Class 7 Science Chapter-12 Earth, Moon, and the Sun Extra Questions and Answers
Q1. Rashmika noticed that coconut tree shadows were long in the morning but short in the afternoon. Explain why this happens.
Ans: The Sun appears to move across the sky from East to West during the day. This happens because the Earth rotates from West to East. As the Sun’s apparent position in the sky changes, the length and direction of shadows also change — long in the morning and evening, short at noon.
Q2. If the Earth rotates from West to East, why do the Sun, Moon, and stars appear to move from East to West?
Ans: When we ride a merry-go-round turning anti-clockwise, objects around us appear to move in the opposite (clockwise) direction. Similarly, as the Earth rotates from West to East, all celestial bodies appear to move in the opposite direction — from East to West.
Q3. How did Leon Foucault demonstrate the rotation of the Earth?
Ans: In the middle of the nineteenth century, Foucault used a long pendulum with a heavy bob suspended from a high ceiling. As the pendulum swung, its plane of swing appeared to change, giving the first simple demonstration of the Earth’s rotation. A 22-metre Foucault pendulum hangs in the Constitution Hall of India’s new Parliament building in New Delhi.
Q4. Why does the Pole Star (Dhruva Tara) appear almost stationary in the night sky?
Ans: The Earth’s axis of rotation points very close to the Pole Star. So while all other stars appear to move around it due to the Earth’s rotation, the Pole Star itself appears nearly fixed in the sky.
Q5. What is the difference between rotation and revolution?
Ans:
- Rotation: The motion in which all parts of an object move in circles around an imaginary line (axis) passing through it. The Earth rotates on its axis in about 24 hours.
- Revolution: The motion of an object around another object. The Earth revolves around the Sun in about 365 days and 6 hours along its orbit.
Q6. The night sky looks different in March compared to December. Why?
Ans: As the Earth revolves around the Sun, we look in different directions in space in different months. Hence, different stars rise at sunset in different months, and the patterns of stars visible at a fixed time of night gradually change over the year.
Q7. Why do summer days have longer daytime hours than winter days in the Northern Hemisphere?
Ans: The Earth’s axis is tilted. In June, the Northern Hemisphere is tilted towards the Sun, so it receives sunlight for more than 12 hours during one rotation — days are longer. In December, the Northern Hemisphere is tilted away from the Sun and receives sunlight for less than 12 hours — days are shorter.
Q8. “The Earth is closer to the Sun in summer, so summers are hot.” Is this reasoning correct? Give the actual reason for seasons.
Ans: No, this reasoning is incorrect. The Earth is actually closest to the Sun in January. Seasons occur because of (i) the tilt of the Earth’s axis of rotation and (ii) the spherical shape of the Earth, which causes the same sunrays to be spread over a smaller or larger area in different hemispheres, and changes the duration of daylight.
Q9. Why do the effects of seasons appear less prominent in the southern states of India?
Ans: Places near the Equator always receive about 12 hours of sunlight and 12 hours of darkness, and the intensity of sunrays changes very little across months. So seasonal effects are not very prominent near the equator.
Q10. What happens at the North Pole on 21 March and 22 September?
Ans: On 21 March (spring equinox), the Sun rises at the North Pole and remains visible continuously for six months, setting on 22 September. The South Pole experiences the opposite. Thus, polar regions get six months of continuous sunshine followed by six months of darkness.
Q11. The Moon is much smaller than the Sun. How can it completely block the Sun during a total solar eclipse?
Ans: This is due to perspective. Just as a small thumb held at arm’s length can cover a friend’s entire head, the Moon, though much smaller, is much closer to the Earth than the Sun. From the Earth, the Moon appears large enough to cover the entire disc of the Sun.
Q12. Why is a total solar eclipse visible only from a small part of the Earth, while a lunar eclipse is visible from a much larger area?
Ans: During a solar eclipse, the Moon’s shadow falls on only a small area of the Earth’s surface, so only observers in that small region see totality. During a lunar eclipse, the Earth’s shadow falls on the Moon itself, which is visible to everyone on the night side of the Earth — a much larger area.
Q13. Why does the Moon appear dark red during a total lunar eclipse?
Ans: When the Moon completely enters the Earth’s shadow, direct sunlight is blocked. The Moon’s bright disc appears dark red and stays that way until it moves out of the Earth’s shadow. (Unlike a solar eclipse, it is safe to watch a lunar eclipse with the naked eye.)
Q14. List the safety precautions to follow while viewing a solar eclipse.
Ans:
- Never look at the Sun directly during an eclipse — even then, sunlight is intense enough to damage the eyes and cause blindness.
- Do not view it through sunglasses, binoculars, or telescopes.
- Attend viewing events organised by planetaria or astronomy clubs, which provide specialised eye protection.
- A safe method is to project the Sun’s image using a mirror (set up by a teacher), taking care that the reflected light never enters anyone’s eyes.
Q15. Abhay in India was awake during the day, but his uncle in the USA was sleeping. What explains this difference?
Ans: As the Earth rotates from West to East, sunlight falls on only one half of the Earth at a time. When India is on the side facing the Sun (daytime), the USA is on the dark side facing away from the Sun (night-time). Earth’s rotation thus causes opposite times of day at different longitudes.
Q16: When viewed from above the North Pole, in which direction does the Earth rotate, and in how much time does it complete one rotation?
Ans: When viewed from above the North Pole, the Earth rotates in the anti-clockwise direction, that is, from West to East. The Earth completes one rotation in about 24 hours. The axis of rotation passes through the geographic North Pole and the South Pole.
Q17: Why do Mercury and Venus, though larger than the Moon, never block the sunlight from reaching the Earth?
Ans: Mercury and Venus appear very small compared to the Sun because they are much farther from the Earth than the Moon is. Apparent size depends on both actual size and distance. Since their apparent sizes are much smaller than the Sun’s, they cannot block the sunlight. For example, when Venus passes between the Sun and the Earth, it appears only as a tiny black dot against the bright face of the Sun — this rare event is called a Transit of Venus.
Q18: What is the difference between a total solar eclipse and a partial solar eclipse?
Answer:
- Total solar eclipse: When the Moon completely blocks the Sun, the Moon’s shadow falls on a small area of the Earth’s surface. This area is in total darkness, and no part of the Sun can be seen from there. It remains dark for only a few minutes because the Moon’s shadow keeps moving across the surface of the Earth.
- Partial solar eclipse: In areas where the Moon blocks only some regions of the Sun, observers see a partial solar eclipse. Daylight begins to return as the Moon starts moving away from the front of the Sun.
Q19: What did the Bhil and Pawara communities of the Tapi Valley use the stars for?
Ans: The Bhil and Pawara are indigenous communities from the Tapi Valley in western India. They used the appearance of certain patterns of stars in the sky as markers for the arrival of monsoon rain.
Q20: How can you observe the apparent movement of stars caused by the rotation of the Earth using the Big Dipper (Saptarishi)?
Ans: On an early evening between March and May, identify the Big Dipper (Saptarishi) and the Pole Star (Dhruva Tara), if visible. Note down your location, date, and time, and draw the orientation of the Big Dipper with respect to the Pole Star or a fixed tree or building on the ground. After two hours, observe again and draw its new position. Repeat once more after another two hours. You will find that the Big Dipper appears to move around the Pole Star, because the Earth’s axis of rotation points very close to the Pole Star, making it appear nearly stationary while all other stars appear to move around it.