

[In order to get the most out of this discussion of what moon phases actually are, the order that we perceive them in, and why, I would suggest opening Fig. 1 or Fig. 3 in a new tab, or saving the picture to your computer so that it is handy for you to refer to as we go through this article. It will make everything clearer as we proceed.]

Fig.1 – A great diagram of the Moon revolving around the Earth as the orbits the Sun. Image Author: Orion 8 [Wikimedia Commons]

Night and Day on the Moon

The Moon is always 50% illuminated by the Sun. All of the planets and moons of our solar system are hanging in space, gravitationally locked to one another, to the Sun, and to whatever the most massive and/or nearest body is to us. Therefore there is nothing blocking the Earth or the Moon from the Sun’s rays (except during eclipses) meaning that half of each planet in our system is always illuminated, and half always in darkness. In other words, one half in a state of night, the other half in a state of day.

This is true for the Moon as well, as Fig. 2 below illustrates. Therefore the moon phases are not a product of the Sun illuminating portions or slivers of the Moon at a given time, because half of the Moon is always illuminated (except during lunar eclipses), nor are moon phases the result of the Earth’s shadow upon the moon’s surface. It is the position of the Moon as in the four major alignments of the Earth, Sun, and Moon as I described above, which result in the apparent Moon Phases that we perceive, which tell us very specifically where the Moon is in relation to the Earth and the Sun, and can even tell us fairly specifically where the Sun is even if it is below the horizon somewhere on the opposite side of the Earth, just by looking.

Fig. 2 – The above image displays how the Earth and the Moon are both consistently half-illuminated by the light from the Sun.

The Moon Phases

There is no ‘beginning’ to the lunar cycle, so there is really no moon phase that we can make an argument for as being ‘first’. However it is conventional to consider the New Moon phase as the start of the lunar cycle, so let us begin our exploration of the cycle of moon phases from the New Moon phase, as that is also where we traditionally measure the lunar month from.

As you can see, there is an angular measure of the lunar phases in degrees in the table above. These values are measured looking down on the Earth-Moon system, and measuring 0°/360° as the direction towards the Sun, then from that starting point of our reference system, 90° is counter clockwise to the left, 180° as the direction opposite the Sun, and 270° being in the direction of the Earth’s orbit around the Sun, and then back to 0°/360°.

Primary and Intermediate Moon Phases

The Moon Phases are divided into 8 phases: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Third Quarter, Waning Crescent, and back to New Moon. These 8 moon phases can also be further divided into 4 primary phases and 4 intermediate phases.

The primary phases are the major positions of alignment of the geometry of the Earth-Moon-Sun system. They are the alignments of: New Moon (0°/360°), First Quarter (90°), Full Moon (180°), and Third Quarter (270°). They differ from the intermediate phases because they are moments of alignment which occur specifically at the Sun-Earth-Moon angles noted at above.  Between the primary phases there is about 7 days 9 hours (7.38 days) which is the origin of our 7 day week, which, along with our months, were once based on lunar calendars, and are truly ancient divisions of time going back to the earliest of cultures: the Sumerians.

The intermediate phases on the other hand have no specific angle where an alignment is reached, but rather a transition period between the major phase alignments. That is why the angles in the table above for the intermediate phases are ranges of angles between each primary phase. This will become clearer as we move forwards.

Fig 3 – The 8 Major Moon Phases of the Lunar Cycle. Image by Andrei Stroe [Wikimedia Commons]

New Moon

The New Moon is the name for the moment in the Moon’s orbit around the Earth when the Moon and the Sun are in perfect alignment (from geometric center to geometric center) as viewed from Earth. In other words, when the Moon is in the daytime sky and reaches its zenith, its highest point in the sky, when the Sun also reaches its zenith, that is, at noon.

