Earth, Moon, And The Sun

Hello students, welcome to today's science class. I'm so happy to see all of you here today. Today we're going to learn about something really wonderful and fascinating — we are going to talk about our Earth, our Moon, and the Sun. This is Chapter 12 of your science textbook, and I promise you, by the end of this lesson, you will look at the sky in a completely different way. You will understand why the Sun rises and sets, why we have different seasons, and what happens during an eclipse. So let's begin our journey into space, shall we?

Now students, let me start with a story. Imagine a 12-year-old girl named Rashmika from Kanniyakumari in Tamil Nadu. Every morning, she used to cycle to school, and she noticed something very interesting. She saw that the shadows of coconut trees were very long in the morning, but by afternoon, when she was returning from school, the shadows had become much shorter. She thought about this and realized that the size of shadows changes because the Sun moves across the sky during the day. But then she remembered something she had learned in her previous class — that the Earth moves around the Sun. So she got confused. She wondered — does the Sun move in the sky? Or does the Earth move? Have you ever thought about this, students? Today we will answer this question together.

Let's start with Section 12.1, which is about the Rotation of the Earth.

Now, you all know that the Sun rises in the East and sets in the West. But have you ever wondered why this happens? Let me help you understand this with a simple activity that you can actually try at home or in your school playground.

Imagine you are sitting on a merry-go-round at a park. Ask someone to turn it slowly in the anti-clockwise direction. Now, while you are sitting on this moving merry-go-round, look around you. What do you see? Do the objects around you appear to be moving? In which direction do they appear to be moving?

When you turn in the anti-clockwise direction, the objects around you appear to turn around you in the opposite direction — that is, in the clockwise direction. This is exactly what happens when we view from the Earth. The Sun appears in the East, moves across the sky from East to West, and disappears in the West. But does this mean the Sun is actually moving in the sky? Or is it that the Earth itself is turning around, and the Sun just appears to move?

The fact is, students, the Sun appears to be moving because we view it from the Earth, which is actually turning around itself. This turning of the Earth around its own axis is called rotation.

Now, let me explain how the Earth rotates. Have you ever watched a top spinning around its spindle? Or a fan spinning? Or have you tried spinning a ball? All these objects spin around an imaginary line passing through their center. In the same way, the Earth also spins on its own axis in space. The Earth's axis of rotation passes through its geographic North Pole and the South Pole. The Earth completes one rotation in about 24 hours. That's why we have a day of 24 hours, students!

Now, when viewed from the top of the North Pole, the Earth is rotating in the anti-clockwise direction — that is, from West to East. This is very important to remember. The Earth rotates from West to East.

Let me give you another activity to understand this better. Take a globe to represent the Earth. Place a small sticker to mark your location on it — let's say we mark Delhi or Mumbai or your own city. Now, while viewing from above the North Pole, slowly rotate the globe on its axis in the anti-clockwise direction. Observe how your location turns around and finally comes back to its original position, completing one rotation. This is exactly what happens in real life. When you complete one full rotation, one day has passed.

So students, let me recap what we have learned so far. Rotation is the motion of an object in which all its parts move in circles around an imaginary line that passes through it. This line is called the axis of rotation. The Earth rotates on its axis from West to East, and it takes about 24 hours to complete one rotation. This rotation causes the Sun to appear to rise in the East and set in the West.

Now, let's understand how day and night occur due to this rotation. For this, we need to do another activity. Use a torch to represent the Sun. Go to a relatively dark room. Now shine light from the torch, placed at some distance — say about 1.5 metres — on the globe. What do you notice? You will see that half of the globe receives light from the torch, while the other half stays dark. It is daytime in the half that receives light, and nighttime in the other half.

Now, in India, sunrise first occurs in the eastern part and then in other parts. While looking at the eastern part of India on the globe, rotate the globe in one direction and then in the opposite direction. Can you tell me in which direction the globe should rotate so that light falls on the eastern part of India first? The light falls on the eastern part of India first when the globe rotates from West to East with respect to the North-South axis of the globe. This confirms that the Earth rotates from West to East.

Now, while rotating the globe from West to East, observe your location. Does it go through a cycle of day and night? Yes, it does! Sunrise occurs as your location moves into light, and sunset occurs as it moves into darkness.

So students, the Earth's rotation from West to East causes the day-night cycle. The side facing the Sun experiences daytime, while the other side is dark and experiences night. This is why when it is day in India, it is night in the United States of America. That's why when Abhay in India was having lunch, his uncle in the USA was sleeping because it was nighttime there. We'll discuss this more when we solve the exercises.

