Planet Earth: Solar System, Globe, Motions, Maps, Landforms, Interior, Air and Water (Class VI-VII Geography)

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The earth in the solar system

The solar system has the Sun, eight planets, their moons, asteroids and meteoroids; in 2006 the International Astronomical Union reclassified Pluto as a dwarf planet. From the Sun the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The earth, third from the Sun at about 150 million km, is called the blue planet because about two-thirds of its surface is water, and its shape, slightly flattened at the poles, is called a geoid. The Moon, about 384,400 km away, takes about 27 days both to go round the earth and to spin once on its axis, so we always see the same side.

Latitudes, longitudes and time

The equator (0°) divides the earth into the Northern and Southern Hemispheres. The Tropic of Cancer (23½° N), Tropic of Capricorn (23½° S), Arctic Circle (66½° N) and Antarctic Circle (66½° S) mark the heat zones: the Torrid Zone between the tropics gets the overhead midday sun at least once a year, with the Temperate and Frigid Zones towards the poles. The Prime Meridian (0°) passes through Greenwich in London. The earth turns 360° in about 24 hours, so 15° of longitude equals one hour and 1° equals 4 minutes. Indian Standard Time is the local time of 82½° E, which passes near Mirzapur, and is 5 hours 30 minutes ahead of Greenwich Mean Time.

Motions of the earth

Rotation on the axis takes about 24 hours and causes day and night. The axis makes an angle of 66½° with the plane of the orbit. Revolution round the Sun along an elliptical orbit takes 365¼ days, so every fourth year is a leap year of 366 days, with 29 days in February. Tilt and revolution cause the seasons: on 21 June the Sun is overhead at the Tropic of Cancer and the Northern Hemisphere has its longest day (summer solstice); on 22 December it is overhead at the Tropic of Capricorn (winter solstice in the north); and on 21 March and 23 September it is overhead at the equator, so day and night are equal everywhere (equinoxes).

Maps

A map represents the earth or a part of it on a flat surface, drawn to scale. Physical maps show natural features, political maps show countries, states and cities, and thematic maps show one theme such as rainfall or roads; a globe is a true model of the earth but cannot show much detail. Every map has three components: distance (shown by the scale), direction and symbols, and a large-scale map shows a small area in detail. Conventional colours are used: blue for water, brown for mountains, yellow for plateaus and green for plains. A sketch has no scale, while a plan is a large-scale drawing of a small area.

Realms and major landforms

The earth has four realms: the lithosphere, hydrosphere, atmosphere and biosphere, where land, water and air meet to support life. Asia is the largest continent and Australia, an island continent, the smallest; the Pacific is the largest ocean and holds the Mariana Trench, nearly 11 km deep. Air is about 78 per cent nitrogen and 21 per cent oxygen. Fold mountains may be young (Himalayas, Alps) or old (Aravalli, Urals); block mountains, such as the Vosges, form where large areas break and are displaced vertically; and volcanic mountains include Kilimanjaro and Fujiyama. The Tibet plateau is the highest, at 4,000 to 6,000 m, and the Chota Nagpur plateau is rich in minerals.

Inside the earth and the changing surface

The crust is about 35 km thick under continents, of silica and alumina (sial), and about 5 km under oceans, of silica and magnesium (sima). The mantle extends to about 2,900 km, and the core, of nickel and iron (nife), has a radius of about 3,500 km. Igneous rocks form when molten magma cools (granite, basalt), sedimentary rocks form from deposited layers and may hold fossils, and metamorphic rocks form under heat and pressure (clay to slate, limestone to marble). Endogenic forces cause volcanoes and earthquakes, which start at the focus below the epicentre and are recorded by a seismograph. Exogenic forces wear the surface by weathering and erosion: rivers form meanders, ox-bow lakes and deltas, sea waves form arches and stacks, glaciers leave moraines and wind builds sand dunes.

Air, water and natural vegetation

Weather occurs in the troposphere, on average 13 km high; the stratosphere, up to 50 km, holds the ozone layer; meteorites burn up in the mesosphere; and the ionosphere helps radio transmission. Weather is the day-to-day state of the atmosphere, and climate its average over many years. Winds are permanent (trade winds, westerlies, easterlies), seasonal (monsoons) or local (the loo). About 97.3 per cent of the earth's water is in the salty oceans. Tides rise twice a day, with spring tides at full moon and new moon, and fishing grounds are rich where warm and cold currents meet, as the Gulf Stream and the Labrador Current do near Newfoundland. Vegetation depends on temperature and moisture: forests, grasslands, thorny bushes in deserts, and mosses and lichens in the tundra.

Key facts

Planets from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune; Pluto became a dwarf planet in 2006; Venus is called the earth's twin.
Tropic of Cancer 23½° N, Tropic of Capricorn 23½° S, Arctic Circle 66½° N, Antarctic Circle 66½° S; the Torrid Zone lies between the two tropics.
1° of longitude = 4 minutes and 15° = 1 hour; Indian Standard Time is based on 82½° E and is 5 hours 30 minutes ahead of Greenwich Mean Time.
Solstices: 21 June (Sun overhead at the Tropic of Cancer) and 22 December (overhead at the Tropic of Capricorn); equinoxes: 21 March and 23 September.
Map components: distance (scale), direction, symbols; colours: blue water, brown mountains, yellow plateaus, green plains; a sketch has no scale, a plan is large-scale.
Crust: about 35 km under continents (sial) and about 5 km under oceans (sima); mantle to about 2,900 km; core of nickel and iron (nife).
Rocks: igneous (granite, basalt), sedimentary (sandstone; may hold fossils), metamorphic (clay to slate, limestone to marble).
Atmosphere: troposphere (average 13 km, weather), stratosphere (ozone, aeroplanes), mesosphere (meteorites burn up), thermosphere (ionosphere), exosphere.
Temperate grasslands: prairies (North America), pampas (South America), steppes (Central Asia), velds (South Africa), downs (Australia); tropical grasslands are savannas.
In class: use a torch and a tilted ball to show day and night and the seasons, and draw a plan of the classroom to scale before moving on to atlas maps.

Quick revision

8 planets, Pluto a dwarf planet since 2006; earth third from the Sun, a geoid; tropics at 23½°, polar circles at 66½°; 1° = 4 minutes, IST at 82½° E (5 hours 30 minutes ahead of GMT); rotation 24 hours, revolution 365¼ days; solstices 21 June and 22 December, equinoxes 21 March and 23 September; map = scale, direction, symbols; crust sial and sima, core nife; troposphere 13 km; 97.3 per cent of water in oceans.

Practise this topic

All 10 lessons on Social Studies and its Pedagogy (Paper II)