The Earth

Solar System, Latitudes, Longitudes and Earth's Movements

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Overview

The Earth is a foundational topic in geography that appears consistently in TET-2 Social Science papers. Questions typically test factual recall (distances, sizes, names of planets) as well as conceptual understanding (why seasons occur, how time zones work, what causes day and night).

This topic connects physical geography to map reading and climate studies. A clear grasp of latitudes, longitudes and earth's movements helps students understand later topics like climate zones, ocean currents and Indian geography. For the exam, focus on the solar system's basic structure, the grid system of the earth, and the two primary motions—rotation and revolution—along with their consequences.

Expect 3–5 direct questions from this topic, often involving diagrams or simple calculations related to time zones.

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Key Concepts

• **The Solar System** consists of the Sun, eight planets, dwarf planets, moons, asteroids and comets. The Sun contains about 99.8% of the solar system's mass and provides light and heat to all planets.

• **Order of planets** from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Remember: "My Very Educated Mother Just Served Us Noodles."

• **Earth's unique position**: Third planet from the Sun, at an average distance of about 150 million km (1 Astronomical Unit). This distance allows liquid water to exist—the "Goldilocks zone."

• **Latitudes** are imaginary horizontal lines running east-west, measuring distance north or south of the Equator (0°). They range from 0° to 90° N/S.

• **Longitudes** are imaginary vertical lines running north-south, measuring distance east or west of the Prime Meridian (0°). They range from 0° to 180° E/W.

• **Rotation** is the spinning of Earth on its axis (west to east), completed in approximately 24 hours. It causes day and night.

• **Revolution** is Earth's orbit around the Sun, completed in approximately 365¼ days. Combined with the 23.5° axial tilt, it causes seasons.

• **The axis of Earth is tilted at 23.5°** to the plane of its orbit. This tilt is the primary reason for seasonal variation, not Earth's distance from the Sun.

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Formulas / Key Facts

| Item | Value / Fact | |------|--------------| | Distance from Sun | ~150 million km (1 AU) | | Earth's diameter | ~12,742 km | | Rotation period | 23 hours 56 minutes 4 seconds (sidereal day) | | Revolution period | 365 days 6 hours (365¼ days) | | Axial tilt | 23.5° | | Equator | 0° latitude; divides Earth into Northern and Southern Hemispheres | | Prime Meridian | 0° longitude; passes through Greenwich, UK | | International Date Line | 180° longitude (with deviations); crossing it changes the calendar date | | Total latitudes | 181 (0° to 90° N and 0° to 90° S, including Equator) | | Total longitudes | 360 (0° to 180° E and 0° to 180° W, with 0° and 180° counted once) | | Time zone calculation | 1° longitude = 4 minutes; 15° longitude = 1 hour | | Indian Standard Time (IST) | Based on 82.5° E longitude (passing near Mirzapur, UP); IST = GMT + 5 hours 30 minutes |

**Important Latitudes:**

• Tropic of Cancer: 23.5° N
• Tropic of Capricorn: 23.5° S
• Arctic Circle: 66.5° N
• Antarctic Circle: 66.5° S

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Worked Examples

### Example 1: Time Zone Calculation **Problem:** When it is 12:00 noon at Greenwich (0° longitude), what is the time at a place located at 75° E?

**Solution:**

• Difference in longitude = 75° – 0° = 75°
• Time difference = 75 × 4 minutes = 300 minutes = 5 hours
• Since the place is east of Greenwich, time is ahead.
• Time at 75° E = 12:00 + 5:00 = **5:00 PM**

### Example 2: Identifying Hemisphere **Problem:** A city is located at 28° N, 77° E. In which hemispheres does it lie?

**Solution:**

• 28° N means north of the Equator → **Northern Hemisphere**
• 77° E means east of Prime Meridian → **Eastern Hemisphere**
• The city lies in the Northern and Eastern Hemispheres. (This is New Delhi's approximate location.)

### Example 3: Season Explanation **Problem:** Why does the Northern Hemisphere experience summer in June?

**Solution:**

• In June, the North Pole is tilted towards the Sun due to Earth's 23.5° axial tilt.
• The Sun's rays fall more directly on the Northern Hemisphere.
• Days are longer; more solar energy is received.
• Result: **Summer in Northern Hemisphere, winter in Southern Hemisphere.**
• On June 21 (Summer Solstice), the Sun is directly overhead at the Tropic of Cancer.

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Common Mistakes

1. **Confusing rotation with revolution**

• Wrong: "Rotation causes seasons."
• Correct: Rotation causes day and night; revolution (with axial tilt) causes seasons.

2. **Thinking Earth is closer to Sun in summer**

• Wrong: "Summer happens because Earth is nearest to the Sun."
• Correct: Seasons result from axial tilt, not distance. Earth is actually closest to the Sun (perihelion) in January, during Northern Hemisphere winter.

3. **Mixing up latitude and longitude directions**

• Wrong: "Latitudes run north-south."
• Correct: Latitudes run east-west (horizontally) but measure north-south distance. Longitudes run north-south (vertically) but measure east-west distance.

4. **Forgetting to add/subtract time correctly for east/west**

• Wrong: Subtracting time for places east of a reference point.
• Correct: Places to the east are ahead in time; places to the west are behind.

5. **Confusing the International Date Line with the Prime Meridian**

• Wrong: "Date changes at 0° longitude."
• Correct: The Prime Meridian (0°) is the reference for time. The International Date Line (180°) is where the calendar date changes.

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Quick Reference

• **Planet order:** Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• **Rotation = Day/Night (24 hrs); Revolution = Seasons (365¼ days).**
• **Axial tilt = 23.5°** — the cause of seasons.
• **1° longitude = 4 minutes of time; 15° = 1 hour.**
• **IST = GMT + 5:30** (based on 82.5° E).
• **Equator = 0° latitude; Prime Meridian = 0° longitude.**