Now, imagine you are standing on the Equator on the Earth, watching the sky during one rotation of the Earth while it rotates from West to East. What will you observe? Let me tell you what happens.

In the morning, when you face North with your back to the South, the morning Sun appears in your view to your right in the East. Around noon, you have rotated to a new position where the Sun is now over your head. By evening, you have rotated further. The Sun has started to disappear from your view to the left, towards the West. And then the stars become visible in the night sky.

Due to the rotation of the Earth, the Sun appears to rise in the eastward direction, move across the sky from East to West, and set in the westward direction. Then the night begins, and the stars become visible in the sky.

Now students, I want to tell you something really interesting. Since the Earth is rotating, shouldn't the stars also appear to move in the sky like the Sun? Yes, indeed! Let us look at the stars in the night sky to see the effect of Earth's rotation.

Here's an activity you can try. On an early evening between March and May, identify the Big Dipper, which is also known as Saptarishi, and the Pole Star, which is called Dhruva Tara in Hindi, if visible. Note down your location and date of your night sky observations. Draw the orientation of the Big Dipper in the sky with respect to the Pole Star, or a fixed tree or building on the ground in a direction towards the Big Dipper if you cannot see the Pole Star. Mark the time of your observation along with your sketch.

After two hours, observe the Big Dipper again. Has it moved? Again, draw its orientation and note down the time. Repeat this step after two more hours. What do you observe? You will see that the Big Dipper appears to move around the Pole Star. This is because the Earth's axis of rotation points very close to the Pole Star in the Northern Hemisphere. Therefore, the Pole Star appears nearly stationary in the sky from the Earth. All the stars appear to move around it. Just like the Sun, the Moon also appears to rise in the eastward direction and set in the westward direction because the Earth rotates from West to East.

Now students, let me tell you some fascinating facts. In the nineteenth century, a scientist named Leon Foucault used a long pendulum to give the first simple demonstration of the Earth's rotation. This pendulum is known as the Foucault pendulum in his honour. It consists of a long string with a heavy bob, suspended from a high ceiling. And here's something really cool — a Foucault pendulum with a length of 22 metres has been hung from a skylight in the Constitution Hall of the new Parliament building in New Delhi, India! It symbolises the integration of the idea of India with the vastness of the cosmos. So next time you visit Delhi, do ask your parents to take you to see it!

Also, ancient Indian astronomers, including Aryabhata, had noticed the daily apparent motion of celestial objects such as the Sun, Moon, planets, and stars. Aryabhata was a famous mathematician and astronomer of ancient India who wrote an important treatise called Aryabhatiya around the fifth century CE. He explained the apparent motion of stars due to the rotation of the Earth. His stated value for the time taken by the Earth to complete one full rotation about its axis is around 23 hours 56 minutes 4.1 seconds, which is impressively close to the currently accepted value. Isn't that amazing, students? Indian scientists were making these discoveries over 1,500 years ago!

Now, some of you might have noticed that the stars and constellations that appear in the East at sunset change during different months. You were told to look for certain stars and constellations at certain times of the year in Grade 6. But why do different stars appear in the night sky over the course of a year? This brings us to our next section — Section 12.2: Revolution of the Earth.

While rotating on its own axis, the Earth also revolves around the Sun. This movement is different from rotation. Revolution is the motion of an object around another object. The path an object takes while revolving around another object is called its orbit. If viewed from the top, the orbit of the Earth around the Sun is nearly circular. The Earth completes one revolution around the Sun in about 365 days and 6 hours. That's why we have a year with 365 days, and every four years we have a leap year with 366 days to account for those extra 6 hours each year.

Now, let's understand how the changing view of the night sky is related to the Earth's revolution. Every evening, the Sun sets in the westward direction, and the night sky becomes visible. We know that this occurs due to the Earth's rotation. But as the Earth also revolves around the Sun continuously, the stars seen in the night sky after sunset gradually change over a year as we look in different directions. You can notice this change by looking at the pattern of stars at a fixed time of the night on days separated by a month.

Now, let's talk about something that affects all of us — seasons! You must have noticed that we go through a cycle of seasons every year. Have you ever wondered why we have summer and winter? Is it related to the revolution of the Earth around the Sun in some way? You might have also noticed that days are longer in summer than in winter.

Here's the answer, students. The Earth's axis of rotation is not upright with respect to the orbit, but is tilted. The Earth maintains this tilt as it orbits around the Sun. This tilt of the Earth's axis and the spherical shape of the Earth gives rise to seasons. Let me explain how.

