Earth, Moon, and the Sun – Case-based Questions with Answers
Class 7
Science
Chapter 12
Earth, Moon and the Sun — Case-Based Questions & Answers (NCERT-aligned)
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Topic: Rotation — Day and Night
Case 1
Riya lives in a town where the Sun appeared to rise in the east at 6:00 a.m. and set in the west at about 6:00 p.m. She hears that the Earth completes one rotation in 24 hours.
Q1: Explain why Riya sees day and night and why the Sun appears to move across the sky from east to west.
Answer: The Earth rotates about its axis from west to east once every ~24 hours. Because of this rotation, different parts of Earth face the Sun at different times — the side facing the Sun experiences day and the opposite side experiences night. The apparent motion of the Sun from east to west is due to the Earth's eastward rotation.
Q2: If the Earth did not rotate, how would day and night be affected?
Answer: Without rotation, one side of Earth would face the Sun continuously (permanent day) and the other side permanent night. There would be no regular cycle of day and night.
Topic: Rotation — Speed & Observations
Case 2
A student notes that the Earth rotates faster at the equator than near the poles.
Q: Why does a point on the equator travel a longer distance in 24 hours than a point near the poles?
Answer: Points on the equator are farther from the Earth's axis, so during one rotation they travel a larger circular path. Since the rotation period (~24 h) is the same everywhere, linear speed at the equator is higher. Near the poles the circular path is very small, so linear speed is lower.
Topic: Apparent Motion & Shadows
Case 3
A sundial's shadow length changes during the day. At noon the shadow is shortest.
Q: Relate the changing shadow to Earth's rotation.
Answer: As the Earth rotates, the Sun’s apparent position in the sky changes. When the Sun is highest (around local noon), the sunlight falls most directly, producing the shortest shadow. In morning and evening, lower Sun angle creates longer shadows.
Topic: Time & Longitude
Case 4
Two students in different longitudes observe local noon at different times.
Q: Why does local noon occur at different times at different longitudes?
Answer: Local noon is when the Sun is highest in the sky at that longitude. Because Earth rotates eastward, places to the east experience local noon earlier than places to the west. Longitude determines the local solar time.
Topic: Revolution — Yearly Motion
Case 5
A teacher tells her students that the Earth revolves around the Sun in approximately 365 days.
Q1: What is meant by Earth's revolution, and how long does it take?
Answer: Revolution is the Earth's movement around the Sun along its orbit. It takes about 365¼ days to complete one revolution, which is why an extra day is added every 4th year (leap year).
Q2: Does Earth's revolution alone cause seasons?
Answer: No. Seasons are caused by a combination of Earth's revolution around the Sun and the tilt of its axis. Revolution changes which hemisphere is tilted toward the Sun at different times of the year.
Topic: Tilt of Axis & Seasons
Case 6
During summer in India (Northern Hemisphere), days are longer and nights shorter.
Q: Explain, using Earth's tilt, why days are longer in summer in the Northern Hemisphere.
Answer: Earth's axis is tilted by ~23.5°. During Northern Hemisphere summer, the North is tilted toward the Sun. This tilt causes sunlight to fall more directly and keeps the hemisphere illuminated for a longer portion of the 24-hour rotation, producing longer days and shorter nights.
Topic: Seasons — Hemispheric Opposites
Case 7
A student asks why India has summer when Australia has winter.
Q: Explain this using Earth's revolution and tilt.
Answer: When the Northern Hemisphere is tilted toward the Sun (e.g., June), it is summer in the north and winter in the southern hemisphere. Six months later, the situation reverses as Earth moves to the opposite side of its orbit, making it summer in the Southern Hemisphere.
Topic: Intensity of Sun's Rays
Case 8
Two regions at different latitudes receive different amounts of solar energy in summer.
Q: Why do regions near the equator get more intense sunlight compared to higher latitudes?
Answer: Near the equator, the Sun's rays strike more directly (steeper angle), concentrating energy over a smaller area. At higher latitudes, rays arrive at a slant, spreading energy over a larger area and so are less intense.
Topic: Seasonal Changes in Temperature
Case 9
A seaside town has milder seasonal variations than an inland city at the same latitude.
Q: While axial tilt and revolution explain seasons, what reason might explain milder seasons at the coast?
Answer: Water has a high heat capacity and moderates temperature changes. Oceans absorb and release heat slowly, so coastal regions experience smaller temperature swings, making seasons milder than inland areas.
Topic: Axis Tilt — Consequences
Case 10
Imagine Earth's axis had no tilt (0°). Consider the effect on seasons and climate.
Q: Predict how seasons and yearly temperature patterns would change if Earth's axis were not tilted.
Answer: With zero tilt, both hemispheres would receive roughly the same sunlight year-round. There would be no regular seasons (no summer/winter cycle); day lengths and solar angles would remain nearly constant throughout the year, drastically changing climate and ecosystems.
Topic: Phases of the Moon — Observations
Case 11
Arun notices that the Moon looks different every few nights — sometimes fully lit, sometimes a thin crescent.
Q: Explain why the appearance (phases) of the Moon changes.
Answer: Phases of the Moon occur because as the Moon revolves around Earth, different portions of its sunlit half are visible from Earth. The relative positions of Sun-Earth-Moon determine how much of the lit side we see (new, crescent, quarter, gibbous, full).
Topic: New Moon & Full Moon
Case 12
Two friends ask when a new moon and a full moon occur in relation to Sun-Earth-Moon positions.
Q: Describe the Moon's position relative to Earth and Sun during new moon and full moon.
Answer: New moon: Moon lies roughly between Earth and Sun; the sunlit side faces away from Earth so Moon is not visible. Full moon: Earth lies between Sun and Moon; the sunlit side faces Earth, making the Moon appear fully lit.
Topic: Time Differences in Moonrise
Case 13
The Moon rises about 50 minutes later each day.
Q: Explain why the Moonrise time shifts each day.
Answer: The Moon takes about 27.3 days to orbit Earth (sidereal month) and about 29.5 days to complete its phases (synodic month). As it moves eastward around Earth each day, Earth must rotate ~50 minutes more for the Moon to appear on the same meridian, so Moonrise is roughly 50 minutes later each day.
Topic: Observing Moon Phases
Case 14
A student sees a half-lit Moon exactly at sunset.
Q: Which phase is likely being observed and why does it appear at sunset?
Answer: A half-lit Moon (first or third quarter) is visible around sunset (first quarter) or around sunrise (third quarter). At first quarter the Moon is about 90° east of the Sun and is high in the sky at sunset, appearing half-lit.
Topic: Solar Eclipse — Basic Idea
Case 15
A community experiences a sudden darkening during daytime and learns a solar eclipse occurred.
Q: Describe when a solar eclipse occurs and why it is observed only in certain regions.
Answer: A solar eclipse happens when the Moon comes between the Sun and Earth, blocking the Sun's light. Because the Moon's shadow falls on a small part of Earth's surface, only observers in that path (umbra or penumbra) see the eclipse. Elsewhere only a partial or no eclipse is visible.
Topic: Types of Solar Eclipses
Case 16
Students hear terms: total, partial, and annular solar eclipse.
Q: Distinguish among total, partial, and annular solar eclipses.
Answer:
- Total: Moon fully covers the Sun as seen from a location in the umbra — sky darkens significantly.
- Partial: Only part of the Sun is covered — seen from penumbra regions.
- Annular: Moon is too far from Earth to cover the Sun completely; a bright ring of the Sun remains visible around the Moon.
Topic: Lunar Eclipse — Basic Idea
Case 17
One night, people see a reddish Moon and learn it’s a lunar eclipse.
Q: Explain when a lunar eclipse occurs and why the Moon may appear reddish.
Answer: A lunar eclipse occurs when Earth comes between the Sun and Moon and the Moon passes into Earth's shadow. Earth's atmosphere scatters shorter wavelengths of light (blue) and allows red/orange light to reach the Moon, giving it a reddish hue (often called a "blood moon").
Topic: Visibility of Eclipses
Case 18
A friend asks why lunar eclipses are visible from an entire night-side of Earth, unlike solar eclipses.
Q: Why can many people watch a lunar eclipse but only a few can watch a solar eclipse?
Answer: During a lunar eclipse, Earth's shadow covers the Moon and this shadow is large enough that everyone on the night side of Earth can see the event. For a solar eclipse the Moon's shadow on Earth is small, so only a narrow region experiences the full eclipse.
Topic: Calendar, Leap Year & Time
Case 19
A child wonders why we add one day in February every fourth year (leap year).
Q1: Explain the reason for a leap year.
Answer: Earth takes about 365.25 days to orbit the Sun. The extra ~0.25 day each year accumulates, so every 4 years we add 1 day (0.25×4 = 1 day) to keep the calendar aligned with seasons. Hence, a leap year has 366 days.
Q2: Why does the calendar need this correction?
Answer: Without corrections, calendar months would slowly drift relative to the seasons; for example, summer months would shift to different times of the year over centuries.
Topic: Orbits, Satellites & Observations
Case 20
A class discusses natural and artificial satellites. They learn the Moon is Earth’s natural satellite.
Q1: Define a satellite and give an example of a natural and artificial satellite.
Answer: A satellite is any body that orbits a larger body. The Moon is a natural satellite of Earth. Artificial satellites are human-made objects put into orbit for communication, weather monitoring, GPS, etc. (e.g., Indian satellites like INSAT or remote sensing satellites).
Q2: Why do we see phases for the Moon but not for most artificial satellites?
Answer: The Moon’s phases are easily noticed because it is large, bright, and relatively distant with a long orbital period compared to small artificial satellites which are close, move quickly across the sky, and often reflect sunlight in brief flashes rather than showing gradual phases visible to the naked eye.
Note: These case-based questions are strictly aligned with NCERT Class 7 Chapter 12 topics — Earth’s rotation and revolution, tilt of axis, day & night, seasons, phases of the Moon, eclipses, and basic satellite concepts. Use them for classroom practice, revision, and CBSE-style exam preparation.