The Earth and the Solar System - Geography Form 1 Notes

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  • Solar system is the group of heavenly bodies comprising the sun and the nine planets.

The Origin of the Solar System


A theory is a set of reasoned ideas intended to explain facts or events

  1. Passing Star Theory
    • A star with a greater gravitational pull passed near the sun
    • It attracted large quantities of gaseous materials from the sun
    • The materials split, cooled and condensed to form planets
    • The planets were set in orbit by the passing star

    • Doesn’t explain the origin of the sun and star.
    • Minimal chance of a star approaching another
    • High temperature, gaseous material drawn from the sun would disperse than condense
    • The gases should have followed the star since it had a greater gravitational pul
  2. Nebula Cloud Theory
    • There was a slowly rotating cloud of dust and gas called Nebula
    • As the nebula rotated it flattened into a disc with high concentration of materials at the centre
    • The high concentration of material at the centre condences to form the sun.
    • Gases such as ammonia and methane condense to form larger planets that is Jupiter, Saturn, uranius and Neptune
    • Closer to the centre of Nebula, particles of dust and gas bumping into each other formed the inner smaller planets.

    • The origin of nebular is not explained.
    • Cause of rotation is not mentioned.
    • All planet should rotate in the direction of the rotation of the nebula that is west to east but venus and uranius rotates from west to east.

Composition of the Solar System

  1. The Sun
    - It’s the centre of the universe.

    • It’s a star.
      A star is a heavenly possessing its own light which it transmits.
      Nebula/galaxy is a cluster of stars.
      The earth is in a galaxy called The Milky Way.
    • It’s made of very hot gases mainly hydrogen (70%) and helium (30%). Has a diameter of 1392000km.
    • Surrounded by a layer of gas which has boiled from its surface which is called corona.
    • Rotates on its own axis in anticlockwise direction.
    • Has gravitational pull which holds all the planets in orbit around it. An orbit is a path which a planet or a satellite follows around a star or a planet. 
    • Temperature at its centre is the surface is 6000◦c.
    • Radiates solar energy which is very important for all forms of life on the earth.
  2. The Planets
    - Planets are large spherical celestial/heavenly bodies in space.
    - There are 8 planets in our solar system.
    the planets
    • Spherical in shape.
    • Don’t have their own light but reflect it from the sun.
    • Revolve around the sun in anticlockwise direction.
    • Have their own force of gravity
    • Only one is known to support life.

    The following are the planets arranged in order from the one nearest to the sun.

    • Nearest from the su
    • Its 58m km from the sun
    • Has no satellites
    • Takes approximately 88 earth days to revolve around the sun

    • 2nd planet from the sun
    • It’s 108m km from the sun
    • One of the brightest planetsCan be seen clearly with naked eyes
    • Takes approximately 225 earth days to revolve around the sun
    • Slightly smaller than the earth
    • Has no satellites
    • Together with the earth they are called twin planets due to having many similarities Earth
    • The 3rd planet from the sun
    • The earth and the heavenly bodies make the universe
    • The only planet that supports life
    • The home of man
    • Approximately 149m km from the sun
    • Takes 365 ¼ days to revolve around the sun
    • Has one satellite, the moon

    • Also called The Red Planet because when it’s observed through a telescope it appears reddish.
    • The 4th from the sun
    • Slightly smaller than the earth
    • Approximately 228m km from the sun
    • Takes 687 earth days to revolve around the sun
    • Between Mass and Jupiter there are small celestial bodies called planetoids.
    • Has 2 satellites.

    • 5th planet from the sun
    • Approximately 778m km from the sun
    • Largest in the universe
    • Rotates on its own axis at very fast speed
    • Has flattened poles due to its fast speed of rotation
    • Has very thick layers of ice on its surface
    • takes 12 earth years to revolve around the sun
    • Has 13 satellites

    • 6th planet from the sun
    • Second largest planet
    • Approximately 1427m km from the sun
    • Takes 29 ½ earth years to revolve around the sun
    • Has 3 rings around it, a ring of clouds, a ring of fine dust and a ring of ice particles revolving around it at different speed.
    • Has 8 satellites

    • 7th planet from the sun
    • About 4 times bigger than the earth
    • Approximately 2870m km from the sun
    • Also rotates very fast
    • Also has flattened poles due to fast speed of rotation
    • It appears greenish foe being surrounded by methane gas
    • Has 5 satellite
    • Takes 84 earth years to revolve around the sun


    • One of the farthest from the sun
    • 8th planet from the su
    • Approximately 4497m km from the sun
    • Has 2 satellites
    • Takes 165 earth years to revolve around the sun
    • Very similar in size, colour and character with Uranus
  3. Other Celestial Bodies

    Natural Satellites
    • Any natural heavenly body that orbits around a planet e.g. moon for earth 

    • Also called minor planets.
    • Are small fragments of rocks left going around the sun when the solar system was formed
    • Found between Mass and Jupiter
    • Are 1500 in number
    • They sometimes collide with each other and planets due to Jupiter’s gravitational pull causing them to move in erratic orbits.