Technically speaking, this moment of crossing is called the New Moon. During the few days around the New Moon, the Moon will be rising and setting with the Sun – rising at around 0600, reaching zenith at 1200, and setting at 1800 – and these two bodies will move together through the sky although with a slight lag, as each day after the New Moon, the Moon will appear to move away from the Sun roughly towards the left. That is why it is also called conjunction, which is a word that refers to two things “joining or coming together in space or in time”. In this case it is implying that the Sun and the Moon pass one another in alignment.

This phase is not actually visible most of the time firstly because it happens at Midday when the Sun is highest in the sky and thus veils the Moon in its glare, but also because the Sun is illuminating the dark side of the Moon which is never visible from Earth, so no reflected light from the lunar surface will reach us. During the New Moon the Moon is for all intents and purposes invisible to us. The best way to describe this phase and what is meant specifically by conjunction, is in one of the most brilliant examples of phenomena that occur at New Moon conjunction: the solar eclipse.

A Total Solar Eclipse.
Image Author: Ipicgr (Pixabay)

Waxing Crescent

After the New Moon the Moon continues on its path counterclockwise around the Earth, appearing to move left across the sky away from the Sun day by day, moving across the sky at about 14° of arc per 24 hours (which is about the width of your fist at arm’s length). Because the angle between the Sun and the Moon is now getting larger, we begin to see a sliver of light from the day side of the Moon because of this change in perspective. This phase is loosely called the waxing crescent.

Waxing is a word which means to “cover”, “increase” or “get bigger”. Therefore a “waxing crescent” literally mean’s something along the lines of “the crescent that is getting bigger”, or in other words, the “increasing crescent of light” that gets larger and larger over a two week period towards Full Moon. As you can probably guess, everything between New Moon and Full Moon is the waxing part of the lunar cycle.

During this phase the Moon is essentially visible all day from shortly after sunrise, and remaining in the sky later and later as we move towards First crescent and Full Moon because the angle between the Sun and Moon in the sky is increasing because of the Moon moving away from the Sun in its counterclockwise orbit around the Earth (See Fig. 1).

Because the New Moon is essentially invisible, it is not surprising that some cultures refer to the New Moon phase as the period when the Moon appears visible as the tiniest of slivers on the left-hand side of the Sun a day or two after the moment of conjunction (what we call the first sliver of the intermediate waxing crescent phase).

First Quarter

First quarter is the first primary phase after New Moon. The Moon moves through the sky about 14 degrees every day, which means that after about 7 days and 9 hours the Moon will have traveled 90 degrees across the sky.

This is called the First Quarter phase because technically speaking we see 1/4 of the Moon illuminated, but it is common to call these Half Moon’s because half of the visible face is illuminated. At this moment through the lunar cycle the Moon is technically behind the Earth in its orbit around the Sun, on the direct opposite side as the Earth’s direction of motion. And it is 90 degrees away from the Sun in the sky.

During the First Quarter phase the Moon will be visible low in the Eastern sky around Noon, high in the sky at around 1800, and it will set in the Western sky around midnight. So it will be fairly prominent all day.

Waxing Gibbous

After passing through primary First Quarter phase, the Moon enters its intermediate transition phase of Waxing Gibbous. This is the period after First quarter when the illuminated face on the Moon becomes convex (appearing to bulge from the nice line of light and shadow it made at First Quarter) until just before it becomes a Full Moon.

During this phase the Moon will be rising from just after Noon and setting just after Midnight, rising and setting about 50 minutes later each night. At First Quarter the Sun is separated from the Moon in the sky by 90°, which during the waxing gibbous phase changes from 91° to 179°, technically speaking, because the Moon-Sun separation in the sky at 90° and 180° mark the alignments of the primary phases, which is the distinguishing factor between primary and intermediate phases. As this angle between the Sun and Moon increases due to the Moon’s movement away from the Sun towards the opposite side of the Earth, we can see more of the illuminated face of the Moon (which is whatever face that’s facing the Sun) which is why the illuminated face of the Moon appears to be getting bigger.

