The Moon, our neighbour in space (for educators)
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The Moon is part of our daily lives, but it is not easy to represent its movement in space and its position in relation to Earth and the Sun. It's not surprising that it can become confusing at times.
Do you have a hard time explaining concepts related to those movements, such as phases and eclipses? Well, you are not alone! It is not easy, for kids or educators, to visualize these phenomena in three dimensions, even though that is necessary in order to properly understand them.
We will cover a few concepts, which are sometimes misunderstood, related to the visible phenomena associated with the movements of Earth and the Moon (!). We suggest elements that may capture children's attention (magnifying glass icon) and answer a few questions that children often ask (?). Many other questions and answers can be found on the page for youth.
The Moon and Earth in motion
In order to properly visualize the movements of the Moon and Earth within the solar system, children need to develop their visuospatial abilities so they can create a mental representation of the Sun–Earth–Moon system in three dimensions.
Magnifying glass icon to suggest elements that may capture children's attention: Modelling is an essential part of teaching these concepts and can be done in several ways: manipulating objects, using your body to simulate the movements, virtual modelling, etc. 3D animations are provided a little further ahead, and activities are a good way to complement the teaching of these concepts.
Let's start by visualizing Earth and the Moon in space. Earth completes one rotation on its axis in 23 hours and 56 minutes. This rotation causes night and day. When we are facing the Sun, it is day, and when we are on the dark side, it is night. It is because of this rotation that celestial bodies, including the Sun and the Moon, rise in the east and set in the west. This east-to-west movement is an illusion created by Earth rotating on its axis from west to east.
Magnifying glass icon to suggest elements that may capture children's attention: This principle can easily be demonstrated through a motor activity. In this activity, children face a light source, then slowly spin: when they see the light, it is day, and when they don't, it is night.
The Moon revolves around Earth. This orbital movement is called revolution. It takes 27.3 days to complete one revolution. We say that the Moon is orbiting Earth: it is Earth's gravity that keeps it in that movement. Because the Moon is moving fast enough, it does not fall to Earth and remains in orbit.
The Moon's orbit is an ellipsis, but it looks like a nearly perfect circle. The distance between the Moon and Earth varies slightly throughout its orbit. On average, the Moon is 384,000 km from Earth, which is approximately 30 times the diameter of Earth (therefore, Earth could be placed 30 times within the Earth–Moon distance).
Movements of Earth and the Moon. Animation created using WorldWide Telescope. (Credit: Canadian Space Agency [CSA].)
Magnifying glass icon to suggest elements that may capture children's attention: It is easy to create a model of the Earth–Moon distance in a classroom. With a standard globe of Earth (30 cm diameter), the Moon would be the approximate size of a baseball. Ask the children where they think the Moon should be placed for the model to be to scale. They will probably be surprised to learn that the distance is nine metres, approximately the length of a classroom!
!: exclamation point: covering concepts related to the visible phenomena associated with the movements of Earth and the Moon: When the Moon is at its closest point to Earth and it is full, the term supermoon is sometimes used. However, it must be noted that the difference in size in the sky is minimal and very difficult to notice. There are many hoaxes and doctored photos of this phenomenon circulating on social media. Don't fall for it!
The full moon always appears larger on the horizon, no matter how close it actually is to Earth. It's an illusion! To prove it, try this trick and encourage the children to do it when they are looking at the Moon. Whether it is tiny and very high in the sky, or very large on the horizon: stretch your arm out in front of you and try to cover the Moon with your little finger. It works every time!
!: exclamation point: covering concepts related to the visible phenomena associated with the movements of Earth and the Moon: While it moves around Earth, the Moon is just as often on the night side as on the day side of Earth. Contrary to what many believe, the Moon can be seen in the sky during the day!
?: question mark: answers questions that children often ask: You may have heard about the far side of the Moon. What is it? The Moon is always facing the same way toward Earth; this is called the near side. On the other side, there is a side we never see, called the far side.
