Our Home: Planet Earth

Earth is the third planet of the solar system according to distance from the Sun. It has an atmosphere to support life due to its suitable size and distance from the sun. Size of this planet is not a perfect sphere but a little flattened on both ends or the poles and thus a bulge is made slightly around the centre or the equator. You can compare Earth’s shape to that of an orange This type of shape refers is usually called ‘ablate spheroid’ or ‘geoid’ meaning earth-shaped.

ORIGIN OF THE EARTH

There are various theories trying to explain the origin of the Earth. Some theories are Evolutionary/ natural/ monistic hypothesis. these theories states that the system of planets have evolved. Others are the cataclysmic/catastrophic dualistic.  these theories states that the some sudden and violent event in the space, like the collision or close approach of two stars.

MOVEMENT OF THE EARTH

There are two movements of The Earth:

Rotation:

• Rotation means the spinning of the Earth on an imaginary axis line joining the two poles that is the North and South. The axis of the Earth is tilted at an angle of 23.5 degrees.
• The Earth rotates from West to East and completes one rotation in 24 hours.
• Earth spins with a speed of 1,700 km per hour at the equator and 850 km per hour.at 60 degrees parallel. This speed is almost zero at the poles.
• Rotation is important because it causes day and night on the Earth.
• Day occurs at the places of the Earth facing the Sun, while night at the places which are away from the Sun.

Revolution:

• Revolution is the movement of Earth around the Sun in a fixed elliptical path or orbit.
• It is completed by Earth in 365 and ¼ days called an Earth year.
• Generally, in a calendar year only 365 days are counted and one-fourth day is eliminated. It is added to every fourth year as an extra day i.e. as 29^th This fourth year is known as leap year.
• Earth’s revolution path is an elliptical shape, that’s why the distance between the Sun and the Earth is minimum (about 152 million km) on January 3. This situation is called Perihelion. At July 4, Earth is at farthest distance from the Sun (197 million km). This situation is called Aphelion.
• Earth revolves around the sun with an average speed of 107,000 km/hr or 29.72 km/sec.
• Earth’s revolution with a rotation on tilted axis causes the different seasons on this planet.

Eccentricity:

• Earth follows a cycle of changing the shape of the orbit in every 90,000 to 100,000 years. Thus, it may be more elliptical to a more circular path around the Sun between this period.

EQUINOXES

• It is the time when the plane of Earth’s equator passes through the center of the Sun.
• In this situation sun is right over the head in the sky at the equator.
• Duration of day and night is equal on all places of Earth from poles to equator.
• This situation happens twice each year i.e. around 20 March and 23 September.

SOLSTICE SUMMER

• Summer solstice is the time when one of the Earth’s poles has its maximum tilt toward the Sun. it s also called midsummer.
• Winter solstice is the time when one of the Earth’s poles has its maximum tilt away from the Sun
• Both happen once in each hemisphere every year.
• The Sun reaches its highest position in the sky in the Hemisphere experiencing summer solstice. That day experiences the longest period of daylight. While other hemisphere experiences opposite conditions.
• For example, if Southern hemisphere experiences summer solstice, then sun is right overhead in the sky at 23 ½ S. It is longest day and shortest night in southern hemisphere. Whereas, at same time northern hemisphere experiences winter solstice, it is lowest daily maximum elevation in the sky and it is shortest day and longest night in northern hemisphere.

SEASONS

A year on the Earth is divided into seasons on the basis of changes in weather, ecology, and the amount of daylight. Seasons occur as the consequence of Earth’s orbit around the Sun and Earth’s rotation on a tilted axial.

Astronomical Seasons

These are seasons on the Earth in relation to the Sun. These seasons are:

• Winter: in this season the sun is in southern hemisphere and begins to shift toward northern hemisphere. It begins on December 21-22.
• Spring: in this season sun is located above the equator in its journey toward northern hemisphere. It begins on March 21-22.
• Summer: in this season sun is in the northern hemisphere and begins to be shifted towards southern hemisphere. It begins on the June 20-21.
• Fall: in this season sun is located above the equator in its journey toward southern hemisphere. It begins on September 21-22.

Meteorological Seasons

These are seasons on the basis of the variation in temperatures. Each season is of three months:

• Winter: First week of December to the last week of February.
• Spring: First week of March to the last week of May.
• Summer: First week of June to the last week of August.
• Fall: First week of September to the last week of November.

