Latitudes, Longitudes and Time: Grid, IST and Date Line

The correct answer is B, 82 degrees 30 minutes east. This meridian passes near Mirzapur in Uttar Pradesh and runs roughly through the middle of the country, so it was adopted as the standard meridian of India. Its local time, which is five hours and thirty minutes ahead of Greenwich Mean Time, is kept as Indian Standard Time everywhere from Gujarat to Arunachal Pradesh.

Option A, 75 degrees east, passes through the western part of the country and is often offered because it sounds like a round figure. Option C, 88 degrees 30 minutes east, lies close to Kolkata, which once kept its own local time before standardisation. Option D, 90 degrees east, is the standard meridian used by Bangladesh, six hours ahead of Greenwich. The half hour in the Indian offset comes directly from the thirty minutes in the chosen meridian, since half a degree equals two minutes of time.

The correct answer is B, four minutes. The earth completes one rotation of 360 degrees in 24 hours, that is 1,440 minutes, and dividing 1,440 by 360 gives four minutes for every degree of longitude.

Option C, fifteen minutes, reverses the relationship that candidates most often remember, which is that the earth turns fifteen degrees in one hour; fifteen is the number of degrees, not of minutes. Option D, one hour, is the time for fifteen degrees, not for one. Option A, one minute, would mean a full rotation in six hours, which is plainly wrong. Every numerical question in this chapter rests on this single conversion, so keep both forms ready: one degree equals four minutes, and fifteen degrees equal one hour. The half degree that appears in Indian Standard Time therefore accounts for exactly two minutes.

The correct answer is C, Odisha. The Tropic of Cancer passes through eight states, namely Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram. Odisha lies to the south of that line and is therefore wholly within the tropical belt.

Option A, Jharkhand, and option B, Chhattisgarh, both lie on the line in central and eastern India, and option D, Tripura, is one of the two north-eastern states it crosses, the other being Mizoram. Examiners set this question in both directions, sometimes asking which state the line passes through and sometimes which it does not, and the usual wrong options are Odisha and Bihar, both of which lie just outside it. A simple way to hold the list is to run from west to east: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura, Mizoram.

The correct answer is A, the equator. A great circle is one whose plane passes through the centre of the sphere and which therefore divides it into two equal halves. Among the parallels only the equator satisfies this, which is why it separates the northern and southern hemispheres and is the longest parallel of all.

Option B, the Tropic of Cancer, and option D, the Tropic of Capricorn, lie 23.5 degrees away from the equator, so their planes are offset from the centre and the circles they form are smaller. Option C, the Arctic Circle, at 66.5 degrees north, is smaller still, because parallels shrink steadily towards the poles. Meridians behave differently: every meridian and its opposite meridian together make a full great circle, which is why the shortest air route between two distant cities follows a great circle and often looks curved on a flat map.

The correct answer is B, Greenwich, the site of the old Royal Observatory near London. The meridian running through it was accepted as the zero meridian, from which all longitudes are counted east and west, and its local time is Greenwich Mean Time, the reference for the world's standard times.

Option A, Paris, is a strong distractor because France long used its own prime meridian through the Paris Observatory before the international agreement. Option C, Rome, and option D, Washington, were never adopted as the zero meridian, although several countries did use national meridians of their own in the nineteenth century. Remember the pair of extremes for this chapter: the equator is the reference for latitude and the Greenwich meridian is the reference for longitude, and the meridian directly opposite Greenwich, at 180 degrees, is the one the International Date Line broadly follows.

The correct answer is C, 66.5 degrees north. The Arctic Circle marks the latitude beyond which the sun stays above the horizon for the whole of at least one day in midsummer and below it for the whole of at least one day in midwinter, the phenomenon known as the midnight sun and the polar night.

Option A, 23.5 degrees north, is the Tropic of Cancer, the northern limit of the overhead sun. Option B, 45 degrees north, is merely the midpoint between the equator and the pole and has no special astronomical meaning. Option D, 90 degrees north, is the North Pole itself, the highest latitude that exists. The two numbers to fix are 23.5 and 66.5, and note that they add up to 90, since the tilt of the earth's axis of 23.5 degrees decides both of them.

