Looking for authentic, 100% textbook-grounded DAV Class 6 Social Science Chapter 1 Solutions: The Planet Earth and the Solar System? Based strictly on the prescribed DAV Board textbook We and Our World, this comprehensive guide provides fully explained solutions for Section A (Tick the correct option with reasons), Section B (Fill in the blanks), Section C (Match the following), Section D (Short Answer Questions), Section E (Detailed Long Answer Questions), Value-Based Questions (VBQ), HOTS, structured planetary comparison tables, and authentic DAV Board Examination Previous Years’ Questions (2018-2024) with step-by-step marking schemes.
Section A: Tick (✓) the Correct Option
1. Which one of the following is the nearest star to the Earth?
(a) Alpha Centauri
(b) Pole Star
(c) The Sun
(d) Sirius
Answer: (c) The Sun
Detailed Justification: The Sun is the central star of our Solar System, situated approximately 150 million kilometres (149.6 million km) from the Earth. The next nearest star system, Proxima Centauri / Alpha Centauri, is about 4.24 light years (over 40 trillion kilometres) away.
2. Which is the third nearest planet to the Sun?
(a) Venus
(b) Earth
(c) Mars
(d) Mercury
Answer: (b) Earth
Detailed Justification: In terms of increasing distance from the Sun, the planetary order is: (1) Mercury, (2) Venus, (3) Earth, (4) Mars, (5) Jupiter, (6) Saturn, (7) Uranus, and (8) Neptune. Therefore, Earth is the third planet from the Sun.
3. Which planet is known as the “Earth’s Twin” due to its similar size and shape?
(a) Mars
(b) Jupiter
(c) Venus
(d) Saturn
Answer: (c) Venus
Detailed Justification: Venus is frequently termed ‘Earth’s Twin’ or ‘sister planet’ because its diameter (12,104 km), mass (about 81.5% of Earth’s), and gravitational pull are remarkably close to those of Earth, though its thick carbon dioxide atmosphere produces a runaway greenhouse effect.
4. All celestial bodies that shine in the night sky and have their own heat and light are called:
(a) Planets
(b) Satellites
(c) Stars
(d) Asteroids
Answer: (c) Stars
Detailed Justification: Stars are colossal self-luminous celestial bodies composed predominantly of hydrogen and helium gases. Deep within their cores, continuous thermonuclear fusion converts hydrogen into helium, releasing enormous quantities of heat and light radiant energy.
5. The Asteroid Belt is located between the orbits of:
(a) Earth and Mars
(b) Mars and Jupiter
(c) Jupiter and Saturn
(d) Venus and Earth
Answer: (b) Mars and Jupiter
Detailed Justification: Millions of rocky, irregularly shaped fragments called asteroids orbit the Sun in a well-defined annular ring between the orbits of Mars (the outermost terrestrial planet) and Jupiter (the innermost gas giant).
Section B: Fill in the Blanks
- The vast, boundless expanse of space which includes all matter, energy, galaxies, and celestial bodies is called the Universe.
- A massive gravitational cluster containing billions of stars, dust clouds, and interstellar gas is termed a Galaxy (our solar system belongs to the Milky Way or Akash Ganga).
- The linear distance traversed by a ray of light travelling through a vacuum in one complete solar year (at 300,000 km/s) is called a Light Year (approximately 9.46 × 1012 km).
- All eight major planets revolve around the Sun in fixed, flattened paths known as elliptical orbits.
- The Moon is the solitary natural satellite revolving around the planet Earth at a mean distance of 384,400 km.
Section C: Match the Following
| Column A (Celestial Entity) | Column B (Accurate Scientific Identification) |
|---|---|
| 1. Ursa Major (Great Bear / Saptarishi) | Prominent Northern Hemisphere Constellation |
| 2. Proxima Centauri | Nearest star to Earth after the Sun (approx. 4.24 light years away) |
| 3. Sirius (Dog Star) | Brightest star visible in the night sky from Earth |
| 4. Neil Armstrong | First human astronaut to step onto the lunar surface (21 July 1969, Apollo 11) |
| 5. Ceres | Largest object located within the asteroid belt classified as a Dwarf Planet |
Section D: Short Answer Questions
Question 1: What constitutes the Solar System?
Answer: The Solar System constitutes the Sun as its central gravitational anchor, along with eight revolving planets, over 200 known natural satellites (moons), thousands of dwarf planets (such as Pluto and Ceres), millions of asteroids, meteoroids, and icy comets. All these heavenly bodies are bound in perpetual elliptical orbits by the Sun’s immense gravitational force.
Question 2: State three scientific reasons why Earth is considered a unique planet in the universe.
Answer: Earth is the only known celestial body supporting diverse biological life due to three crucial factors:
1. Optimum Distance from the Sun (Goldilocks Zone): Being 150 million km from the Sun, Earth experiences moderate ambient temperatures (average 15°C) – neither scorchingly hot like Mercury nor perpetually frozen like Neptune.
2. Presence of Liquid Water: Liquid water covers roughly 71% of Earth’s surface (the hydrosphere), acting as a universal solvent and chemical medium essential for metabolic survival.
3. Protective Life-Sustaining Atmosphere: Earth’s atmosphere contains balanced proportions of oxygen (21%) for aerobic respiration, carbon dioxide (0.04%) for plant photosynthesis, and an ozone layer in the stratosphere that blocks destructive ultraviolet solar radiation.
Question 3: Why do we always observe only one side of the Moon from the Earth?
Answer: The Moon revolves around the Earth in approximately 27 days and 8 hours (27.3 days) and completes one full rotation on its own axis in precisely the exact same period of time. Because the Moon’s orbital period matches its rotational period (a gravitational phenomenon called synchronous rotation or tidal locking), the exact same lunar hemisphere remains permanently turned toward Earth.
Question 4: Distinguish clearly between Stars and Planets.
Answer: Below is a detailed scientific comparison:
| Feature | Stars | Planets |
|---|---|---|
| Source of Light | Self-luminous; produce their own heat and light through nuclear fusion. | Non-luminous; merely reflect sunlight falling upon their surfaces. |
| Size & Composition | Colossal, extremely hot celestial bodies made of glowing gases. | Comparatively much smaller bodies made of rock, metal, or compressed gas. |
| Twinkling Effect | Twinkle noticeably due to atmospheric refraction of distant point-source light. | Do not twinkle because their larger apparent angular size averages out refraction. |
| Motion | Appear fixed relative to one another in constellations over centuries. | Constantly wander along elliptical orbits around their parent star. |
Question 5: What are Asteroids and Meteoroids? How do they differ?
Answer:
• Asteroids: Asteroids are rocky, metallic bodies left over from the formation of the solar system about 4.6 billion years ago. Most of them are concentrated in the vast Asteroid Belt between Mars and Jupiter.
• Meteoroids: Meteoroids are smaller pieces of rock or dust broken off from asteroids or comets. When a meteoroid enters Earth’s upper atmosphere at high velocity, friction with air molecules heats it to incandescence, producing a bright streak of light called a meteor (shooting star). If a fragment survives the fiery passage and strikes Earth’s surface, it is known as a meteorite.
Section E: Long Answer Questions
Question 1: Explain the formation of the Solar System and describe the Sun as the head of the family.
Answer:
1. Nebular Hypothesis (Origin): Approximately 4.6 billion years ago, a massive rotating cloud of interstellar gas and cosmic dust called a solar nebula collapsed under its own gravity. As the cloud contracted, it spun faster and flattened into a protoplanetary disk. The central dense mass accumulated most of the material and ignited nuclear fusion, giving birth to the Sun, while the cooler outer rings accreted into planets, moons, and asteroids.
2. The Sun as the Master of the Solar System:
• Central Gravitational Anchor: The Sun accounts for roughly 99.86% of the total mass of the entire solar system. Its tremendous gravitational pull keeps all eight planets, moons, and asteroids revolving in stable, predictable paths.
• Ultimate Source of Energy: All climatic, weather, biological, and hydrological processes on Earth depend on solar radiant energy. Plant photosynthesis, winds, ocean currents, and the water cycle are entirely powered by the Sun.
• Scale and Composition: The Sun’s diameter is about 1.39 million kilometres (roughly 109 times that of Earth), and it can hold more than 1.3 million Earths inside it. It is composed of 74% hydrogen, 24% helium, and traces of heavier elements, with surface temperatures near 6,000°C and core temperatures exceeding 15,000,000°C.
Question 2: Classify the planets of our solar system into Terrestrial (Inner) and Jovian (Outer) planets. Present their distinct characteristics in a structured comparison table.
Answer: The eight planets are divided by the Asteroid Belt into two distinct groups:
| Characteristic | Inner / Terrestrial Planets | Outer / Jovian Planets |
|---|---|---|
| Member Planets | Mercury, Venus, Earth, Mars (4 planets). | Jupiter, Saturn, Uranus, Neptune (4 planets). |
| Position Relative to Sun | Located between the Sun and the Asteroid Belt; close to the Sun. | Located beyond the Asteroid Belt; very far from the Sun. |
| Composition & Density | Solid rocky surfaces with metallic iron-nickel cores; high density (3.9 to 5.5 g/cm3). | Gaseous and liquid envelopes of hydrogen, helium, methane, and ammonia; low density (0.7 to 1.6 g/cm3). |
| Size & Mass | Comparatively small radii and smaller masses. | Giant bodies with huge radii (Jupiter is the largest planet). |
| Ring Systems | Possess zero planetary rings. | All four outer planets possess ring systems (Saturn has spectacular visible rings). |
| Natural Satellites (Moons) | Very few moons (Mercury: 0, Venus: 0, Earth: 1, Mars: 2). | Large families of moons (Saturn and Jupiter have over 90 confirmed moons each). |
Value-Based Question (VBQ) with Ethical Evaluation
Contextual Scenario:
While observing the night sky through a telescope during an astronomy workshop, Rohan remarked: “Out of billions of stars and planets in our galaxy, Earth is the only home where living beings can breathe and flourish. Yet humans indiscriminately pollute its rivers, cut down forests, and choke its air.”