Full Moon

Once the Moon travels in its orbit until it is on the opposite side of the Earth from the Sun (please refer to the diagram of your choice), ultimately culminating in that 180° of separation from the Sun in the sky, where the Moon is at its furthest point away from the Sun in that lunar cycle with the Earth directly between, this is when Full Moon occurs.

Once again, the Full Moon is a moment of alignment. It is the moment of opposition, when the Moon is precisely on the opposite side of the Earth from the Sun. This is when lunar eclipses occur, such as the one that just happened in January 2018, because the Earth was positioned just right between the Sun and the Moon at that time so the Earth cast it’s shadow upon the Moon. At Full Moon the Moon rises at 1800 (6:00 PM) and will set at 0600 (6 AM), reaching its zenith (highest point in the sky) at midnight. However, the exact moment of full moon won’t necessarily occur at midnight in your location.

Around the Full Moon phase the Moon is predominantly visible at night. It rises in the evening, and sets early in the morning. Visually speaking, it is not always easy to tell which night/day the Full Moon occurs on. Usually when I see a Moon that appears to be full, I will consider it as being Full Moon ±1 day.

Waning Gibbous :: Returning To The Sun

Now that we have explored the first half of the lunar cycle, the second half will be far easier to understand because it is the same moon phases just in reverse, because the Moon is moving in its orbit around Earth back towards the Sun. Thus it will start off Full, and as it moves closer and closer towards the Sun, the angle which we can view the illuminated sun-ward lunar face becomes smaller and smaller (wanes) transitioning from Waning Gibbous, to Third Quarter, Waning Crescent, and finally New Moon again where the cycle begins anew once the Moon crosses the vertical axis that the Sun is on.

Continuing on from this Full Moon, when the Moon rises at 1800, reaches zenith at 000, and sets at 0600, the Moon will continue rising about 50 minutes later each day, moving about 14° of arc every 24 hours. Because the Moon is moving closer to the Sun, its illuminated face is becoming less visible to us because we cannot any longer see it straight on like we could at Full Moon, thus the light appears to shrink.

Third Quarter

Once the Moon reaches 270° this is the primary phase of Third Quarter. Now the Moon is directly in the path of the Earth’s orbit around the Sun in the direction of our motion. At Third Quarter, the Moon once again appears to be half full, but this time the illumination waned from more to less, or from gibbous to quarter, as opposed to from crescent to quarter. The Moon will rise at midnight at Third Quarter, reach zenith at 0600, and set at noon. So as the Sun rises the Moon will be around its highest point in the sky, and as the Sun reaches zenith, the Moon will be setting over the western horizon. During this phase the Moon will be visible predominantly during the daytime.

Waning Crescent: The Cycle Resets

After Third Quarter, our perspective on the illuminated lunar surface becomes smaller and smaller as the Moon appears to chase down the Sun in the sky, approaching it from the right (moving left across the sky towards the Sun), gaining on it by about a fist’s width each day.

As we approach New Moon, the Moon will rise closer and closer to 0600 meaning that it will now be up for most of the day, and will be seen as a sliver close to the Sun on its right-hand side, with the sliver getting smaller and smaller the closer that it gets. Until eventually it will be too close to the Sun to be visible. Yet if you are vigilant and determined, you can catch your last glimpses of the thinnest of waning crescents if you look on the horizon at dawn just above where the Sun will rise.

Moon Phases Forever

Once the Moon crosses the vertical axis that the Sun is on, either below, above, or directly in front of the Sun (as in a solar eclipse) the cycle resets and begins anew.

This lunar cycle will continue for as long as the Sun is shining. For as long as the Earth is orbiting the Sun, and for as long as the moon is still hanging in the sky orbiting the Earth, they will continue this perpetual dance through the moon phases that is the lunar cycle. I find it worth mentioning that all of these dynamics could not exist without a spherical Moon, Earth, and Sun.

These moon phases are complex and beautiful dynamics of light and shadow, and of perspective. They have been occurring since before humans were on Earth, and they will continue long after we are gone.