In June, the Northern Hemisphere is tilted towards the Sun while the Southern Hemisphere is tilted away from the Sun. Due to the spherical shape of the Earth's surface, a given amount of sunrays are spread in a smaller area in the Northern Hemisphere compared to the Southern Hemisphere. So that area is heated more. Further, the Northern Hemisphere receives sunlight for more than 12 hours in June. So, the Northern Hemisphere experiences more intense sunlight, which lasts for a longer time, causing the summer season.

In December, the situation is opposite in the Northern Hemisphere. It is tilted away from the Sun, so it experiences winter season with sunlight for a shorter time. The seasons and length of daytime are reversed in the Southern Hemisphere as compared to the Northern Hemisphere. There, it is winter in June and summer in December.

Now, let me tell you about some important days. In the Northern Hemisphere, the longest day occurs around 21 June. This is known as summer solstice. After the summer solstice, the duration of a day becomes shorter while that of a night becomes longer. The shortest day and longest night in this Hemisphere occur around 22 December, known as the winter solstice. Around 21 March and 23 September, the daytime lasts for 12 hours. In the Northern Hemisphere, these days are called the spring and the autumn equinox, respectively.

At the North Pole, the Sun rises in the East direction on the equinox day — 21 March — and remains continuously in the sky for six months. The Sun sets on 22 September. The South Pole experiences the opposite behaviour. The polar regions thus experience continuous sunshine for six months followed by a six-month period of darkness.

On the equator, there is always 12 hours of sunlight and 12 hours of darkness. There is little difference in the intensity of the sunrays falling on the equator in different months. Thus, for the southern states of India that lie close to the equator, like Kerala and Tamil Nadu, the effect of seasons is not very prominent.

Now students, I want to clear a common misconception. Some people think that seasons occur because the Earth is closer to the Sun in summer and farther in winter. But this is not true! The difference in distances is very small and is not the reason for seasons. In fact, the Earth is closest to the Sun in January, which is winter in the Northern Hemisphere! So remember, seasons occur because of the tilt of the Earth's axis of rotation and its spherical shape, not because of the distance from the Sun.

Now let's move on to Section 12.3: Eclipses.

Day and night cycle, seasons, the life on the Earth — so much is dependent upon the Sun. Could the light from the Sun get blocked by the two planets which are revolving between the Earth and the Sun? The planets Mercury and Venus appear very small compared to the Sun and never block the entire light from the Sun reaching us. However, you may be surprised to know that the Moon can do that! The Moon is a natural satellite of the Earth, and it revolves around the Earth as the Earth revolves around the Sun.

Let's first understand about solar eclipses. At certain times, the Moon can come in between the Sun and the Earth in a way that obstructs the light from the Sun from reaching us. This is known as a solar eclipse. You may wonder how the Moon, which is smaller than the Sun, blocks the light coming from the Sun that we see in the sky.

Here's an activity to understand this. Ask your friend to stand in front of you at a distance of about 5 metres. Consider his head to be the Sun. Now close one eye and show a thumbs up with your outstretched hand towards your friend. Are you able to cover the entire head of your friend with your thumb?

You can cover the entire head of your friend with the help of your thumb, even though your thumb is much smaller than the actual size of your friend's head. How could it be? The size of any object as seen by your eye — also known as apparent size — depends upon both its actual physical size and its distance from you. The thumb being much closer to you compared to your friend, the apparent sizes of your thumb and your friend's head as seen by you are similar.

The apparent sizes of the Moon and the Sun in the sky are similar when viewed from the Earth. This is so because though the Moon is much smaller in physical size than the Sun, the Moon is much closer to us compared to the Sun. Therefore, the Moon can appear to cover the entire Sun as viewed from the Earth.

Though the planets Mercury and Venus are much larger than the Moon in size, they are also much farther from the Earth compared to the Moon. Thus, their apparent sizes are very much smaller than the Sun, and they cannot block the Sun.

Now, let's understand the geometry of a solar eclipse. During a solar eclipse, the shadow of the Moon falls on a small area on the surface of the Earth. This area is in total darkness, and no part of the Sun can be seen from there. The observers in this area witness a total solar eclipse. In areas where the Moon partially blocks out only some regions of the Sun, the observers see a partial solar eclipse.

During a total solar eclipse, for a few minutes, it turns dark during the day as no sunlight reaches the Earth. Due to the Earth's rotation and the motion of the Moon in its orbit, the Moon's shadow moves across the surface of the Earth. Thus, the total solar eclipse is visible only for a few minutes.