    • Heavenly bodies which appear to have a head and a long tail
    • Made of ice, dust and frozen gas
    • The head is made of many particles of dust, rock and frozen gases.
    • Their tail is made of gases and points away from the sun.
    • Move around the sun in extremely long and oval orbits
    • Their orbits cross the earth’s orbits.

    • Small heavenly body which strays from its orbit in the solar system and enters the earth’s atmosphere at very high speed.

    • A meteoroid which is burning out due to friction after entering the earth’s atmosphere.

    • Remains of a meteoroid which have reached the earth’s surface or incompletely burnt up meteoroid.
    • When they fall they sink into the ground forming craters
    • They are rich in iron.

    The Moon

    • A natural satellite
    • Receives its light from the sun and reflects it onto the earth.
    • It revolves around the earth
    • Takes 29.5 days to complete one revolution around the earth
    • Its orbit is almost circular
    • As it revolves around the sun it appears in various shapes ranging from crescent/new moon, half moon, gibbons moon and full moon.
    • Has gravitational pull which causes the rising and falling of the ocean level
    • As the moon orbits around the earth it creates an event called eclipse.

    • Phenomenon occurring when the rays of the sun are blocked from reaching the earth or the moon.
      Solar and Lunar Eclipse
      Solar/Sun Eclipse
    • The moon comes between the earth and the sun
    • The moon’s shadow is cast on the earth
    • The earth appears to be covered by darkness.

      Lunar/moon Eclipse
    • The earth comes between the moon and the sun
    • The earth’s shadow is cast on the moon
    • The moon appears to be covered by darkness

The Origin of the Earth

  • A star with a greater gravitational pull passed near the sun
  • It attracted large quantities of gaseous materials from the sun
  • The materials split, cooled and condensed
  • Heavier materials collected at the Centre to form the core
  • Less dense materials collected around the core to form the mantle
  • The lightest materials formed the crust


The Shape of the Earth

  • The shape of the earth is called geoid/ovoid/oblate spheroid due to being an imperfect sphere by being wide at the equator and flat at the poles.
    the shape of the earth

Forces that are Responsible for the Spherical Shape of the Earth

  1. Force of gravity- pulls every part of the earth towards the centre giving it a rounding effect.
  2. Centrifugal force- causes the earth to bulge outwards
  3. The centripetal force-causes the flattening at the pole.

Proofs/Evidence that the Earth Is Spherical

  1. If one moves towards the east in a straight line he will end up where he started; this is called circumnavigation.
  2. Satellite photographs taken from space show that the earth is like a sphere.
  3. Places in the east see the sun earlier than those in the west.
  4. When a ship is approaching the smoke is seen first, then the mast and finally the whole ship.
  5.  All the planets are spherical so the earth being one of them is also spherical.
  6. During the moon eclipse the earth casts a spherical shadow on the moon.
  7. The earth’s horizon appears curved when observed from a very high point like a tower.

The Movement of the Earth

  • There are 2 movements of the earth namely:
    1. Rotation of the earth on its axis
    2. Revolution of the earth around the sun

Rotation of the Earth

  • Movement/spinning of the earth on its own axis in an anticlockwise direction (west to east).
  • Rotates through 360.
  • Takes 24 hours (day) to complete 1 rotation.

Effects of Rotation of the Earth

  1. Causes day and night because at any one time one side of the earth faces the sun (day) and the other remains in darkness (night).
  2. Causes deflection of winds and ocean currents( in the north hemisphere to the left and in the south hemisphere to the right).
  3. It causes rising and falling of ocean tides.
  4. It causes variation in the speed of air masses.
  5. Causes time difference between meridians 150.

Calculation of Local Time

  • The time recorded in places within the same longitude.
  • A longitude is an imaginary line running from north to south which shows how far east or west a place is from the prime meridian.
  • Greenwich Meridian (0◦) longitude is the point of reference when calculating time.
  • Time is gained towards the east and lost towards the west.


  1. Suppose the time at GWM is 12 noon what is the local time at Watamu 40E?

    Time gained=40×4=160min=2 hours
    40min Local time at Watamu is 12.00+2.40=14.40-1200=2.40pm.
  2. At Dar-es-Salaam 40◦E time is 12pm what is the time at Ecuador 40E?
    Ecuator is behind in time =12.00-4=8am.
    If the places are on the same side subtract the degrees to get the difference and add or subtract from the reference time depending on which side the place is.

