Grade - 8 (Science ) :- Earth and Space

                 Unit :- 21             Earth and Space

Some hypothesis about origin of the  Earth :

1.)George Wafan’s hypothesis:  According to the hypothesis propounded by George Wafan’s ,the earth along with other planets and satellites was formed when a comet revolving around the universe struck the sun millions years ago i.e.the broken pieces of round mass are planet and the central parts of that mass is the sun.

2.)Jeans Jeffery or Tidal hypothesis: According the hypothesis propounded by Jean and Jeffery a big star was orbiting around the sun  came closer to the sun and developed a tide due to its own attraction of the star and the material splitted out from the sun while cooling and planet , satellite were formed.

3.Nabular hypothesis (Kant-Laplace nebular hypothesis): This hypothesis was first propounded by German philosopher Kant and latter it was modified by French mathematician Laplace,According to this hypothesis whole of our solar system  including other heavenly bodies was formed in the form of Nebula i.e. large mass of dust particle and gases.

4. Formation of Solar system from the Milky way Galaxy : According to this hypothesis the sun was formed by the accumulation of small particles present in the milky way galaxy due to gravitational force .Remaining members of the solar system i.e. planets ,satellites ,comets etc. were formed by  the same process as the formation of the sun .

Causes of evolution of life on the Earth :

i)Earth has suitable temperature for the survival of living organism .

ii)Earth has sufficient amount of water essential for survival of living organism  .

iii)Earth has sufficient amount of life supporting gases i.e. Oxygen, carbon dioxide, nitrogen etc.

Note:

Estimated age of earth=about 4.5 billion year

Diameter of earth=12756.3Km

Mass of earth=6×10²⁴kg

Area of surface=5097×10⁵Sq.km

Major gases.N_2,O₂,CO₂

Number of satellite=1(Moon)

Axis :The imaginary line passing through the centre of earth that connects the north pole and south poles is called axis .

Diurnal motion or rotation :The motion of earth on its own axis is called diurnal motion .One complete round on its axis is called  one rotation .Earth takes about 24 hours to make complete one rotation on its axis .It is causes of day and night on the earth .

Orbit :The elliptical path in which earth revolves around the sun is called orbit .Earth takes about 365.6 days to make complete one revolution on its orbit .

Effects on Earth due to tilted axis of the rotation of the earth with orbital plane :The axis rotation of earth is tilted at an angle of 66.5⁰ with its orbital plane .It has following effects on the earth :

i)Days and night are not equal except in the equatorial region .Some days are longer that night and days are shorter than night .

ii)Change of seasons takes place .

iii)The rays of the sun do not fall perpendicularly  in a place all the time .It affects the climate of that place .

iv)The sun rises and sun set do not always takes place from the same place .

v)The height of the sun from the earth in the afternoon change every day .

Season: The division of the year marked by changes in weather ,ecology,and hours of daylight is called season .Seasons is divided  into  4 parts on the earth surface :

1)Summer (Grisam )       2.)Autumn (Sarad)

3.Winter (Sishir )              4.)Spring (Basant )

i)The axis of rotation of the earth is tilted at an angel of 66.5⁰

and ii) the earth is not always at the same distance from the sun due to its elliptical orbit .Due to these reasons change of seasons takes place on the earth .

There are four seasons. They are:

1. Summer: On December 21^st .(Winter solstice)it is the shortest day and longest night. On those days northern hemisphere experiences, coldness and southern hemisphere experience hotness. On those days northern hemisphere experiences winter season whereas southern hemisphere experiences summer season.

2. Winter: On June 21st,(Summer solstice) .it is the shortest night and longest day. Northern hemisphere faces nearer towards the sun. So northern hemisphere experiences the longer day and shorter nights. On these days, northern hemisphere experiences hotness and southern hemisphere experience coldness.

3. Autumn: On March 21st, (Spring equinox) the rays of the sun again lie on the equator. Day and night have a same interval of time. So, there will be no extreme hotness or coldness. On those days northern hemisphere experiences, spring season whereas southern hemisphere experiences the autumn season.

4. Spring: On September 23rd,(Autumn equinox) the rays of the sun exactly lie on the equator. That day both hemisphere experiences equal interval of day and night. So, there will be no extreme hotness or coldness. On those days, northern hemisphere experiences autumn season whereas southern hemisphere experiences spring season.

Phase of moon :The changing of shape of moon every day is called  phase moon .

Causes of phases of moon :

i)The moon is non luminous body and reflect the sun light .

ii)The moon revolves around the earth in it’s elliptical orbit .

Bright half or waxing moon :The period of a month in which bright parts of moon increases is called bright half .

Dark half or waning moon :The period of a month in which dark parts of moon increases is called dark half .

Phase of moon :

i)New moon: When the moon forms an angle of 0⁰ from the earth’s plane  of the earth’s orbit the whole dark parts of moon faces towards the earth any parts of moon not seen is called  New moon(Aunshi).

ii)Waxing crescent :After 24 hours of new moon a little bright part of the moon faces toward the earth and minimum part of moon is seen is called waxing crescent .

iii)First Quarter :When  the moon form an angle 90⁰ from the plane of earth’s orbit bright part of moon is seen in the form of hemisphere is called first quarter .

iv)Waxing gibbous :When moon from an angle of 168⁰ from the plane of  earth’s orbit ,a very little part of moon remains dark is called waxing gibbous .

v)Full moon :When moon form an angle of 180⁰ from the plane of earth’s orbit the moon is seen completely is called full moon .

vi)Warning gibbous :When moon form an angle of  192⁰  with the plane of earth ‘s orbit very little part of moon is covered with darkness is called warning gibbous .

vi)Last quarter :When moon form an angle  of 270⁰ with plane of earth’s orbit, the moon is seen in the form of hemisphere is called last quarter .

vii)Warning crescent :When moon form an angle of 348⁰ with the plane of earth’s orbit ,a very little part of moon is seen in the form hemisphere is called warning  crescent .

Synodic month :The time periods of full new moon to  full new moon is called synodic month .It is about 29 ½  days .It is also called lunar month .12 lunar month makes one lunar year .

Sidereal month :The time taken by moon to complete one revolution around the earth or own its orbit is called sidereal month .It is about 27.33 days .