Near side and far side of the Moon. The two sides have different geological properties. (Credit: NASA/Goddard/Arizona State University)
!: exclamation point: covering concepts related to the visible phenomena associated with the movements of Earth and the Moon: When children learn that the Moon is always facing the same way toward Earth, they may think that it does not spin on its axis. Actually, it does rotate, but its rotation period is exactly equal to the time required to complete one revolution around Earth (27.3 days). This is called synchronous rotation. Many of the moons in the solar system have synchronous rotation and always show the same side to their planet. The best way to understand it is to demonstrate it. Take a sphere and make it spin on its axis (rotation) while making it revolve around an object (revolution). We realize that if the two actions take the same amount of time to complete, the same side is always visible from the centre of revolution.
It must be noted that there is no side of the Moon that always remains in the dark. The far side is lit by the Sun just as often as the near side.
This animation shows the difference between the Moon's actual movement, in synchronous rotation (on the right), and what would happen if the Moon did not rotate (on the left). From Earth, the near side is always visible during synchronous rotation. (The objects in this animation are not to scale, and Earth's rotation speed is sped up for the purposes of demonstration.) (Credit: CSA)
Phases of the Moon
One of the phenomena most often covered in schools is also one of the least understood: the phases of the Moon. It is easy to observe that the Moon appears to change shape every day. This phenomenon can be explained, but not as easily as you may think.
!: exclamation point: covering concepts related to the visible phenomena associated with the movements of Earth and the Moon: You have likely heard that the phases of the Moon are caused by Earth's shadow. However, this explanation is inaccurate: Earth's shadow has nothing to do with the phenomenon of Moon phases. We will come back to this when discussing lunar eclipses.
To properly understand the phases of the Moon, the Sun must be added into our visualization of Earth and the Moon in space.
?: question mark: answers questions that children often ask: How does the Moon shine? The answer to this question is essential to understanding the phenomenon of Moon phases. The Moon does not shine on its own. We can see it because it reflects the light of the Sun. Just like Earth or any other orb placed in front of the Sun, half of the Moon is lit and the other half is in the dark. These are the Moon's night and day. If children can understand that Earth always has one day side and one night side, they can easily extend that understanding to the Moon.
The phases of the Moon, seen from space. Animation created with WorldWide Telescope. (Credit: CSA)
The phases of the Moon, as seen from Earth, are caused by the regular movement of the Moon around Earth. Based on the Moon's position, we can see its day or night side. During a full moon, we are facing the day side of the Moon, whereas during a new moon, we see its night side. All of the other phases exist between these two extremes.
Phases of the Moon. From left to right: waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent. (Credit: Scientific Visualization Studio, NASA)
The Moon goes through each of the phases shown above within a 29.5-day cycle. This cycle is slightly longer than one revolution of the Moon around Earth. To return to the same Moon–Earth–Sun alignment, for instance from one new moon to another (lunation), Earth's movement around the Sun, which adds a few days to the cycle, must be taken into consideration. This cycle is the basis for our calendar.
Magnifying glass icon to suggest elements that may capture children's attention: You can help children visualize the phases of the Moon by asking them to look at a ball when they are outside on a sunny day.
!: exclamation point: covering concepts related to the visible phenomena associated with the movements of Earth and the Moon: Some phases are not as well known as others. A gibbous moon, for instance, represents the step between a quarter moon (when the face is 50% lit) and the full moon (when the face is 100% lit). Even the term "quarter
" can be confusing because we can see half of the lit face of the Moon!
Magnifying glass icon to suggest elements that may capture children's attention: It might be a good idea to ask children to look at the Moon as often as possible throughout the course of one month and to keep an observation journal. This would allow them to observe day-to-day changes, weather permitting. They will probably notice some interesting details, like the fact that the Moon is not visible at the same time every day. Starting with the new moon, the waxing phases can be seen in the evening, from the crescent moon to the full moon. The waning phases are more difficult to see, because they are visible early in the morning. Remind your students to look up at the sky at all hours of the day!
According to popular belief, the full moon has an effect on birth rates and crime rates. Many studies have compared these statistics with the Moon phase calendar. Final result: the full moon does not have any effect.
Eclipses
Once the children understand the phases of the Moon, you can teach them about eclipses, which happen when Earth, the Moon and the Sun are perfectly aligned.
