International Date Line Class 9 Geography Notes Maharashtra Board


International Date Line Class 9 Geography Notes Maharashtra State Board

We bid adieu to the year 2016 and welcomed the New Year 2017 on Saturday midnight. Similarly, other countries also welcomed the New Year. A table showing the days and times of the welcome of New Year in different countries and cities according to the Indian Standard Time is given.

Different meridians have different timings at the same time. For example, if it is noon at the prime meridian, then it is 8 am at 60°W and 1600 hrs at 60°E. Sunil and Meenal have to note these timings. Take the help of the figure alongside.

International Date Line Class 9 Geography Notes Maharashtra Board 1
Sunil and Meenal embark on their journey to circumvent the earth from 0° meridian at noon on Thursday. During their journey, they cross each other at 180° meridian and move forward. When Sunil returns to 0° meridian after his journey, he thinks it is Friday. When Meenal returned at 0° meridian, she thought it was Wednesday.

International Date Line Class 9 Geography Notes Maharashtra Board 2
Earth’s rotation, revolution, the sunrise, and the sunset are natural phenomena. Man has studied the rotation speed, direction, and shape of the earth and prepared the chronometric (time-keeping) systems for his purpose. Considering the shape of the earth, he defined the graticule on the earth. He coordinated between the time of the earth’s rotation and the meridians of the graticule. The earth takes 24 hours to make one rotation. (Earth takes 24 hours to make a round of 360° around itself).

International Date Line Class 9 Geography Notes Maharashtra Board

Earth rotates from west to east. Therefore, in terms of time, the eastern part of the earth is ahead of the western part. Keeping this in mind, let’s think about Sunil’s journey. Sunil is moving towards the east. Therefore, the time will move ahead at different meridians. For example, at 90° E meridian, it will be Thursday at 6 p.m. Sunil moves eastwards and according to him, it will be Thursday midnight at 1800 meridian. According to him, as he continues, it will be Friday after he crosses the 180° meridian. At 90° W, it will be Friday 6 a.m. and therefore after the full round of the earth when Sunil meets Meenal at 0° meridian, it will be Friday noon.

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Now consider Meenal’s journey. As she moves westwards, she will see that time is moving backward. For example, at 30° W, it will be 10.00 hrs in the morning on Thursday, while it will be 8 am on Thursday at 60° W, and it will be midnight at 180° meridian. If Meenal continues ahead, then it will be Wednesday at 6 p.m. at 90° E. When she completes her full journey and returns to 0° meridian, then according to her it should be Wednesday afternoon. There is a debate between Meenal and Sunil when they meet after their respective journeys. According to Meenal, it should be Wednesday and according to Sunil, it should be Friday. Then where does Thursday go? The following method was used to come to a solution.

The solution to the Confusion of Days:
When you cross the 1800 meridian, some precautions need to be taken. A Meridian of 180° is reached after 12 hours from Prime Meridian if you go from east or west. Accordingly, an adjustment or change in day and time is made. According to the convention, the start (and end) of a date is considered to be at 180° meridian. Therefore, the following points were taken into consideration while drawing the International Date Line (IDL).

  • The direction of travel
  • The current day and date

While traveling from Japan to America, we need to consider the existing day and date even after crossing the IDL. For example, if it is Thursday 25th December, then Thursday 25th December only should be considered. On the contrary, while going from America to Japan, and crossing the IDL, one needs to add a day to the date. So if it is Thursday, 25th December then it will be Friday, 26th December. It will be clear from the following example and figure.

Shyamrao left for America (which lies to the east of the IDL) from Japan (from the west of the IDL) on 1st at 1 pm on Monday. After traveling for 24 hours on a flight, he reached America and saw the date there. It was Monday 1st, 1 pm. Shyamrao left for Japan (to the West of IDL) from America (from the East of IDL) on Friday 5th at noon. He reached Japan after 24 24-hour flight and the date was Saturday 6th at noon.

International Date Line Class 9 Geography Notes Maharashtra Board

We can consider that concerning the IDL, the day on the Earth starts in the West and ends in the East. It is the same day only at 12 ‘o’clock midnight at the 180° meridian. For the countries lying to the east of it i.e. the USA, Chile, etc., it is the end of the day while for the countries lying to the east of it i.e. Japan, Australia, etc. it is the start of a new day. Now a question may come to your mind, why don’t the dates change concerning 180° meridian then? Why is the IDL required?

The flight by American plane UA-840 awakened our curiosity. This plane left Shanghai (China) on 1st January 2017 and crossed the Pacific Ocean to reach the western coast of America at San Francisco on 31st Dec 2016. The reverse change in date about the IDL is evident.

International Date Line:
Our 24-hour day starts at midnight. Because of the earth’s rotation, midnight occurs at different times at different places. As the earth is spherical, every place has a place to its east. Then at what location to the east does the day start on the earth? To get an answer to many questions like this, representatives of many nations got together, under the leadership of American professor Davidson in the year 1884 and decided on the International Date Line. The line was drawn opposite to the Greenwich Prime Meridian. i.e. concerning the 180° meridian. It is an international convention to change the day and time while crossing this line.

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An attempt has been made to make the IDL pass through the Pacific Ocean completely. Had it passed through land or some islands, the people there would have had to follow different dates and timings because dates would have been different on their eastern and western parts. Also, it would have been difficult to know when one crosses the IDL on land and when the date changes on the calendar. Therefore, the IDL is not a straight line like the 180° meridian. At places, it turns east while at other places, it turns west. The IDL has been changed as and when required. The last change was made in 2011. Nevertheless, the maximum part of the IDL coincides with the 180° meridian.

International Date Line Class 9 Geography Notes Maharashtra Board 4
The Importance of IDL:
The IDL brings coordination between international airlines, transportation services, and economic and trade activities. The IDL has been carved out of the necessity of coordinating time and date. It is also important in today’s modern era and rapidly happening global developments. We can keep track of all the calculations of day and time accurately with the help of IDL in the case of global transportation esp. concerning airways. It is only because of the IDL that the schedules of the traffic worldwide are organized properly.

Maharashtra State Board Class 9 Geography Notes International Date Line can be used for revisiting and reinforcing previously learned content.

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