Phases of the Moon

We’ve been looking at the Moon for as long as we’ve been living on the Earth.  Our planet’s only natural satellite has been an endless font of fascination, mystery, and beauty.

While modern man is still celebrating its handful of trips to the lifeless lunar surface, the ancients were experts in its path across the night sky.  They diligently observed the wandering celestial body and used it many phases as a calendar.

Nowadays, our phones keep us temporally organized and our numerous screens—handheld and otherwise—hold our attention far better than the Moon.  For most of us, there’s no practical reason to know anything about the lunar phases. 

Still, understanding our orbiting neighbor allows us to relate to our ancestors and see what they saw.  It also tells us more about our cosmos.   After all, our word “month” comes from the word “moon.”

In all phases. View assumed from the center of the earth.  (Geometrically impossible, but best demonstration of  monthly libration caused by moon’s motion By Tomruen 
[Public domain], via Wikimedia Commons

Eight Lunar Phases

Lunar phases refer to the shapes of the illuminated portion of the Moon.  These shapes are caused by the positions of the Moon relative to the Sun as viewed by observers on Earth.

There are four main phases and four intermediate phases.  Each phase refers to the amount of illumination the Sun casts on the lunar surface.

The four main phases of our Moon are new moon, first quarter, full moon, and third quarter. 

For This Article…

Obviously, the Moon is a sphere, but from our perspective on Earth it appears as a disc.  The side of the moon facing us is tidally locked by Earth’s gravity. 

Therefore, we only see about 59 percent of the Moon’s surface.  The other side—the side that’s perpetually facing away from Earth—is colloquially called the “dark side of the Moon.”

For the rest of this article, when we talk about the shapes of the moon we are referring to the two-dimensional perspective of a viewer on the surface of Earth. 

Furthermore, our viewer is standing in the Northern Hemisphere.  For viewers in the Southern Hemisphere, everything is “flipped” 180 degrees.  So, a first quarter moon in the Northern Hemisphere is a third quarter moon in the Southern Hemisphere.

Your latitude will affect the orientations of the shapes of the Moon.  Depending on which hemisphere you’re in, moving toward the equator will cause the Moon to rotate (i.e. the ends of the crescent will open more upward or open more downward). 

At the equator, the lunar terminator (the boundary between the dark hemisphere and illuminated hemisphere) will appear horizontal.

Lastly, the number of lunar phases explained in this article belong to the West.  Other cultures may have a different number of lunar phases. 

Four Main Phases

The names “new moon,” “first quarter,” “full moon,” and “third quarter” can be a little misleading.  A new moon is when the Moon’s surface is dark.  A full moon is when the entire surface is illuminated.

First quarter and third quarter moons are when the lunar surface is half illuminated.  The “quarter” in the name of these two phases doesn’t refer to an amount of illumination, but where the satellite is in its orbit around Earth. 

During the phases of new moon, first quarter, full moon, and third quarter, the Moon’s and the Sun’s ecliptic longitudes differ by zero degrees, 90 degrees, 180 degrees, and 270 degrees respectively. 

Each of the four main phases of the Moon lasts for approximately seven days.  These durations vary because the Moon’s orbit is elliptical.

Another term for a first quarter and a third quarter moon is half moon.

An “old moon” is not a new moon.  It’s a waning sliver—basically the last moments before a new moon.

Four Intermediate Phases 

Between each of the four main phases of the Moon, are the four intermediate phases.  The four intermediate phases are waxing crescent, waxing gibbous, waning gibbous, and waxing crescent.

Crescent and gibbous refer to the shape of the illuminated part of the moon.  “Crescent” is a sickle-shaped figure.  A “gibbous” is a protuberant or a shape that’s greater than a semicircle but smaller than circle.

“Waxing” means the amount of illuminated surface area is decreasing.  “Waning” means the amount of illuminated surface area is decreasing.

For those in the Northern Hemisphere, the right side of the Moon is always waxing.  In other words, if the right side is illuminated, then the Moon is getting brighter.  If the right side is dark, then the Moon is getting darker. 

The reverse is true for the Southern Hemisphere.

Here are the eight phases of the moon in order:

  1. New Moon
  2. Waxing Crescent
  3. First Quarter
  4. Waxing Gibbous
  5. Full Moon
  6. Waning Gibbous
  7. Third Quarter
  8. Waning Crescent

Lunation

After a waxing crescent, the lunar cycle is complete, and it begins again with a new moon.   Each complete cycle is called a “lunation.”  The number of days from the new moon to whatever phase the Moon is currently in is called the Moon’s “age.”

The eight phases of the Moon take 29.53 days to complete.  This duration is called a synodic month. 

It’s possible to have two full moons in a month (a phenomenon that occurs, on average, every 2.7 years).  The second full moon in a month is called a “blue moon.”

Full Moon

A fully illuminated moon appears to last for several days.  Actually, the moon’s surface is 100 percent illuminated for just one. 

The previous and following days have such a high level of illumination that it’s extremely difficult to notice the difference. 

Even two nights before, as well as two nights after, the surface is still illuminated between 93 and 97 percent and may appear to be full.

Libration

If you view a time-lapse video of the different lunar phases, you might notice the Moon getting slightly larger.  This is due to the eccentricity of the Moon’s orbit around Earth—it’s not always the same distance away.

Viewing a time-lapse video may also reveal another phenomenon called libration.  Libration is a perceived wobbling of orbiting bodies.  

This wobbling is caused by the eccentricity of the moon’s orbit around Earth, the slight inclination of the moon’s axis, and the small daily oscillation of the Earth’s rotation.

Top Image Credit:
Diagram illustrating various phases of the Moon in their order of appearance starting from the New Moon and progressing through Crescent, First Quarter, and Gibbous to reach the Full Moon. It is followed by Gibbous, Last Quarter and Crescent to complete full circle at the New Moon again. The gray circle around the Earth shows the Moon’s orbit. The dotted gray line illustrates moon’s trajectory. The solid ivory line passing through the earth is indicative of Earth’s orbit around the Sun. Notice that this graphical representation can be misleading, as the Earth & Moon’s path around the Sun is actually always concave towards the Sun. Reference The Royal Geographical Society – (ISBN 0540084050)
By Fresheneesz~commonswiki via Wikimedia Commons