The correct answer is C, the 180 degree meridian, which lies in the Pacific Ocean opposite Greenwich. The line does not follow it exactly: it bends east and west to keep the eastern tip of Siberia, the Aleutian Islands and several Pacific island groups on one side, so that no country is left with two dates at the same moment.

Option A, the zero degree meridian, is the Prime Meridian at Greenwich, the starting line for longitude, not for dates. Option B, 90 degrees east, and option D, 90 degrees west, are ordinary meridians with no special role. The rule to remember for the crossing is that a traveller going from east to west across the line adds a day, while one going from west to east repeats the day just passed. This is why a voyage westward around the world appears to lose a calendar date unless the line is accounted for.

The correct answer is B, 181. There are 90 parallels in the northern hemisphere, 90 in the southern hemisphere and the equator itself, which belongs to neither, making 181 in all. Note that the poles are points rather than circles, although they are counted as the ninetieth parallel in each hemisphere.

Option A, 180, is what you get by forgetting the equator, and it is the commonest mistake in this question. Option C, 360, is the number of meridians at one degree intervals, 180 to the east and 180 to the west of the Prime Meridian, and option D, 361, mixes the two counts together. Keep the pair straight: 181 parallels and 360 meridians. The reason for the difference is that a meridian runs only from pole to pole and its opposite half is counted separately, while a parallel is a complete circle on its own.

The correct answer is C, about 111 kilometres. The circumference of the earth along a meridian is close to 40,000 kilometres, and dividing that by 360 degrees gives roughly 111 kilometres for one degree of latitude. Because the angle is measured at the centre of the earth, this figure holds almost everywhere from the equator to the poles.

Option A, 51 kilometres, and option B, 88 kilometres, are distances between meridians at high latitudes, since meridians draw closer together as they approach the poles; at the equator the gap between two meridians is also about 111 kilometres, and it falls to zero at the poles. Option D, 150 kilometres, has no basis. A useful related figure is that one minute of latitude equals one nautical mile, that is 1,852 metres, which is why sailors measure distance in nautical miles.

The correct answer is B, 6:30 in the morning. Indian Standard Time is the local time of 82 degrees 30 minutes east. Multiplying 82.5 degrees by four minutes gives 330 minutes, that is five hours and thirty minutes, and since India lies east of Greenwich its clock is ahead by that much. Subtracting five and a half hours from 12 noon leaves 6:30 am.

Option A, 5:30 am, would follow from an offset of six and a half hours, which belongs to no Indian meridian. Options C and D place Greenwich ahead of India, reversing the rule that places to the east see the sun first and therefore run ahead. The direction is the part most often lost in the exam hall, so fix it with a simple sentence: east is ahead, west is behind. The same calculation in reverse tells you that noon at Greenwich is 5:30 in the evening in India.

The correct answer is B. Every meridian runs from the North Pole to the South Pole, so all meridians are semicircles of the same length and all of them converge at the two poles. That is exactly what distinguishes them from parallels.

Option A describes parallels, not meridians: parallels are full circles that never meet. Option C is wrong in both directions, since meridians are equal in length and it is the distance between them, not the meridians themselves, that changes; the gap is widest at the equator at about 111 kilometres and falls to zero at the poles. Option D is a near miss that examiners like: a single meridian is only half a circle, and it is a meridian taken together with its opposite meridian that forms a complete great circle. The equator, by contrast, is a great circle on its own.

The correct answer is B, Mirzapur, in Uttar Pradesh. The meridian of 82 degrees 30 minutes east passes near this town, and its local time is kept as Indian Standard Time across the whole country.

Option A, Nagpur, is often chosen because the city is known as being near the geographical centre of the country, but the standard meridian does not run through it. Option C, Jabalpur, and option D, Kanpur, are other central Indian towns offered to confuse the candidate. The meridian was chosen because it runs through the middle of the country in terms of longitude, so that the difference between the clock and the sun stays within roughly an hour at either end, even though India spans nearly 30 degrees of longitude from the Rann of Kachchh in the west to Arunachal Pradesh in the east.