VARYING LENGTHS OF DAY AND NIGHT

• As seasons, variation in lengths of day and night also occur as the consequence of Earth’s orbit around the Sun with the Earth’s rotation on a tilted axis,
• The Sun is vertically overhead at the Tropic of Cancer in 21^st June each year i.e. summer solstice. This time northern hemisphere experience the longest day and shortest night in its every part. Towards the North Pole, the length of day time increases. Even beyond 66^1/2 degrees N the region has 24 hrs. light for six months.
• On 22^nd December, the same conditions prevail in the Southern hemisphere when the Sun is vertically overhead at the Tropic of Capricorn i.e. winter solstice.

LONGITUDES AND LATITUDES

• Latitudes are imaginary circles parallel to the Equator.
• Longitudes are also imaginary lines but running from the geographical North Pole to the geographical South Pole intersecting the Equator. Longitudes are used to calculate time. Difference between every longitude is 4 minutes.
• Latitudes and Longitudes are measured in degrees (°) and for more precision, degrees are divided into minutes (′).
• Distance between the latitude is same everywhere, while distance between the longitude is decreases toward poles.

TIME ZONE

• Due to the legal, commercial, and social purposes, various regions of the globe, sometimes countries adopt a uniform standard time for them known as time zone.
• Time zones do not follow longitude, but boundaries of regions or country because it is convenient for commercial or other communication to keep the same time.

PRIME MERIDIAN AND INTERNATIONAL DATE LINE

• Longitude running through the Greenwich, London, UK, is taken as Prime Meridian(0° longitude). It is a reference point of time zones. All countries have to adopt time zone according to this longitude. All the places located east of Prime Meridian have time ahead of it and all the places located west have time past of it according to their chosen longitude for standard time. For example, India has chosen 82 ½ degree east for its standard time zone. Thus, India’s time is 5 hours and 30 minutes ahead of London.
• The International Date Line is located halfway around the world from the prime meridian (0° longitude) or about 180° east (or west) of Greenwich, London, UK. It is also known as the line of demarcation.

ECLIPSES

An eclipse takes place when a planet or moon obstructs the way of sunlight.

There are two types of eclipses observed on Earth

• Solar Eclipse: it occurs when moon comes between the Earth and the Sun and casts its shadow on the Earth.
• Lunar Eclipse: It occurs when the Earth comes between the Sun and the Moon and the Earth casts its shadow on moon.

INTERNAL STRUCTURES OF THE EARTH

• It is not possible for human to go inside the depth of Earth or send a mission inside the Earth to study. That’s why we have to rely on different sources. These are indirect and direct sources of information which tell the details about the internal structure of the Earth.
• Direct Sources: direct sources are the rock founds on the Earth and volcanic eruptions.
• Indirect Sources: indirect sources are meteors, change in temperature and pressure, gravity, magnetism, seismic waves.
• On the basis of these sources, the internal structure is divided into three layers, these are .

Crust

• This is the outermost solid part.
• Its average thickness is 8-40 km. In ocean its thickness is less (around 5-10 km) and in continent its thickness is more(around 30-35 km).
• It is made up of Silica and Aluminum, that’s why it is also called SIAL.
• The discontinuity between hydrosphere (lower crust) and upper crust is referred to as the Conrad Discontinuity.

Mantle

• It is the layer below the crust.
• Its thickness is about 2900 km.
• Discontinuity between the crust and mantle is known as the Mohorovich Discontinuity or Moho discontinuity.
• It is made up of Silica and Magnesium, that’s why it is also called SIMA.
• the major source of magma is located between the Mantle in the depth of 80-200 km. It is called Asthenosphere and it is the highly viscous part of the mantle layer..
• The discontinuity between upper mantle and the lower mantle is known as Repetti Discontinuity.
• Part of the mantle that lies just above the core is called Mesosphere.

Core

• It is the innermost layer.
• The Guttenberg’s Discontinuity separates it from mantle.
• Its major constituents are Nickel and Iron, that’s why this layer is also called
• It has two layers –

outer core- the outer core is in liquid state

inner core-the inner core is in solid state.

• Lehmann Discontinuity is located between the upper core and the lower core.
• Core is also called as Barysphere.