Class 6 Social Science Chapter 1: Locating Places on the Earth

Looking straight down

Stand at a window above a courtyard and look down. Walls become lines. A tree becomes a round blob. That downward view is what a map gives you. A map draws an area as though you were floating directly above it. This is why a map of your street shows no house fronts. Fronts face sideways. From above you see roofs, roads and the gaps between them, and the gaps guide you.

Big or small, still a map

The area a map covers is up to whoever draws it: one village, one state, all of India, or the whole Earth. Nothing about the drawing changes when the area grows. What changes is how much must be left out. A village map can mark every lane and well. A map of India keeps the rivers, the borders and the big cities. Deciding what to drop is half of a map maker's work.

An atlas holds many maps

An atlas is a book, or any collection, of maps bound together. That word is worth being careful with. People sometimes use atlas and map as though they meant the same thing. They do not. One map shows one area. An atlas turns from a page on the Himalayas to a page on rainfall to a page on the oceans. Each of those pages is a separate map. So checking the atlas really means finding the right map inside it.

Physical maps

A physical map shows what nature put there. Mountains, plateaus, plains, rivers, lakes and oceans are its business. Height is shown by colour: greens for low land, browns as the ground rises, blue for water. Open it to learn whether a journey crosses a mountain range or follows a valley. It will not tell you which state you are in. Borders are not natural features, so a physical map ignores them.

Political maps

A political map shows what people decided. Countries, states, districts, borders and capital cities are its business. Its colours are chosen only to keep neighbours apart on the page, because a state is not orange in real life. This map answers different questions from the physical one. Which country is this? Which state? Where is its capital? The land underneath may be mountain or desert. A political map will draw a border straight across both without saying so.

Thematic maps

A thematic map picks one subject and shows only that. It may cover where rice is grown, or monsoon rainfall, or the languages people speak. Because it carries one thing, it carries it well. It usually keeps a faint outline of borders or coastlines underneath, just enough to show you where you are looking. Everything else goes into the one subject.

What every map needs

Whatever kind it is, a map has to answer three questions to be any use. How far is it? Which way is it? What is that little mark? Those three are the components of a map: distance, direction and symbols. Distance comes from the scale, direction from the north arrow, and every mark's meaning from the symbols. A drawing that answers none of the three is a picture of a place rather than a map of it.

Scale: paper and ground

How does a whole city fit on one sheet? Through the scale, which is the rule tying paper to ground. Suppose a city map is drawn at 1 cm = 500 m. Then every centimetre you measure on that paper stands for five hundred metres of real road. Two centimetres between the station and the bank means a kilometre of walking. The scale is printed so that you never have to guess, and it is the first thing worth finding on a new map.

The scale bar

Some maps state the scale in words and some draw it. Open the map of India in this book and look at the bottom left corner. A short printed ruler carries 500 above it and km beside it. That ruler measures 2.5 cm, and those 2.5 cm stand for 500 kilometres of ground. A drawn bar has one clear advantage. Photocopy the page and the bar changes size with the map, so it stays honest.

Using the scale

The method never changes. Measure on the map in centimetres, then multiply by what one centimetre is worth. On a map at 1 cm = 10 m, a playground drawn 4 cm long is 40 m long. On a map at 2.5 cm = 500 km, a river measured at 5 cm runs 1,000 km. Five centimetres is twice the bar, and twice 500 is 1,000. The arithmetic is easy. The mistake to avoid is forgetting which scale this map uses.

Which way is north?

Directions on a map start from north, and north is at the top of almost every map you will meet. From there the rest follow clockwise: east on the right, south at the bottom, west on the left. These four are the cardinal directions, or cardinal points. Most maps print only one arrow, marked N. