Our World — Earth as a Planet
Overview
Earth is the third planet from the Sun and the only known planet to support life. It is part of the solar system, which is located in the Milky Way galaxy. Understanding Earth's shape, size, and the grid system of latitudes and longitudes is fundamental to geography. This chapter covers Earth as a planet, its motions, and the system of coordinates used for locating places on its surface.
Earth in the Solar System
| Aspect | Details |
|---|---|
| Position from Sun | Third (after Mercury and Venus) |
| Shape | Oblate spheroid (flattened at the poles, bulging at the equator) |
| Equatorial diameter | 12,756 km |
| Polar diameter | 12,714 km (43 km less than equatorial) |
| Circumference at equator | 40,075 km |
| Circumference through poles | 40,008 km |
| Mass | 5.97 x 10^24 kg |
| Rotation period | 23 hours 56 minutes 4 seconds (sidereal day) |
| Revolution period | 365.25 days |
| Average distance from Sun | 149.6 million km (1 AU) |
| Planet | Diameter (km) | Distance from Sun (million km) |
|---|---|---|
| Mercury | 4,879 | 57.9 |
| Venus | 12,104 | 108.2 |
| Earth | 12,756 | 149.6 |
| Mars | 6,792 | 227.9 |
| Jupiter | 142,984 | 778.5 |
| Saturn | 120,536 | 1,432 |
| Uranus | 51,118 | 2,867 |
| Neptune | 49,528 | 4,515 |
Shape of the Earth
| Evidence | Explanation |
|---|---|
| Ship's visibility | A ship disappears hull-first over the horizon |
| Lunar eclipse | Earth's shadow on the Moon is curved |
| Satellite images | Photographs from space clearly show a spherical Earth |
| Circumnavigation | Sailing around the world (Ferdinand Magellan, 1519–1522) |
| Different stars | Different constellations are visible from different latitudes |
| Coriolis effect | Deflection of winds and ocean currents due to Earth's rotation |
'The Earth is not a perfect sphere. It is an oblate spheroid — flattened at the poles and bulging at the equator due to centrifugal force from its rotation.'
Latitudes
| Term | Definition |
|---|---|
| Latitude | Angular distance north or south of the Equator, measured in degrees |
| Equator | 0° latitude; divides the Earth into Northern and Southern Hemispheres |
| Tropic of Cancer | 23.5° N |
| Tropic of Capricorn | 23.5° S |
| Arctic Circle | 66.5° N |
| Antarctic Circle | 66.5° S |
| North Pole | 90° N |
| South Pole | 90° S |
Important Parallels of Latitude
| Parallel | Degrees | Significance |
|---|---|---|
| Equator | 0° | Divides Earth into North and South |
| Tropic of Cancer | 23.5° N | Northernmost limit of direct Sun's rays |
| Tropic of Capricorn | 23.5° S | Southernmost limit of direct Sun's rays |
| Arctic Circle | 66.5° N | 24-hour daylight/sunlight in summer/winter |
| Antarctic Circle | 66.5° S | 24-hour daylight/sunlight in summer/winter |
Heat Zones of the Earth
| Zone | Latitudinal Extent | Climate |
|---|---|---|
| Torrid Zone | 0° to 23.5° N and S | Hottest; direct Sun's rays |
| Temperate Zone | 23.5° to 66.5° N and S | Moderate; slanting rays |
| Frigid Zone | 66.5° to 90° N and S | Coldest; extremely slanting rays |
Longitudes
| Term | Definition |
|---|---|
| Longitude | Angular distance east or west of the Prime Meridian |
| Prime Meridian | 0° longitude; passes through Greenwich, London (UK) |
| 180° meridian | Opposite the Prime Meridian; roughly the International Date Line |
Important Longitudes
| Longitude | Significance |
|---|---|
| 0° (Prime Meridian) | Reference line for time zones |
| 82.5° E | Standard Meridian of India (IST) |
| 180° | International Date Line (approximately) |
Time Zones
Greenwich Mean Time (GMT)
GMT is the mean solar time at the Royal Observatory in Greenwich, London. It serves as the world's time standard.
Indian Standard Time (IST)
| Aspect | Detail |
|---|---|
| IST longitude | 82.5° E (passes through Mirzapur, Uttar Pradesh) |
| Difference from GMT | IST is 5 hours 30 minutes ahead of GMT |
| Calculation | 82.5° x 4 minutes = 330 minutes = 5 hours 30 minutes |
Calculation of Time
Since the Earth rotates 360° in 24 hours:
- 15° of longitude = 1 hour of time
- 1° of longitude = 4 minutes of time
- Eastward = add time; Westward = subtract time
International Date Line
| Aspect | Detail |
|---|---|
| Location | Approximately 180° longitude |
| Purpose | To adjust the calendar date when crossing the Pacific |
| Deviation | The line zigzags to avoid dividing island groups |
| Crossing westward | Add one day (you lose a day) |
| Crossing eastward | Subtract one day (you gain a day) |
'When you cross the International Date Line from east to west, you 'lose' a day. When you cross from west to east, you 'gain' a day.'
Rotation and Revolution
| Motion | Definition | Period | Effects |
|---|---|---|---|
| Rotation | Earth spinning on its axis | 24 hours (approximately) | Day and night; Coriolis effect; tides |
| Revolution | Earth orbiting the Sun | 365.25 days | Seasons; varying day length |
Seasons
| Season | Northern Hemisphere | Southern Hemisphere |
|---|---|---|
| Summer solstice | June 21 (longest day) | December 22 (shortest day) |
| Winter solstice | December 22 (shortest day) | June 21 (longest day) |
| Equinox (Spring) | March 21 | September 23 |
| Equinox (Autumn) | September 23 | March 21 |
Self-Test
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Fill in the blank: Earth's shape is an ______. (Answer: oblate spheroid)
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True or False: The Equator is at 90° latitude. (Answer: False — 0°)
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Match: (a) Tropic of Cancer — 23.5° N; (b) Prime Meridian — 0°; (c) Arctic Circle — 66.5° N. (Answer: All correct)
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Calculate: If it is 12 noon at GMT, what is the time at 82.5° E? (Answer: 12 + 5:30 = 5:30 PM)
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Explain: What is the International Date Line and why does it zigzag? (Answer: It is the line at approximately 180° longitude where the date changes. It zigzags to avoid dividing island groups under the same political jurisdiction.)
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Critical thinking: Why does the Earth have seasons? (Answer: Due to the 23.5° tilt of Earth's axis combined with its revolution around the Sun, causing varying angles of sunlight throughout the year.)
Summary
Earth is a unique planet in the solar system — the only one known to support life. Its oblate spheroid shape, axial tilt, and rotation-revolution motions create the conditions for life and produce the seasons, day and night, and time zones. The grid of latitudes and longitudes allows precise location of any place on Earth. Understanding these fundamentals is essential for all further study of geography. For ICSE students, this chapter provides the foundational concepts for physical and human geography.
This chapter is aligned with the ICSE Class 9 2025–26 Geography syllabus prescribed by the Council for the Indian School Certificate Examinations (CISCE).