Questions for Ethical Reflection:
1. What makes planet Earth exceptionally fragile and irreplaceable?
Answer: Earth is the only known planetary haven equipped with a delicate, balanced biosphere. Its protective atmospheric ozone layer, pure freshwater reservoirs, and moderate thermal envelope took hundreds of millions of years to stabilize. Unlike science fiction, humans currently have no alternate planet capable of sustaining human civilization.
2. What fundamental values and environmental duties must students practice to preserve Earth?
Answer: Students must cultivate (a) Ecological Stewardship by conserving freshwater and minimizing plastic waste, (b) Intergenerational Responsibility by planting native trees to replenish oxygen, and (c) Energy Conservation by turning off unnecessary lights and using sustainable transport to reduce carbon emissions.
Higher Order Thinking Skills (HOTS) Questions
HOTS 1: If the distance between the Earth and the Sun were halved, what catastrophic changes would occur on the Earth?
Answer: If the distance were halved (from 150 million km to 75 million km), Earth would receive four times more solar irradiance (governed by the inverse-square law). This would cause:
1. Rapid vaporization of Earth’s oceans, rivers, and polar ice caps into steam.
2. A severe runaway greenhouse effect similar to Venus, elevating surface temperatures above 300°C to 400°C.
3. Complete extinction of liquid-water dependent biological life and destruction of the protective ozone layer.
HOTS 2: Why do astronomers describe looking at distant stars through telescopes as ‘looking back in time’?
Answer: Light travels at a finite velocity of 300,000 kilometres per second. When light leaves a distant star such as Proxima Centauri (4.24 light years away) or the Andromeda Galaxy (2.5 million light years away), it takes years or millions of years to reach Earth’s telescopes. Consequently, astronomers observe the celestial body not as it exists right now, but precisely as it appeared millions of years ago when the light originated.
Authentic DAV Board Examination PYQs (2018-2024)
Q: The rays of the Sun reach the Earth’s surface in about _______ minutes.
Official Answer & Step: 8 minutes and 20 seconds (approx. 500 seconds).
Calculation: Distance = 150,000,000 km ÷ Speed of light (300,000 km/s) = 500 seconds = 8 minutes and 20 seconds. [1 Mark]
Q: What is a Constellation? Name any two prominent constellations visible in the night sky and draw a neat diagram of Ursa Major (Saptarishi).
Official Marking Scheme:
• Definition (1 Mark): A recognizable group or pattern formed by a cluster of prominent stars in the night sky is called a constellation.
• Examples (1 Mark): (1) Ursa Major (Great Bear / Saptarishi), (2) Orion (The Hunter).
• Description of Ursa Major (1 Mark): It contains seven bright stars arranged in the shape of a ladle or question mark. The two pointer stars at the outer edge point directly north toward the Pole Star (Dhruva Tara).
Q: Analyze the key characteristics that led to the reclassification of Pluto from a major planet to a dwarf planet in 2006.
Official 5-Point Answer:
1. In August 2006, the International Astronomical Union (IAU) established three scientific criteria to define a full major planet.
2. A planet must (a) orbit the Sun, (b) have sufficient mass to assume a nearly round (hydrostatic equilibrium) shape, and (c) have cleared its orbital neighbourhood of other debris.
3. Pluto successfully fulfilled the first two criteria but failed the third: its eccentric orbit cuts across Neptune’s orbit and lies embedded in the crowded icy Kuiper Belt.
4. Pluto’s mass is only about 0.07 times the mass of other objects in its orbit, unlike Earth whose mass is 1.7 million times the remainder of its orbit.
5. Hence, the IAU reclassified Pluto as a Dwarf Planet along with Eris, Ceres, Haumea, and Makemake. [5 Marks]
Frequently Asked Questions (DAV Class 6 SST Ch 1 FAQs)
Q1. Why is Venus called the morning and evening star?
Because of its reflective sulfuric acid clouds and proximity to Earth, Venus appears exceptionally bright in the western sky just after sunset and in the eastern sky just before sunrise.
Q2. What is a light year and why is it used?
A light year is the astronomical unit measuring the distance light travels in one year (approx. 9.46 trillion km). It is used because interstellar distances are far too enormous to express in conventional kilometres.
Q3. What is the difference between revolution and rotation?
Rotation is the spinning of a planet on its own central axis (causing alternating day and night), whereas revolution is the movement of a planet around the Sun in its elliptical orbit (causing seasonal changes).