Now, students, this is very important — you must never look directly at the Sun during a solar eclipse! Even during the eclipse, the Sun is intense enough to damage the eyes and cause blindness. Thus, directly viewing a solar eclipse must be strictly avoided. Also, do not view it through sunglasses, binoculars, or telescopes. The best way to observe a solar eclipse is to participate in events organized by planetaria and astronomy clubs, where they provide specialised eye protection for solar viewing.

Now let's talk about lunar eclipses. As the Moon revolves around the Earth, sometimes the Earth can block the sunlight from reaching the Moon. This is known as a lunar eclipse. On such days, we see the Earth's shadow falling on the full disc of the Moon.

When the Moon is completely in the Earth's shadow, it is called a total lunar eclipse. The bright disc of the Moon starts to appear dark red in colour and stays that way until the Moon moves out of the shadow of the Earth. This is why people sometimes call it a "Blood Moon." When part of the Moon is in the Earth's shadow and the rest of the Moon is visible, it is called a partial lunar eclipse. Unlike a solar eclipse, we can safely watch the eclipsed full Moon with our naked eye.

Now, let me answer some questions that you might have. Why can lunar eclipses be seen from a large part of the Earth when they happen, but total solar eclipses can be seen by only a small part of the Earth? This is because during a lunar eclipse, the Earth casts a large shadow on the Moon, and this shadow can be seen from anywhere on the Earth where the Moon is visible above the horizon. But during a solar eclipse, the Moon's shadow falls on only a small area of the Earth because the Moon is much smaller than the Earth.

Now students, let's solve the exercises from your textbook. I want you to pay close attention because these are important questions that can come in your exams.

Question 1: In Fig. 12.17, how many hours of sunlight do the North Pole and the South Pole receive during one rotation of the Earth?

This is a great question about the poles. During one rotation of the Earth, which is 24 hours, the North Pole receives sunlight for all 24 hours in June, as we learned. Similarly, in December, the North Pole is in darkness for all 24 hours. The same is true for the South Pole, but in opposite seasons. So in June, the South Pole is in darkness for 24 hours, and in December, it receives sunlight for 24 hours.

Question 2: Fill in the blanks

(i) Stars rise in the East and set in the West.

(ii) Day and night are caused by the Earth's rotation.

(iii) When the Moon fully covers the Sun from our view, it is called a total solar eclipse.

Question 3: State whether True or False

(i) Lunar eclipse occurs when the Sun comes between the Earth and the Moon. — FALSE. Lunar eclipse occurs when the Earth comes between the Sun and the Moon.

(ii) Sunrise happens earlier in Gujarat than in Jharkhand. — TRUE. Gujarat is to the west of Jharkhand, so the Sun rises earlier in Gujarat as the Earth rotates from West to East.

(iii) In Chennai, the longest day occurs on the summer solstice. — TRUE. Chennai is in the Northern Hemisphere, so the longest day occurs around 21 June, which is the summer solstice.

(iv) We should watch the solar eclipse directly with our naked eye. — FALSE. We should never watch a solar eclipse directly with our naked eye as it can damage our eyes.

(v) Seasons occur due to the tilt of Earth's axis of rotation and its spherical shape. — TRUE.

(vi) The Earth's revolution around the Sun causes day and night. — FALSE. The Earth's rotation causes day and night. Revolution causes the change in seasons and the changing view of the night sky over the year.

Question 4: Padmashree saw the Orion constellation nearly overhead at 8 pm yesterday. When will she see Orion overhead today?

This is about the rotation of the Earth. The Earth rotates once in 24 hours. So if she saw Orion overhead at 8 pm yesterday, she will see it overhead at 8 pm today. But wait, there's a small correction. Actually, due to the Earth's revolution around the Sun, the stars rise about 4 minutes earlier each day. So technically, she would see Orion overhead at about 7:56 pm today. But for most practical purposes in Class 7, we can say around 8 pm.

Question 5: Nandhini saw a group of stars rising at midnight on 21 June. When will she see the same group of stars rising at midnight next year?

The Earth takes about 365 days and 6 hours to complete one revolution around the Sun. So after one year, the same group of stars will rise at about the same time, which is midnight. So she will see the same group of stars rising at midnight on 21 June next year.

Question 6: Abhay noticed that when it was daytime in India, his uncle who was in the USA was generally sleeping as it was night-time there. What is the reason behind this difference?

This is because of the Earth's rotation. The Earth rotates from West to East. India is located to the east of the USA. So when India faces the Sun (daytime), the USA faces away from the Sun (nighttime). As the Earth rotates, different parts of the Earth face the Sun at different times. That's why when it is day in one country, it is night in another country on the opposite side of the Earth.

Question 7: Four friends used the following ways to see the solar eclipse. Who among them was being careless?

