The moon is one of the celestial objects which act as the natural satellite of the earth. The moon orbits the earth continuously, thereby completing its one orbit around the earth in every 27.32 days. As the moon orbits around the earth, it will seem to appear in different shapes with regarding to different positions in the sky. We may wonder, how the moon actually changes its shape in the sky at regular intervals. The science is, that the moon does not actually change its shape; it all happens due to the position of the moon with respect to the earth (i.e., us) and the sun, which we call it as a phase change.
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Credit (Different shapes of Moon): By Hamed Rajabpour and Nariman Ghorbani - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=67278168 |
The different position of the moon around the earth also makes it to rise at different time above the horizon which makes us to appear at different shapes in the sky. A phase change is the process of the stage of the moon by which it appears to us in different forms. This phase change depends upon the amount of the moon’s surface facing the sunlight with respect to earth. If we receive more amount of sunlight reflected from the moon (i.e., moonlight) we can see the maximum amount of the moon, and if we receive less amount of sunlight reflected from the moon we see less amount of the moon. This phase change happens all over the side of the moon but we can able to see this at only on one side of the moon called the near side; because the moon always faces its one side to the earth. To know, how the moon always faces its one side to the earth please visit my previous blog,
https://sciencetopic03.blogspot.com/2021/11/why-does-moon-always-faces-its-one-side.html
Thus depending upon the amount of sunlight falling on the moon; the appearance of the moon changes to our view. This phase change of the moon happens periodically in the sky. Now let’s look at the process of the phase change of the moon briefly,
Phase Changes of the Moon:
Depending upon the position of the moon with respect to the earth and the sun the phase changes can be named as:
- New Moon
- Waxing Crescent
- First Quarter Half Moon
- Waxing Gibbous
- Full Moon
- Waning Gibbous
- Third Quarter Half Moon
- Waning Crescent
From the below figure we can see that at first the moon is placed in-between the sun and the earth. This position of the moon is called the new moon. In other words, we can say that the side we are facing (near side) is completely dark and the side we never face (far side), is completely lit.
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New Moon Stage |
Thus the amount of sunlight falling on the near side of the moon is zero. When we see the moon at this stage we can’t see it, even though it was directly to our eye. But during this position, the brightness of the sun cast a moon’s shadow towards the earth. So we should be able to experience the moon’s shadow, though we can’t able to see the moon. But many times during the new moon stage, we can’t see the moon or the moon’s shadow. This is because the shadow casted by the moon rarely collides with the earth causing a solar eclipse. A solar eclipse is a type of rare event where the light emitted by the sun is partly or fully covered by a new moon. To know more about the solar eclipses please visit my blog.
https://sciencetopic03.blogspot.com/2021/12/principle-behind-solar-and-lunar.html
The reason why the shadow of the moon rarely collides with the earth is due to the orbit of the moon with respect to the sun, is tilted at an angle of 5°. So usually during the new moon, the moon passes between the earth and the sun without casting its shadow on the earth. So we can’t see the moon or the moon’s shadow. But during an eclipse time, we can able to see the moon’s shadow but not the moon. You can observe a sample solar eclipse in the below image.
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Solar Eclipse |
Now as the day passes on, the moon continuous orbiting the earth. Now at a certain position when we stop and look at the moon we will be able to see a quarter part of the moon and the balance three quarters (3/4) remain in the shadow part. This shape of the moon is called waxing crescent. The point that needs to be noted is, wherever the moon is in its orbit, half the side of the moon is always lit but how it appears to us with respect to various positions matters. We can absorb the waxing crescent in the below picture.
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Waxing Crescent Stage |
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Waxing Crescent |
Next, as the orbit continues, the moon will be completing its quarter orbit around the earth. When we look at the moon now, it will be half-lit and half non-lit; which means half-bright side and half-dark side. Here, both side belongs to the same side of the moon (i.e., near side) which is split as lit and non-lit side. This shape is called the First quarter half-moon.
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First Quarter Half Moon Stage |
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First Quarter Half Moon |
Again as the orbit is made, most parts of the moon will get into the bright side and we will be able to see three-quarters (3/4) of the moon which means a quarter part of the moon will not be visible to us. This makes our view of the moon to appear it as a waxing gibbous.
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Waxing gibbous Stage |
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Waxing Gibbous |
After the moon completes its half of the orbit around the earth, it will be completely behind the earth thereby causing a full moon. In this position we will be able to see, one side of the moon completely. Now the near side will be completely lit and the far side will be completely dark. This process is completely vice versa to the new moon. If chance, we will be able to see the earth’s shadow casted on the moon by the sun. This chance will occur only if there is an occurrence of a lunar eclipse. A lunar eclipse is a process of casting the earth’s shadow over the moon whereas a solar eclipse during a new moon is the process of casting the moon’s shadow over earth.
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New Moon Stage |
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New Moon |
To know more about the lunar eclipses please visit my previous blog.
https://sciencetopic03.blogspot.com/2021/12/principle-behind-solar-and-lunar.html
A lunar eclipse only occurs in the full moon phase and this eclipse occurs rarely like the solar eclipse due to the moon’s orbiting tilt. So usually the moon passes behind the earth reflecting the maximum of the sunlight to us without passing into the earth’s shadow. When the moon passes into the earth's shadow its brightness will be dimmed.
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Lunar Eclipse |
Now again as the orbit continues, the phase that we have seen so far occurs in an opposite way. When the moon is at this position as shown in the below figure we will be able to see three-quarters of the moon; which is named as waning gibbous. In this position, three-quarters of the moon will be on the brighter side and only a quarter part will be on the darker side.
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Waning Gibbous Stage |
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Waning Gibbous |
As the orbiting of the moon continues, the moon completes three-quarters of its orbit and makes the near side of the moon half lit and half dark. This part is called the third quarter half-moon. Again as the moon orbits and makes its position over to the Waning crescent, its 3/4 part is darkened and only a quarter part is left bright.
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Third Quarter Half Moon Stage |
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Third Quarter Half Moon |
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Waning Crescent Stage |
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Credit (Waning Crescent): By Art Lupinacci - email 2015090710009336, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=42965885 |
And at last, the moon comes back again to the position of the new moon stage where it is between the earth and the sun. Now the moon will be completely not visible to us. Thus the cycle continues and phase change happens over and over again. This whole process takes over 27.32 days.
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Different phases with respect to different positions of the moon |
The reason why we can see the moon only during certain days and not many days is due to the position of the moon around the earth. The different position of the moon around the earth also makes it to appear at different locations in the sky, during the same time of different days. From new moon to full moon, the moon rises later than the sun as the earth rotates from west to east. That is, the moon rises at 12pm at the new moon and gets delayed over each day and rises at 1pm, 2pm, 3pm, and so on. We will be able to see the moon on the east side during its cycle from new moon to full moon; when the moon rises over the horizon at evening time. When the moon takes over the cycle from the full moon to the new moon, the moon gets delayed in rising and rises over the horizon at 9pm, 10pm, 11pm, and so on over each day. By one time when the moon gets to reach the new moon phase, the moon rising gets completely delayed and rises together with the sun. So by this time (i.e. when the moon completes its cycle from the full moon to the new moon) when we look at the sky during the same evening time, the moon will be on the west side. This is the magic of the moon’s orbit.
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