Explanation Local time: Since the earth rotates from west to east, places in the east have their noon earlier than in the West. Consequently, the local times of these places will be different. Now, if each of these places were to observe its own local time, a great deal of inconvenience would result. To remove this difficulty, the local time of some central place is adopted over a certain area or country. This time is called the standard time of that area or country. For example, the local time of Greenwich is the standard time for England. In big countries like Canada or the U.S.A, more than one standard time is used. Indian Standard Time: The standard time of India is the local time of a place 82-1/20 E longitude so that it is exactly 5-1/2 hours in advance of Green witch Mean Time (GMT). This meridian passes close to Allahabad, which is almost in the middle of the country. Another important place almost on this meridian is Mirzapur (U.P). Determination of local time: To determine the local time of a place, one has to find out the true North- South line, drawn on the ground. Then a vertical rod is fixed in the ground. Everyday, when the shadow of the rod coincides with that line, it is taken as 12Oo clock, local time at that place. Now if your watch is set according to this time, it will keep local time. Difference between the local time of Delhi and the standard time: The longitude of Delhi is 77o E and the Indian standard time is reckoned from 82-1/2o E. The difference between these longitudes is 5-1/20. For one degree, there will be a difference of four minutes in time. Hence, for 5-1/20 , the difference in time will be 5.1/2X4 =22 minutes. Since Delhi is in west of the place from where the standard time is calculated, the local time of Delhi is 22 minutes behind the standard time. Error due to circumnavigation: The Earth rotates on its axis from west to east, hence places in the east have their sun rise earlier than places in the west. Now if a man travels around the world from east to west, for every degree of longitude covered, he will have to set back his watch by four minutes. Thus, after covering 3600 , when he returns to the place from he stared, he will have to set back his watch ( 360X4)/60=24 hours. Thus, he will appear to have lost a day. When Francis drake returned to England after his voyage circumnavigating the globe, he thought, it was Saturday, whereas actually it was Sunday. The reason was that he had travelled from east to west, and had under calculated a day. On the ther hand, if a man travels from west to east, for every degree of longitude he will put forwards his watch by four minutes, and for 3600 he will put it 24 hours forward. When he returns to the place from where he started, he will appear to have gained a day. When Capt. Basil Hall reached Manila after circumnavigating the globe, he thought it was Monday, whereas actually it was Sunday. The reason was that he had travelled from west to east, and had to reduce one day. International Date line: In travelling round the world, there occurs an error of one day, To avoid this error, a date line has been adopted, which is so called because ships when crossing this line change their date. The date line is an imaginary line roughly corresponding to 1800 meridian; ships crossing this line from west to east (i.e from China to the U.S.A) repeat a day, that is, if they cross it on Monday, December 10. On the other hand, ships crossing this line from east to west (i.e from the U.S.A to China) drop a day, that is, if they cross it on Monday, December 10, the next day will be counted as Wednesday, December 12. The date line, however, does not everywhere coincide with 1800 meridian, but at places deviates a little from it. The reason for this is that 1800 meridian passes through some groups of islands belonging to the same Government. If the date line were all along the coincident with 1800 meridian, the islands to the east of the line would have a different date from the islands to the west of this line, though under the same administration, and this is inconvenient and absurd. So the date line does not everywhere coincide with 1800 meridian, but turns right and left at some places.