Gist of NCERT - Geography - Motions of the Earth

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Rotation is the movement of the earth on its axis. 

The movement of the earth around the sun in a fixed path or orbit is called Revolution.

The axis of the earth which is an imaginary line, makes an angle of 66° with its orbital plane. The plane formed by the orbit is known as the orbital plane.

The portion facing the sun experiences day while the other half away from the sun experiences night. The circle that divides the day from night on the globe is called the circle of illumination. This circle does not coincide with the axis. 

The earth takes about 24 hours to complete one rotation around its axis. The period of rotation is known as the earth day. This is the daily motion of the earth.

The second motion of the earth around the sun in its orbit is called revolution. It takes 365 days (one year) to revolve around the sun. We consider a year as consisting of 365 days only and ignore six hours for the sake of convenience.

Six hours saved every year are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February. Thus every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year.

The earth is going around the sun in an elliptical orbit.

Since a large portion of the Northern Hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and the shortest night at these places occur on 21st June. At this time in the Southern Hemisphere all these conditions are reversed. It is winter season there. The nights are longer than the days. This position of the earth is called the Summer Solstice.

On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn (23 ° S), a larger portion of the Southern Hemisphere gets light. Therefore, it is summer in the Southern Hemisphere with longer days and shorter nights. The reverse happens in the Northern Hemisphere. This position of the earth is called the Winter Solstice.

On 21st March and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox.

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Gist of NCERT - Geography - Globe : Latitudes and Longitudes

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Globe is a true model (miniature form) of the earth. A needle is fixed through the globe in a tilted manner, which is called its axis. 

Two points on the globe through which the needle passes are two poles – North Pole and South Pole.

An imaginary line running on the globe divides it into two equal parts. This line is known as the equator. 

The northern half of the earth is known as the Northern Hemisphere and the southern half is known as the Southern Hemisphere. They are both equal halves. 

Therefore, the equator is an imaginary circular line and is a very important reference point to locate places on the earth. All parallel circles from the equator up to the poles are called parallels of latitudes. 

Latitudes are measured in degrees.

The equator represents the zero degree latitude.

Besides the equator (0°), the North Pole (90°N) and the South Pole (90° S), there are four important parallels of latitudes –
(i) Tropic of Cancer (23 ° N) in the Northern Hemisphere. 
(ii) Tropic of Capricorn (23 ° S) in the Southern Hemisphere. 
(iii) Arctic Circle at 66 ° north of the equator. 
(iv) Antarctic Circle at 66 ° south of the equator." 

The mid-day sun is exactly overhead at least once a year on all latitudes in between the Tropic of Cancer and the Tropic of Capricorn. This area, therefore, receives the maximum heat and is called the Torrid Zone.

The mid-day sun never shines overhead on any latitude beyond the Tropic of Cancer and the Tropic of Capricorn. The angle of the sun’s rays goes on decreasing towards the poles. As such, the areas bounded by the Tropic of Cancer and the Arctic Circle in the Northern Hemisphere, and the Tropic of Capricorn and the Antarctic Circle in the Southern Hemisphere, have moderate temperatures. These are, therefore, called Temperate Zones. 

Areas lying between the Arctic Circle and the North Pole in the Northern Hemisphere and the Antarctic Circle and the South Pole in the Southern Hemisphere, are very cold. It is because here the sun does not rise much above the horizon. Therefore, its rays are always slanting and provide less heat. These are, therefore, called Frigid Zones (very cold).

Unlike parallels of latitude, all meridians are of equal length. Thus, it was difficult to number the meridians. Hence, all countries decided that the count should begin from the meridian which passed through Greenwich, where the British Royal Observatory is located. This meridian is called the Prime Meridian. Its value is 0° longitude and from it we count 180° eastward as well as 180° westward. 

The Prime Meridian and 180° meridian divide the earth into two equal halves, the Eastern Hemisphere and the Western Hemisphere. Therefore, the longitude of a place is followed by the letter E for the east and W for the west. It is, however, interesting to note that 180° East and 180° West meridians are on the same line.

