The study of the earth’s interior is the subject of Geology, since, the interior part of the earth is not visible directly, knowledge about it is gathered through indirect means. The earth as a whole has been divided into 3 broad zones – Crust, Mantle and Core.
The Average thickness of this upper most layer of the earth is about 30 km. The average density of the upper crust is 2.8 and that of the lower crust is 3.0. This difference in density is believed to be due to the pressure. This discontinuity in density between upper crust and lower crust is known as ‘conrad discontinuity’. Silica and Allumunium are the main constituent elements of the crust. Therefore, it is also known as the SIAL.
At the lower end of the crust the speed of the seismic waves increases suddenly and reaches upto 7.9 to 8.1 km/sec. This results into a discontinuity between the upper mantle and the lower crust, which shows a change in the density of the rocks. This discontinuity was discovered in 1909 by a Russian scientist A. Mohorvicie, therefore, it is named after him as ‘Mono-discontinuity’. The mantle extends UPPER MANTLE” upto a depth of about 2900 km from the Moho-discontinuity,. Silica and Magnesium are the major constituting elements of this layer, hence it is also called SIMA.
Between 100 km to 200 km depth of the upper mantle, the velocity of the seismic waves is slowed down to 7.8 km/sec. Therefore, this zone is is known as the ‘Zone of Low Velocity’. The discontinuity in density between upper mantle and lower mantle is known as Repetti discontinuity.
At the lower end of the lower mantle the velocity of the P waves suddenly increases to 13.6 km/sec. This shows as sudden change in the density of the rocks resulting into a type of discontinuity, known as ‘Weicher Gutenber Dscontinuity’. It is divided into two parts Outer core and inner core. S waves cannot penetrate into the outer core. The density of the core is more than the double of the density of the mantle. In the inner core where the density is at its maximum, the velocity of the P waves is 11.23 km/sec because of its relatively liquid or semi-liquid state due to excessive temperature but because of too much pressure it is in semi-liquid or plastic state. It’s main constituens are nickel and iron, so it is also known as NIFE.
Motions of the Earth
The Earth has two types of motions:
- Rotational Motion
- Revolution or Orbital Motion
Rotation: The Earth rotates on its axis, from west to east like a top. This motion is called Rotation of the Earth.
Revolution: While rotating on its axis, the earth also goes around the sun in an elliptical path and completes one round in 365 days and 6 hours. The elliptical path traced by the earth is called its orbit. This motion of the earth is called revolution.
Earth’s Perihelion and Aphelion
Perihelion: When the earth is at the minimum distance from the sun, while in orbit, this position is known as perihelion. The Earth is closest to the Sun, or at the perihelion, about 2 weeks after the December Solstice, when it is winter in the Northern Hemisphere. When earth is at perihelion, it is about 147 million km (91 million miles) from the Sun. Perihelion always occurs near January 4th
When the planet is furthest away from the Sun, it is at aphelion. The words “aphelion” and “perihelion” come from the Greek language. In Greek, “helios” mean Sun, “peri” means near, and “apo” means away from. Aphelion lands near the 4th of July. The Earth is farthest away from the Sun, at the aphelion point, 2 weeks after the June Solstice, when the Northern Hemisphere is enjoying warm summer months.
Perigee and Apogee
Like the Earth’s orbit around the Sun, the Moon’s path around the Earth is elliptical. The point in the Moon’s orbit that is closest to the Earth is called the Perigee and the point farthest from the Earth is known as the Apogee.
Change in Seasons
The earth moves not only on its axis but also in its orbit around the sun. Therefore, the earth changes its position continuously with respect to the sun. There are four major positions of the earth, while it revolves around the sun.
Position on 21st June: In this position the sun shines perpendicularly over the Tropic of Cancer. This position in called the Summer Solstice. It is the time of summer season in the northern hemisphere, while in the southern hemisphere, it is winter season. The day in the northern hemisphere is longest on 21st June.
Position on 22nd December: In this position the Sun shines perpendicularly over the Tropic of Capricorn. This position is called Winter Solstice. During this period, days are long and nights are short in the southern hemisphere. This positions marks the summers in the southern hemisphere and winters in the northern hemisphere.
Positions on 21st March and 23rd September: In these two positions the Sun shines directly overhead on the Equator. Therefore, half part of all latitudes receives the sun-light at these times. Hence, everywhere, the duration of day and night is equal.. Seasons are also similar in both the hemispheres. These two positions are referred to as Equinoxes. 21st March is called Spring or Vernal Equinox whereas 23rd September is called Autumn Equinox.
Both the Earth and the Moon get light form the Sun. Only one part of the Moon can be seen from the earth because of equal rotational speed of both the earth and the Moon. Illuminated face of the Moon is seen on the Earth once a month on the Full Moon. On the New Moon the Moon is not seen at all because on this night the completely dark Moon faces the Earth.
Lunar Eclipse: When the Earth comes in between Sun and the Moon, the light of the Sun is not able to reach the Moon, rather, the shadow of the Earth falls on the moon. This is called the Lunar Eclipse. Lunar Eclipse always occurs on a Full Moon.
Solar Eclipse: When the Moon comes in between the Sun and the Earth, the light of the Sun is interrupted and shadow of the Moon falls on the Earth. This is called the Solar Eclipse, which occurs on New Moon day.
Night and day
Night and day happen because Earth rotates, or makes one complete turn, every 24 hours. As well as traveling in an orbit around the Sun, planet Earth spins around its axis, an imaginary line going through the North and South Poles. This means that at any one time, half of Earth is facing the Sun and has daytime, while the other half faces away from the Sun, so it has night.
Seasons of Earth
Earth has seasons because it is titled at an angle. This means that as it orbits around the Sun different parts are titled towards the Sun. when the northern hemisphere or top half of the Earth, points to the Sun, this area gets summer. At the same time, the southern hemisphere, or bottom half of the Earth, is pointing away from the Sun and this area experiences its winter.