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

The origin of the Solar System Theories

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
  • Materials would disperse than condense

2. Collision Theory

  • Star with greater gravitational pull passed near the sun
  • It attracted large quantities of gaseous materials
  • The materials split into portions
  • Large portions collided with smaller ones and swept them to form planets. The weaknesses are the same as the passing star’s Theory.

3. Nebula Cloud Theory

  • There was a slowly rotating cloud of dust and gas called Nebula
  • It cooled and began to contact
  • Rotation speed increased and successive rings of gaseous materials were formed.
  • The rings condensed to form planets
  • The central gaseous material remained as the sun Evidence
  • Rotation and revolution of planets in anticlockwise direction Weakness

The origin of nebular is not explained.

4. Supernova/explosion Theory

  • There was a violent explosion of a star.
  • A cloud of dust (nebular) remained from the star.
  • There was fast movement of the cloud due to the force of explosion.
  • Rotation speed increased due to gravitational attraction
  • The cloud flattened into a disc.
  • The matter began to accumulate towards the centre to form a proto-sun which later began to shine.
  • The rest of the cloud collapsed to form planets.


  • Doesn’t explain the origin of the star
  • Doesn’t explain the cause of explosion

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 15m◦c and at the surface is 5500◦c.
  • Radiates solar energy which is very important for all forms of life on the earth.

Planets are large spherical celestial/heavenly bodies in space.

There are 9 planets in our solar system.


  • 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 sun
  • 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 planets
  • Can 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


  • 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 no satellite.


  • 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 16 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 a ring around it
  • Has 18 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 8 satellites
  • Takes 84 earth years to revolve around the sun


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


  • 9th planet from the earth
  • Farthest from the sun
  • The smallest
  • 1/6 the size of the earth
  • Approximately 5900m km from the sun
  • Takes 248 earth years to revolve around the sun
  • Has one satellite
  • Very little is known about it

Other Celestial Bodies Natural Satellites

Any natural heavenly body that orbits around a planet e.g. moon for earth, tritan for Saturn and Triton for Neptune.


  • 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 e.g. Halley’s Comet which appears after every 76 years.


  • 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.

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

Lunar/moon Eclipse

  • The earth comes between the moon and the sun
  • The earths 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 is called geoid/ovoid/oblate spheroid due to being an imperfect sphere by being wide at the equator and flat at the poles.

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.
  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 size of the earth

  • Equatorial diameter-12756km
  • Equatorial circumference-40085km
  • Polar diameter-39995km
  • Surface area of the earth-510×106 km2
  • Water surface-73%.

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 of the earth on its own axis (imaginary line through the centre from N pole to S pole.

  • Rotates through 360◦.
  • Takes 24 hours (day) to complete 1 rotation.
  • Rotates in an anticlockwise direction (west to east).

Effects of Rotation of the Earth

  1. Creates 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 N hemisphere to the left and in the S hemisphere to the right.
  3. It causes rising and falling of ocean tides.
  4. Causes time difference between longitudes.
  5. Takes one hour to go through 15◦.
  6. 4 min to go through 1◦.

Calculation of Local Time

The time recorded in places within the same longitude.

A longitude is an imaginary line running from N to S which shows how far E or W a place is from the prime meridian.

Greenwich Meridian (0◦) longitude is the point of reference when calculating time.

Time is gained towards the E and lost towards the W.


  1. Suppose the time at GWM is 12 noon what is the local time at Watamu 40◦E? Time gained=40×4=160min=2 hours 40min Local time at Watamu is 12.00+2.4
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