The earth rotates 360° in about 24 hours, which means 15° an hour or 1° in four minutes. Thus, when it is 12 noon at Greenwich, the time at 15° east of Greenwich will be 15 4 = 60 minutes, i.e., 1 hour ahead of Greenwich time, which means 1 p.m. But at 15° west of Greenwich, the time will be behind Greenwich time by one hour, i.e., it will be 11.00 a.m. Similarly, at 180°, it will be midnight when it is 12 noon at Greenwich.

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Gist of NCERT - Geography - Solar System

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The sun, the moon and all those objects shining in the night sky are called celestial bodies.

Some celestial bodies are very big and hot. They are made up of gases. They have their own heat and light, which they emit in large amounts. These celestial bodies are called stars. 

The sun is a star.

While watching the night sky, you may notice various patterns formed by different groups of stars. These are called constellations.

One of the most easily recognisable constellation is the small bear or Saptarishi (Sapta-seven, rishi-sages). It is a group of seven stars. 

In ancient times, people used to determine directions during the night with the help of stars. The North star indicates the north direction. It is also called the Pole Star. It always remains in the same position in the sky.

Some celestial bodies do not have their own heat and light. They are lit by the light of the stars. Such bodies are called planets. The word ‘planet’ comes from the Greek word “Planetai” which means ‘wanderers’.

Jupiter, Saturn and Uranus have rings around them. These are belts of small debris. 

The sun, eight planets, satellites and some other celestial bodies known as asteroids and meteoroids form the solar system.

‘Sol’ in Roman mythology is the ‘Sungod’. ‘Solar’ means ‘related to the sun’. The family of the sun is, therefore, called the solar system.

There are eight planets in our solar system. In order of their distance from the sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
 

All the eight planets of the solar system move around the sun in fixed paths. These paths are elongated. They are called orbits. 

Mercury is nearest to the sun. It takes only about 88 days to complete one round along its orbit. 

Venus is considered as ‘Earth’s-twin’ because of its size.

Till recently (August 2006), Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial objects (Ceres, 2003 UB313) discovered in recent past may be called "dwarf planets."

The earth is the third nearest planet to the sun. In size, it is the fifth largest planet. It is slightly flattened at the poles. That is why, its shape is described as a Geoid. Geoid means an earth-like shape.

From the outer space, the earth appears blue because its two-thirds surface is covered by water. It is, therefore, called a blue planet.

Our earth has only one satellite, that is, the moon. Its diametre is only one-quarter that of the earth. It appears so big because it is nearer to our planet than other celestial bodies. It is about 3,84,400 km away from us.

The moon moves around the earth in about 27 days.

Apart from the stars, planets and satellites, there are numerous tiny bodies which also move around the sun. These bodies are called asteroids. They are found between the orbits of Mars and Jupiter.

The small pieces of rocks which move around the sun are called meteoroids. Sometimes these meteoroids come near the earth and tend to drop upon it. During this process due to friction with the air they get heated up and burn. It causes a flash of light. Sometimes, a meteor without being completely burnt, falls on the earth and creates a hollow.

A galaxy is a huge system of billions of stars, and clouds of dust and gases. There are millions of such galaxies that make the Universe.  Our solar system is a part of the Milky Way galaxy which is part of the universe. 

Light travels at the speed of about 300,000 km per second. Yet, even with this speed, the light of the sun takes about eight minutes to reach the earth.

Neil Armstrong was the first man to step on the surface of the moon on 21 July 1969.

A Satellite is a celestial body that moves around the planets in the same way as the planets move around the sun.

A Human-made Satellite is an artificial body. It is designed by scientists to gather information about the universe or for communication. It is carried by a rocket and placed in the orbit around the earth. Some of the Indian satellites in space are INSAT, IRS, EDUSAT, etc.

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