(i) Ravikiran used a solar eclipse goggle. — This is safe.

(ii) Jyothi used a mirror to project the Sun's image. — This can be safe if done properly under supervision.

(iii) Adithya saw the Sun directly with his eyes. — This is CARELESS and dangerous! Looking directly at the Sun can damage the eyes.

(iv) Aruna attended a programme arranged by a planetarium. — This is safe.

So Adithya was being careless.

Question 8: Fill in the circles in Fig. 12.18 appropriately with one of the following: Sun, Moon, Earth.

For a solar eclipse: The Sun is in the center, the Moon is between the Sun and the Earth, and the Earth is at the end. So the order is: Sun, Moon, Earth.

For a lunar eclipse: The Sun is in the center, the Earth is between the Sun and the Moon, and the Moon is at the end. So the order is: Sun, Earth, Moon.

Question 9: The Moon is much smaller than the Sun, yet it can block the Sun completely from our view during a total solar eclipse. Why is it possible?

This is because of apparent size. Although the Moon is much smaller than the Sun in actual size, it is much closer to the Earth than the Sun. So when viewed from the Earth, the Moon and the Sun appear to be of similar size in the sky. This is like how a small thumb can cover a friend's head when the thumb is held close to the eyes. This is why the Moon can completely cover the Sun during a total solar eclipse.

Question 10: The Indian cricket team matches in Australia are often held in December. Should they pack winter or summer clothes for their trip?

They should pack winter clothes! In December, it is winter in Australia because Australia is in the Southern Hemisphere. In December, the Southern Hemisphere is tilted away from the Sun, so it experiences winter with shorter days and cooler temperatures. So the Indian cricket team should pack warm winter clothes.

Question 11: Why do you think lunar eclipses can be seen from a large part of the Earth when they happen, but total solar eclipse can be seen by only a small part of the Earth?

This is because of the sizes of the shadows involved. During a lunar eclipse, the Earth casts a large shadow on the Moon, and this shadow can be seen from anywhere on the Earth where the Moon is above the horizon. That's why about half of the Earth can see a lunar eclipse. But during a solar eclipse, the Moon's shadow on the Earth is very small because the Moon is so much smaller than the Earth. Only the people standing in the small area where the Moon's shadow falls can see the total solar eclipse. That's why total solar eclipses are rare and can be seen only from a small part of the Earth.

Question 12: If the Earth's axis were not tilted with respect to the axis of revolution, explain what would be the effect on seasons?

If the Earth's axis were not tilted, there would be no seasons! The tilt of the Earth's axis is what causes different parts of the Earth to receive different amounts of sunlight at different times of the year. If there were no tilt, all places on Earth would receive the same amount of sunlight throughout the year, and there would be no summer or winter. The days would be of equal length everywhere, and the weather would be relatively constant throughout the year.

Now students, we have covered the entire chapter. Let me give you a summary of everything we have learned today.

We started with the story of Rashmika from Kanniyakumari who noticed that shadows change throughout the day. We learned that this happens because of the Earth's rotation. The Earth rotates on its axis from West to East, completing one rotation in about 24 hours. This rotation causes day and night, and makes the Sun, Moon, and stars appear to rise in the East and set in the West.

We learned about the Foucault pendulum, which is a demonstration of the Earth's rotation, and we even have one in the new Parliament building in Delhi! We also learned about the great Indian astronomer Aryabhata, who understood the Earth's rotation over 1,500 years ago.

Then we learned about the Earth's revolution around the Sun. The Earth takes about 365 days and 6 hours to complete one revolution around the Sun. Because of this revolution, we see different stars in the night sky at different times of the year.

We learned that seasons occur because the Earth's axis is tilted, not because of the distance from the Sun. In June, the Northern Hemisphere is tilted towards the Sun, so it experiences summer with longer days. In December, it's the opposite. We learned about summer solstice, winter solstice, and equinoxes.

Finally, we learned about eclipses. A solar eclipse occurs when the Moon comes between the Sun and the Earth, blocking the Sun's light. A lunar eclipse occurs when the Earth comes between the Sun and the Moon, casting a shadow on the Moon. We learned that we must never look directly at a solar eclipse, but we can safely watch a lunar eclipse with our naked eyes.

We also solved all the exercises from your textbook, and I hope you understood each one of them.

Students, I want you to remember that science is all around us. Every time you see the Sun rise or set, every time you notice the length of the day changing, every time you see the Moon in the sky — you are seeing the concepts we learned today in action. Keep looking up at the sky, keep asking questions, and never stop being curious. That's how scientists make discoveries!

That's all for today, class. Thank you for being such wonderful students. See you next time!