Calculation of Longitude

  1. What is the longitude of place x whose local time is 8 am when local time at GWM is noon?

    Time difference =12.00-8=4 hours
    Since x is behind in time its then 60W.

Standard Time and Zones

  • Standard time is time recorded by countries within the same time zone.
  • Standard time was come up with due to confusion resulting from time changing at every longitude.
  • The world has 24 time zones.

The International Date Line

  • It’s the 180 longitude, its run from north to south but avoid areas with land masses

    Effects of Crossing It
  • One gains time when he crosses it from W to E and has to adjust the clock ahead by 24 hours.
  • One loses time when he crosses it from E to W and has to adjust the clock backwards by 24 hours.

Revolution of the Earth

  • Movement of the earth in its orbit around the sun.
  • It’s in anticlockwise direction.
  • The orbit of the earth’s revolution is elliptical.
  • Takes 365¼ days in a year or 366 days in a leap year (every 4 years).
  • The sun moves from the tropic of cancer to the equator and then towards tropic of Capricorn and back to the tropic of cancer.
  • 21st march and 23rd September are called equinoxes because the length of day and night is equal. The sun is vertically overhead at noon at the equator.
  • 21st June is called summer solstice because its summer in the N hemisphere. The sun is vertically overhead at noon at the tropic of cancer.
  • 22nd December is called winter solstice because its winter in the northern Hemisphere. The sun is vertically overhead at noon at the tropic of Capricorn.
  • Solstice is the period of maximum tilting of the earth towards the sun.
    revolution of the earth

Effects of the Revolution of the Earth

  1. Causes the four seasons summer, autumn, winter and spring due to the movement of overhead sun causing changes in the heat belt.
  2. Causes variation of day and night’s lengths due to the earth’s axis being inclined to the path of revolution at an angle of 60◦.
    • Equinoxes have equal lengths of day and night throughout the world.
    • Summers have longer days and shorter nightsWinters have longer nights and shorter days.
      geography fig 5
  3. Causes changes in the position of the midday sun at different times of the year
  4. Causes lunar eclipse due to revolution bringing the earth in line with the sun and the moon.
    NB: Equinoxes is the time of the year when the sun overhead position at noon is over the equator hence causing equal length of day and night throughout the world.


Latitudes and Longitudes


  • Refers to an imaginary line which is drawn on a map running from west to east and measured in degrees showing how north or south a place is from the equator.


  • Refers to an imaginary line which is drawn on a map running from north pole to south pole and is measured in degrees east or west from the prime meridian

Differences between Latitudes and Longitudes

  1. Latitudes run from west to east while longitudes run from north to south
  2. Latitudes shows how far a place is from the equator while longitudes show how far a place is from the prime meridian
  3. Latitudes are parallel to one another while longitude meet at the poles
  4. The distance between latitude is even around the globe while that between longitudes is longest at the equator and decreases pole wards.

Structure of the Earth

The structure of the earth consist of a series of layers both internal and external, these layers includes

earthstructre no 1

earthstructrure 1


Internal Structure of the Earth

The evidence used to study the earth’s interior are

  1. mining
  2. drilling
  3. quarrying/excavation

 A. Crust/Lithosphere

  • Outermost layer of the earth
  • Made of soils and other loose deposits of sand
  • The dominant rocks are granites.
  • Extends from 16-24 km thick but where mountains occurs it can have a thickness of 70km.
  • Has 2 layers; continental crust and oceanic crust.

    1. Sial

  • Also called continental crust
  • Made of light colored rocks
  • Called sial because it’s made up of silica and aluminum.
  • It is lighter( having a density of 2.7 grams/cc) this makes it float on the oceanic crust.

     2. Sima

  • Also called oceanic crust
  • Mainly made of basaltic rocks which are brittle.
  • Called sima because it is made of silica and magnesium.
  • It has a density of 2.8-3.0gm/cc
  • It is thin ranging from 6km-10km in thickness.

    Mohorovicic Discontinuity (Moho)

  •  A definite zone of discontinuity between the crust and the mantle.
  • Was discovered by Dr. Andrija Mohorovicic in 1909.

B. The Mantle/Asthenosphere

  •  On average its about 2900km thick
  • Its composed of silicate rocks rich in iron and magnesium.(olivine)
  • It has an average density of 3.0-3.3gm/cc
  • Its divided into two layers namely; upper mantle and lower mantle

    Upper mantle

  • Rocks are more elastic than those of sima.
  • Temperature is about 1000c.