Solar eclipses
Solar eclipses happen when the Moon is between the Sun and Earth, during a new moon. If the alignment is just right, the Moon hides the Sun. A solar eclipse typically lasts for a few hours in a given location on Earth, from the moment that location falls under the Moon's shadow to the moment it is out of the shadow. The moment when the Sun is completely hidden by the Moon is called a total eclipse. Total eclipses last for only a few minutes.
If the alignment is not perfect, it is a partial eclipse because only part of the Sun is hidden by the Moon. There are many more opportunities to see a partial eclipse than to see a total eclipse. There are also annular eclipses, where a ring of light from the Sun remains visible during the peak of the eclipse. This phenomenon is possible because the Moon is sometimes further from Earth and appears a bit smaller in the sky.
Magnifying glass icon to suggest elements that may capture children's attention: A solar eclipse is a spectacle that many people describe as one of a kind. There are even people who travel to see as many eclipses as possible! If you have the opportunity to see one, do not miss it and make sure you encourage the children to watch it, too! On , a total solar eclipse will be visible in certain places in eastern Canada, including at the CSA in Longueuil. We will share the details of this eclipse before it happens.
The solar eclipse on , with the Moon hiding the Sun and casting its shadow on Earth. Animation created with WorldWide Telescope. (Credit: CSA)
!: exclamation point: covering concepts related to the visible phenomena associated with the movements of Earth and the Moon: It is interesting for children to realize that total solar eclipses are the result of a great coincidence. Even though the Moon is actually about 400 times smaller than the Sun, it is approximately 400 times closer to us than our star. The two effects complement each other perfectly, so we see both of them as the same size in the sky. This happy coincidence creates an extraordinary spectacle!
The Moon and the International Space Station passing in front of the Sun during the solar eclipse. (Credit: NASA)
Never look directly at the Sun without appropriate protection; its light is so intense that it could permanently damage your eyesight. That is why you must be very careful when watching a solar eclipse. On a typical sunny day, everyone knows not to stare at the Sun. It's dangerous. The same rule applies during a solar eclipse: never watch it with the naked eye.
Magnifying glass icon to suggest elements that may capture children's attention: Luckily, there are many ways to safely watch solar eclipses and enjoy the show. The best-known method is probably to use eclipse glasses. These are special glasses that block over 99.997% of the Sun's light. Careful: they are very different from normal sunglasses, which do not block enough of the intense light. It is extremely dangerous to watch a solar eclipse directly using normal sunglasses.
Another way of watching a solar eclipse is to project the Sun's light onto a surface, like the surface of an eclipse projector. You can then watch the projected light of the Sun on that surface instead of watching the Sun directly. This type of projector can easily be made at school or at home.
Lunar eclipses
Lunar eclipses occur when Earth is between the Sun and the Moon, i.e. during a full moon. When the alignment is just right, Earth's shadow falls on the Moon and it goes dark. Lunar eclipses last for a few hours, from the moment the Moon falls under Earth's shadow until the moment it is out of Earth's shadow. The phase when it is fully covered by Earth's shadow is called a total eclipse. Sometimes, the alignment is not perfect and the eclipse remains partial the entire time.
At the time of the total eclipse, the Moon takes on a reddish hue. This is because a small amount of the Sun's light crosses through Earth's atmosphere before reaching the Moon. Earth's atmosphere mostly absorbs and diffuses ("disperses
") blue light (that is why the sky is blue!). The light that reaches the Moon has been "filtered
" by the atmosphere and is red.
A lunar eclipse seen from different perspectives. Animation created with WorldWide Telescope. (Credit: CSA)
Magnifying glass icon to suggest elements that may capture children's attention: Ask the children to imagine the view an astronaut on the Moon would have during a lunar eclipse. This astronaut would see a solar eclipse because Earth would completely block the Sun!
?: question mark: answers questions that children often ask: Children often wonder why there aren't eclipses every month, when the Sun, Earth and Moon are aligned. It's because Earth's orbital inclination and the Moon's orbital inclination are not exactly the same. Most of the time, during new moons and full moons, the alignment is not perfect in every aspect. Therefore, there is no eclipse.