Class 6 ๐๐๐๐๐ : ๐๐๐๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐๐

### ๐๐๐๐ ๐๐๐ ๐๐๐๐๐๐๐๐๐๐?

color{green}(โข) To fix the position of a place, it is necessary to know something more than the latitude of that place.

color{green}(โข) You can see, for example, that Tonga Islands (in the Pacific Ocean) and Mauritius Islands (in the Indian Ocean) are situated on the same latitude (i.e., 20ยฐ S).

color{green}(โข) Now, in order to locate them precisely, we must find out how far east or west these places are from a given line of reference running from the North Pole to the South Pole.

color{green}(โข) These lines of references are called the meridians of longitude, and the distances between them are measured in โdegrees of longitude.โ

color{green}(โข) Each degree is further divided into minutes, and minutes into seconds.

color{green}(โข) They are semicircles and the distance between them decreases steadily pole wards until it becomes zero at the poles, where all the meridians meet.

color{green}(โข) Unlike parallels of latitude, all meridians are of equal length.

color{green}(โข) Thus, it was difficult to number the meridians.

color{green}(โข) Hence, all countries decided that the count should begin from the meridian which passed through Greenwich, where the British Royal Observatory is located.

color{green}(โข) This meridian is called the Prime Meridian. Its value is 0ยฐ longitude and from it we count 180ยฐ eastward as well as 180ยฐ westward. The Prime Meridian and 180ยฐ meridian divide the earth into two equal halves, the Eastern Hemisphere and the Western Hemisphere.

color{green}(โข) Therefore, the longitude of a place is followed by the letter E for the east and W for the west.

color{green}(โข) It is, however, interesting to note that 180ยฐ East and 180ยฐ West meridians are on the same line.

color{green}(โข) Now look at the grid of the parallels of latitude and meridians of longitude on the globe (Figure 2.6).

color{green}(โข) You can locate any point on the globe very easily if you know its latitude and longitude.

color{green}(โข) For example, Dhubri in Assam is situated at 26ยฐ N latitude and 90ยฐ E longitude.

color{green}(โข) Find out the point where these two lines cut each other.

color{green}(โข) That point will be the location of Dhubri.

color{green}(โข) To understand this clearly draw equidistant vertical and horizontal lines on a paper (Figure 2.7).

color{green}(โข) Label the vertical rows with numbers and horizontal rows with letters, draw some small circles randomly on points where these horizontal and vertical lines intersect each other.

color{green}(โข) Name these small circles as a, b, c, d and e.

color{green}(โข) Let vertical lines represent East Longitudes and horizontal lines as North Latitudes.

color{green}(โข) Now you will see that circle โaโ is located on Bยฐ N latitude and 1ยฐ E longitude.

color{green}(โข) Find out the location of other circles.

### ๐๐๐๐๐๐๐๐๐ ๐๐๐ ๐๐๐๐

color{green}(โข) The best means of measuring time is by the movement of the earth, the moon and the planets.

color{green}(โข) The sun regularly rises and sets every day, and naturally, it is the best time-keeper throughout the world.

color{green}(โข) Local time can be reckoned by the shadow cast by the sun, which is the shortest at noon and longest at sunrise and sunset.

color{green}(โข) When the Prime Meridian of Greenwich has the sun at the highest point in the sky, all the places along this meridian will have mid-day or noon.

color{green}(โข) As the earth rotates from west to east, those places east of Greenwich will be ahead of Greenwich time and those to the west will be behind it (Figure 2.8).

color{green}(โข) The rate of difference can be calculated as follows. The earth rotates ๐๐๐ยฐ in about 24 hours, which means 15ยฐ an hour or 1ยฐ in four minutes.

color{green}(โข) Thus, when it is 12 noon at Greenwich, the time at 15ยฐ east of Greenwich will be 15 ร 4 = 60 minutes, i.e., 1 hour ahead of Greenwich time, which means 1 p.m.

color{green}(โข) But at ๐๐ยฐ west of Greenwich, the time will be behind Greenwich time by one hour, i.e., it will be 11.00 a.m.

color{green}(โข) Similarly, at ๐๐๐ยฐ, it will be midnight when it is 12 noon at Greenwich.

color{green}(โข) At any place a watch can be adjusted to read 12 oโclock when the sun is at the highest point in the
sky, i.e., when it is mid-day.

color{green}(โข) The time shown by such a watch will give the local time for that place.

color{green}(โข) You can see that all the places on a given meridian of longitude have the same local time.

### ๐๐๐ ๐๐ ๐๐ ๐๐๐๐ ๐๐๐๐๐๐๐๐ ๐๐๐๐?

color{green}(โข) The local time of places which are on different meridians are bound to differ.

color{green}(โข) For example, it will be difficult to prepare a time-table for trains which cross several longitudes.

color{green}(โข) In India, for instance, there will be a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam.

color{green}(โข) It is, therefore, necessary to adopt the local time of some central meridian of a country as the standard time for the country.

color{green}(โข) In India, the longitude of ๐๐ยฝยฐ ๐ (๐๐ยฐ ๐๐'๐) is treated as the standard meridian.

color{green}(โข) The local time at this meridian is taken as the standard time for the whole country.

color{green}(โข) It is known as the Indian Standard Time (IST).

color{green}(โข) Kabeer lives in a small town near Bhopal.

color{green}(โข) He tells his friend Alok that they will not be able to sleep tonight.

color{green}(โข) A day and night cricket match between India and England had started at 2 p.m. in London.

color{green}(โข) This means that the match would begin at 7.30 p.m. in India and finish well into the night.

color{green}(โข) Do you know what is the time difference between India and England?

color{green}(โข) India located east of Greenwich at ๐๐ยฐ๐๐'๐ is 5 hours and 30 minutes ahead of GMT.

color{green}(โข) So it will be 7:30 p.m. in India when it is 2:00 p.m. noon in London.

color{green}(โข) Some countries have a great longitudinal extent and so they have adopted more than one standard time.

color{green}(โข) For example, in Russia, there are as many as eleven standard times.

color{green}(โข) The earth has been divided into twenty-four time zones of one hour each.

color{green}(โข) Each zone thus covers ๐๐ยฐ of longitude.

### ๐๐ฑ๐๐ซ๐๐ข๐ฌ๐๐ฌ

๐. ๐๐ง๐ฌ๐ฐ๐๐ซ ๐ญ๐ก๐ ๐๐จ๐ฅ๐ฅ๐จ๐ฐ๐ข๐ง๐  ๐ช๐ฎ๐๐ฌ๐ญ๐ข๐จ๐ง๐ฌ ๐๐ซ๐ข๐๐๐ฅ๐ฒ.

(a) What is the true shape of the earth?

(b) What is a globe?

(c) What is the latitudinal value of the Tropic of Cancer?

(d) What are the three heat zones of the Earth?

(e) What are parallels of latitude and meridians of longitude?

(f) Why does the torrid zone receive maximum amount of heat?

(g) Why is it 5.30 p.m. in India and 12.00 noon in London?

๐. ๐๐ข๐ฅ๐ฅ ๐ข๐ง ๐ญ๐ก๐ ๐๐ฅ๐๐ง๐ค๐ฌ.

(a) The Tropic of Capricorn is located at _________________.

(b) The Standard Meridian of India is ____________________.

(c) The 0ยฐ Meridian is also known as ____________________.

(d) The distance between the longitudes decreases towards___________.

(e) The Arctic Circle is located in the ____________ hemisphere.