    Lower mantle

  • Rocks are like very viscous liquid.
  • Temperature ranges between 1000c to 3000c.

    Gutenberg Discontinuity

  •  A definite zone of discontinuity between mantle and core

C. Core/Barysphere/Centrosphere

  • Its has a radius of 3470km
  • Its divided into two layers namely; inner core and outer core. 

    Outer Core
  •  Its has an average density of 12.0gm/cc
  • Composed of hot molten rocks
  • Made up of nickel and iron
  • Temperatures are up to 3700c. 

    Inner Core

  • A solid mass of iron
  • Its has a density of 16 -17gm/cc.
  • Temperatures are estimated to be 3700c to 5500c.

Why the Interior of Earth Is Very Hot

  1. Due radio-active decay causing most of the heating. The decay of the radio-active material causes explosion occasionally releasing a lot of heat through nuclear fission
  2. Due to great pressure as a result of overlying crustal materials, this intense pressure generates a lot of heat inside the earth.
  3. The earth stills retains the original heat which it had before its started cooling, during cooling the outer parts cool faster than the interior trapping the original heat inside it

External Structure of the Earth

A. The Atmosphere

  • It’s a layer of transparent and odourless gases held to the earth by its gravitational pull.
  • The earth revolves with it because its held onto it by gravity
  • It’s about 330km thick.

Composition of the Atmosphere

  • Gases-exist as a mixture
  • Smoke particles
  • Dust particles
  • Water vapour

The Structure of the Atmosphere

  • It’s divided into 4 layers/zones namely: troposphere, stratosphere, mesosphere and thermosphere.

  • Lowest layer of the atmosphere

  • Contains most of the atmospheric water vapour or clouds
  • Extends 8km at the poles and 13km at the equator
  • Temperature decreases with increase in altitude at a rate of 6.5c per 1000m (normal lapse rate)
  • Pressure falls with increase in height
  • The speed of wind increases with increase in height
  • Contains dust particles
  • There is a zone of transition between troposphere and stratosphere called tropopause,
  • In these zone there is no change in temperature with increase in height, hence experiences zero lapse rate


  •  Extends from the tropopause to a height of 30km

  • The temperature increases with increasing altitude , this is called the negative lapse rate, the temperature increase is caused by the concentration of ozone gas
  • Has ozone layer which absorbs harmful ultraviolet radiation.
  • Winds are light in the lower but increase with height.
  • The atmospheric pressure continues to fall as the air is dry
  • Air is calm so it’s used by passenger jets
  • Limited amounts of water vapour

    There is a zone of transition between stratosphere and mesosphere called stratopause.


  •  Extends from 50-80 km


  • Temperature decreases with increasing altitude, this is called positive lapse rate, the decrease in temperature is because there is no water vapour or ozone gases to absorb the incoming terrestrial radiation
  • Its has the lowest temperature ranging -900c
  • Has very strong winds that at times reaches 3000km/hr

    There is a zone of transition between mesosphere and thermosphere called mesopause .which is an inversion layer.


  •  Extends from 90-120km


  • High radiation is present.
  • Temperature rapidly increases with height, this is due to the over concentration of atomic oxygen which absorb incoming ultra-violet radiation
  • Temperature reaches to a high of over 15000c
  • The pressure is very low.
  • Gases and molecules in this layer exist as ions due to high radiation.

  • Thermosphere has no definite top but merges gradually into the outermost part of the atmosphere called exosphere.
  • Exosphere consists of rare gases like hydrogen and helium.
  • Beyond the atmosphere there is the outer space.
  • Outer space is the universe beyond the atmosphere in which other planets and stars exist.

Composition of the Atmosphere

The atmosphere is composed of the following gases

  1. Nitrogen 78%
  2. Oxygen 20%
  3. Water vapour 1%
  4. Argon 0.94%
  5. Carbon dioxide 0.03%
  6. Inert gases such as neon, krypton and xenon are found in traces (negligible percentages)

Significance of the Atmosphere

  1. Animals and plants breathe in from it oxygen for respiration.
  2. Plants use carbon dioxide from it for photosynthesis.
  3. Water vapour in the atmosphere condenses to form clouds which give us rain.
  4. Ozone layer in the stratosphere shields us from ultraviolet radiation which may cause cancers.
  5. Carbon dioxide and methane in the atmosphere cause global warming through the green house effect.

 B. The Hydrosphere

  • Part of the earth’s surface covered by water masses e.g. oceans, seas, lakes, rivers and even underground water. It comprises 73% of the earth’s surface area.
  • The lower atmosphere, hydrosphere and the upper part of the earths crust are called biosphere meaning the sphere of the earth in which organic liofe exists.
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