Published by: Nuru
Published date: 04 Dec 2021
In this chapter Earth, we discuss about Earth, Moon, season change, theory of origin of earth, rocks, soil, volcano in Grade 8 Science. Reference Notes for 8.
Origin of the Earth:
Origin of the earth is directly related to the origin of the solar system, as the earth is one of the members of the solar system. Many hypotheses have been proposed to explain the formation of the solar system. All these theories can be grouped into the following two general classes.
Some hypotheses about the origin of the earth:
Old planetesimal hypothesis:
This hypothesis state that very large amount of matter was thrown out of the sun due to the collision of the sun and another star. Those hot matters cooled and the other planets along with the earth were formed and the remaining mass was the sun.
Nebular hypothesis:
According to Nebular hypothesis, the sun and the planets were formed from a large whirling cloud of hot gases and dust. As the surface of the cloud cooled by the radiation, a ring of the matter was formed at the equatorial region of it. The ring was escaped out from the surface of the main mass and the first planet was formed. The process was repeated again and again till the entire solar system was formed and the remaining mass became the sun.
New planetesimal hypothesis:
This hypothesis is slightly different from the old planetesimal hypotheses suggested by Buffon. According to this hypothesis, the two stars did not collide but came very close to one another, and due to the gravity of the star, great bulges rose by the sun and the materials remained as large spiral arms and when they cooled, they broke away from the sun and the broken materials combined together to produce the planets and the remaining gaseous matter became the sun.
Tidal hypothesis:
According to this hypothesis, a passing star had exerted a tidal pull on the sun but the effect was to cause a long filament of gases to be drawn from the sun. The outer part of the filament escaped into space, the inner part came back into the sun and the middle part formed a series of round structures of different sizes, which were the planets.
Dust cloud hypothesis:
This hypothesis proposed that the sun and the planets were formed from a large cloud of gases and dust. The light of the star pushed the atoms of the gases and dust to form huge particles which were attracted to each other by the pull of gravity. A huge ball of materials formed the sun and the part of the cloud of dust and gases stayed around the sun and slowly rotated. The huge whirlpools originated in this rotating clouds formed planets. In this way, the earth was originated in its cold stage.
Reason behind evolution of living beings on earth are as follows:
Evolution of earth started before 4.5 billion years ago. When the evolution of earth started there was no suitable atmosphere for living beings. Various changes on the earth have made the atmosphere of the earth suitable for the evolution of living beings. Some reason behind evolution of living beings on earth are:
When we observe the sky daily for some days, we find that the moon changes its shape every day. It is called phases of the moon.
Phases of the moon occur because of following reasons:
When the moon forms an angle of 0 from the plane of earth’s orbit, the whole dark part of the moon faces towards the earth and hence any part of the moon is not seen i.e. a new moon(m1). After 24 hours, when the moon forms an angle 120 with the plane of its orbit, a little bright part of the moon faces towards the earth and a minimum part of the moon is seen bright. It is called waxing crescent (m2). When moon forms an angle of 900, the bright part of the moon is seen in the form of the hemisphere which is called a first quarter (m3). When moon forms an angle of 1680, very little part of the moon remains in the dark which is called waxing gibbous (m4).
When moon forms an angle of 1800 from the plane, the moon is seen completely round, which is called the full moon. When moon forms an angle of 192o from the plane very little part is covered by the darkness and it is called waning gibbous (m6). When moon forms an angle of 2700, the moon is seen in the form of hemisphere again, which is called last quarter (m7). When moon forms an angle of 3840, very little part of the moon is seen in the sky and it is called waning crescent (m8). After completing an angle of 3600 by the moon in its orbit again, the new moon occurs.
Some interesting facts about the moon are as follows:
Earth revolves around the sun in 365 days. Orbit in which earth revolves around is in an ellipse-shaped. So, sometimes earth goes nearer and sometimes further to the sun. When the earth moves nearer to the sun-earth becomes hotter and when the earth moves farther to the sun-earth becomes cooler. Due to this phenomena, earth experiences the different season. There are four seasons. They are:
On June 21st, the rays of the sun lie in the tropic of cancer. So, 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. In these days northern hemisphere experiences hotness and southern hemisphere experiences coldness. In those days, northern hemisphere experiences summer season whereas southern hemisphere experiences winter season.
On September 23, 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. In those days, northern hemisphere experiences autumn season whereas southern hemisphere experiences spring season.
On march 21st, 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. In those days northern hemisphere experiences, spring season whereas southern hemisphere experiences the autumn season.
On December 22, rays of the sun directly lie on the tropic of Capricorn. So, it is the shortest day and the longest night. Southern hemisphere faces nearer towards the sun. In those days, northern hemisphere experiences coldness and southern hemisphere experiences hotness. In those days, northern hemisphere experiences winter season whereas southern hemisphere experiences summer season.
The hard substances which form the Earth's surface are called rocks. They are chief constituents of earth's crust. The rocks are of different kinds. They are igneous rock, sedimentary rocks and metamorphic rocks. Molten hot rocks are called magma. The fossils of organisms are also mixed in the rocks.
Types of rocks are:
1. Igneous rocks:
It is formed by the cooling and hardening of magma. It is usually composed of very large particles. It is harder than sedimentary rocks. Example; Granite, pumice etc.
2. Sedimentary rocks:
It is formed by the compaction and cementing of layers. It is made up of dirt and sediment. It is softer than igneous rocks. Example; Sandstone, Limestone etc.
3. Metamorphic rocks:
It is formed by the effect of heat and pressure on other rocks. It prevents fossilization and likely destroy the fossils. Example: Marble, Diamond etc.
The uses of rocks are as follows:
The erosion of rocks by wind, rain, pressure, heat etc. for a long period of time forms soil. In other words, the physical and chemical breakdown of solid rocks by weathering process forms soil.
Soil is important to us because for the following reasons:
The coming out of molten mass from inside the earth is called volcano. It is a natural process in which hot lava comes out from the interior part of the earth into the earth's crust.
The effects of volcanoes are as follows: