Moon Phases as Viewed from Earth

What You See in the Night Sky

Stars do not change their position in relation to each other. This is why you will always be able to find the Big Dipper and other star patterns.

The Sun and Moon appear to move from east to west along a curved curved path across the sky. This imaginary path is called the ecliptic. This apparent motion of these two celestial bodies in the sky is due to the rotation of the earth. The earth rotates (spins on its axis) toward the east. Thus, in the Northern Hemisphere, celestial bodies appear to move along the ecliptic in the southern sky the eastern horizon to the western horizon.

The zodiac is an imaginary band about 16 degrees wide with the ecliptic running through it. The zodiac is divided into 12 section with each division about 30 degrees apart. There are specific constellation in each of the 12 divisions and they remain in the same order year in and year out. These constellation in order as they appear along the apparent zodiac band that circles the Earth are:

Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn

You see only sections of the sky each night. As the earth moves around the Sun, different sections of the sky are visible. YIKES! There are so many different movements of the Earth and the Moon.

What you see in the nighttime sky is different depending on where the Earth is in its orbit around the Sun as well as where you live on the Earth. But alas, I am getting away from the goal of answering a question from Lori about Moon phases. More about the Zodiac will need to be in another article.

Following is the question I received from Lori:

Can you tell me what causes this particular phase of the moon? At times I look up and see the moon is dark on the west side and bright on the east side even though the sun has just gone down, as it does, to the west. In other words, I would expect that the moon would always be bright on the side closest to the sun if it is reflecting the sun.

Also, when the moon is in it’s various phases it is in that phase for the entire day and can be seen in the sky along with the sun at times – and the sun will be on the east side of it in the morning and on the west side of it at night. Yet this doesn’t change the moon phase.

This is a great puzzle for me, and I would appreciate your thoughts on this.

Answer:

Lori,

I find it easier to understand by using a diagram with the Earth in the center and the path of the ecliptic around it. Remember, I am explaining the apparent movement of the Sun and Moon in the sky each night. In the Northern Hemisphere, the part of the ecliptic visible to you is i
n the Southern sky.

First, I want to identify the four primary moon phases, which are: New Moon, First Quarter (half moon), Full Moon, Third Quarter (half moon with illuminated half being opposite that of the First Quarter phase). The primary moon phases last for a short period of time. This is because the Moon never stops in its path around the Earth. This revolution of the Moon occurs every 29 to 30 days and is called the Lunar Cycle. As the Moon revolves around the Earth, in relation to the Sun, the Moon moves about 12.4 degrees each day, which is 1 degree every minute. The Sun’s apparent diameter in the sky is 0.5 degrees. Thus, the New Moon phase only lasts for about seconds (one-half a minute). But, the New Moon appears to last longer because of the brightness of the Sun’s light, which prevents us from viewing the Sun during the day.

The other primary moon phases also lasts for instant before changing into one of the two intermediate phase, Crescent and Gibbous. The intermediate phases last for about 7.25 days before instantly changing into one of the primary phases. When the illuminated part of an intermediate phase disc increases, the moon phases is said to be waxing. When the illuminated part of an intermediate phase disc is decreasing, the moon phases is said to be waning.

When an intermediate moon phase is waxing, the illuminated part of the moon disc is increasing.
When an intermediate moon phase is waning, the illuminated part of the moon disc is decreasing.

You can identify a waxing or waning moon phase by the side of the moon disc that is illuminated. The illustration shows two examples of a crescent moon phase. In the Northern Hemisphere, the illuminated part of a waxing moon is on the right side. In the illustration, the waxing crescent moon is the first one shown.

Can you identify the waxing gibbous moon in the illustrations to the left? FYI: Half of a Gibbous Moon is illuminated.
Think! Which side of the Moon disc is illuminated, left or right? Ans. The first moon illustration has its left side illuminated, thus it is a Waning Gibbous Moon and the remaining illustration is the Waxing Gibbous Moon.

Now let’s determine why some of the moon phases are waxing and other are waning.

All moon phases from

Now I’ll add the Moon to the diagram and you can see the relative positions of the Sun and Moon during different moon phases.
The New Moon is generally represented by a black circle. But the truth is that you never see the New Moon phase because the Moon is between you and the Sun. Thus, the Moon and the Sun appear to move across the sky together. The light of the Sun prevents you from seeing the Moon.

The New Moon rises with the Sun, travels across the sky from east to west and sets with the Sun.

CAUTION: Never look directly at the Sun. Direct sunlight can damage your eyes and even blind you.

The New Moon

The New Moon is between the Earth and the Sun. Thus:

The New Moon Rises with the Sun
The New Moon Sets with the Sun
The New Moon is not visible in the daytime nor nighttime sky.

The First Quarter Moon Phase

The Moon revolves around the Earth. The time for one complete revolution is called the Lunar Cycle, which takes about 29-30 days. About 7.5 days after the New Moon Phase, when the Sun is at its zenith (highest point, which is noon), the First Quarter Moon phase rises above the eastern horizon. Both the Sun and Moon move across the sky, with the Moon following at about 90 degrees behind the Sun. The illuminated side of the Moon faces the Sun.

The First Quarter Moon rises at noon
The First Quarter Moon is at its zenith when the Sun sets.
The First Quarter Moon sets at midnight.
In the Northern Hemisphere, the right side of the First Quarter Moon phase is illuminated.
The First Quarter Moon phase is visible in both the daytime and nighttime sky.

The Full Moon Phase

About 14.5 days after the New Moon phase, the Moon has moved 180 degrees from the Sun. Thus, the Moon is opposite the Sun in the sky resulting in the side of the Moon facing Earth to be fully illuminated.

The Full Moon rises when the Sun is setting.
The Full Moon is at its Zenith at midnight.
The Full Moon sets as the Sun is rising.
The Full Moon is not visible during the daytime. The Full Moon is only visible from sunset to sunrise.

From the New Moon phase to the Full Moon phase, the Moon appears to be following the Sun across the southern sky. Thus, the Moon is to the east of the Sun and the the west side of the Moon is illuminated. Also, the moon phases are waxing, which means the illuminated part of the Moon visible to viewers on Earth is increases as little each day. From New Moon to Full Moon, the revolution of the Moon along its orbital path moves the Moon farther away from the Sun until the two celestial bodies are opposite each other. After the Full Moon phase, the Moon continues on its orbital

During each lunar month:

the Sun remains in the same place and the Earth-Moon system move about 1/12th of the way around their orbital path around the Sun
the Earth has rotated on its axis 29 times
the Moon has revolved around the Earth one time

The Third Quarter Moon Phase

About 21.75 days after the New Moon phase, the Moon has moved 270 degrees around its orbital path.

After the Full Moon phase, the Moon continues to move in its orbit, which results in the Moon moving toward the Sun. The side of the Moon facing the Sun continues to be illuminated. To a viewer on Earth, the illuminated side of the Moon during the Third Quarter Phase is opposite of that of the First Quarter Moon Phase.

The Third Quarter Moon rises at midnight
The Third Quarter Moon is at its zenith when the Sun rises
The Third Quarter Moon sets at noon

After 29 days, the Moon is again in front of the Sun and thus the New Moon starts another Lunar Cycle.

For each numbered position of the Moon, can you name and make a drawing of the different moon phases by and observer on earth?

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