Why Does The Moon Have Phases? [The Scientific Explanation]

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When you look up at the Moon, you may notice that it regularly appears differently. Sometimes it appears whole and spherical, other time it may appear as a small crescent. This is because the Moon has various phases, which all lead to an apparent changing of its shape and structure.

But Why Does The Moon Have Phases? The Moon is not changing in shape nor structure. Instead, it is our view of the Moon here on Earth that is changing the perception of light due to the shifting positions of our planet, the Moon and the Sun. 

What Causes The Phases of the Moon?

There are common misconceptions around what causes the phases of the Moon. Here are just some of the main ones:

  1. The Earth Blocks the Light that comes from the Sun
  2. The Crescents are caused by shadows of the Earth from the Sun onto the Moon
  3. If the Earth is between the Moon and the Sun, we will not see the Moon.

None of these are actually scientifically correct, lets actually look at what is happening.

Before we can actually understand the phases, we need to know a couple of things.

Firstly, there is only one source of light in the solar system and that is the Sun (which is at the center of the Solar System). The Sun produces all of the light. So both the Earth and the Moon are half illuminated by that one source of light. As the Moon moves around the Earth, our perspective of it changes.

It is therefore our perception of the Moon that provides the various faces.

In total, there are 8 distinct phases of the moon, which occur at different times when the Moon moves around the Earth. Here’s a closer look at them.

Different Phases of the Moon

It is the orbiting of the Moon around the Earth that gives us the impression that the Moon is changing size and shape in the sky. The result is a perception of different angles of light that shine on the Moon’s surface. These are what we call the “Moons Phases”. Of course, the Moon does not create any light by itself, instead it is reflecting the light of the Sun.

Now, the eight phases will always follow in succession to one another, and follow the same pattern and order. Below are the names given for the eight predominant phases of shape and a description what you would expect to see for each.

New Moon: This is when the lighted side of the Moon faces away from the Earth. It will therefore appear dark and ‘unlit’ as the Moon is between us and the Sun.

Waxing Crescent: This is where you start to see the backward C like shape. The crescent begins small and grows larger each day/night, until it reaches the First Quarter Moon.

First Quarter: Here, the right side of the Moon looks illuminated, the left side in darkness. The lighter part of the Moon grows each day until it reaches the Full Moon phase.

Waxing Gibbous: “Waxing” simply means increasing or expanding, so this phase is essentially when the stage in between the First Quarter and Full Moon.

Full Moon: This occurs when the lighted side of the Moon faces the Earth directly. This occurs when the Earth, Sun, and Moon are close to being in a straight line (whereby the Earth is in the Middle). This is also the Moon that appears most bright from the reflecting sunlight.

Waning Gibbous: “Waning” simply means decreasing so this is the exact opposite of the Waxing phase. It essentially means the illuminated Moon Crescent becomes smaller day by day.

Last Quarter or Third Quarter: This occurs when the left side of the Moon is illuminated yet the right is covered in darkness. The lighter part reduces day by day, night by night until it reaches the New Moon phase where the process begins again.

Waning Crescent: This Moon occurs following between the Last Quarter and New Moon. This is the period where the crescent reduces until the New Moon phase is reached.

Different Phases of the Moon

Why Doesn’t The Earth Block The Suns Light?

Now, its logical to question why the Earth does not block the Suns Light so that we cannot see the Moon at all.

The thing is, you would expect this to happen at each full Moon. It makes sense that Moon would go
through the Earth’s shadow.

But here is why this does not happen.

The Moon’s orbit is actually tilted slightly (around 5 degrees) to the orbit of the Earth around the Sun. So
virtually every time you see a full Moon, the Moon is moving just above or just below the shadow.

What Is A Total Lunar Eclipse?

A Total Lunar Eclipse occurs when the Moon is in the Earth’s shadow which it passes through. When in the middle of the shadow, the Moon turns red in color. This is for several reasons:

  1. Refracting: The Earth’s atmosphere is refracting the light into towards the Moon (like a Prism)
  2. Scattering: The blue light is being scattered out by the Earth’s atmosphere, just like a sunset.  This causes the red light to be left preferentially to get through to the Moon.
  3. Dimming: Dust, particularly Volcanic Dust, in the Earth’s atmosphere dims the light which causes a bright or darker red color on the face of the Moon.

Lunar Eclipses are incredibly important to us from a scientific perspective because they enable us to observe and identify other planets orbiting around other stars (these are known as Exoplanets or planets beyond our Solar System).

If we can measure the light that’s going through Earth’s atmosphere and reflecting off the moon, it can give
us an idea of what we might expect to see if the light is coming through the atmosphere of an Exoplanet orbiting
around another star.

So we can use the observations of a Lunar Eclipse around the Earth to infer what the atmosphere of a Exoplanet may be like.

So we can detect things like Ozone or Carbon Dioxide in Earth’s atmosphere by looking at the spectrum that reflects off the Moon.

If we see a similar spectrum when we look at a Exoplanet around another star, we could infer there is Ozone, Carbon Dioxide, Water Vapor or Methane in that atmosphere.

All of these elements together may suggest that there is other life out there in the Universe!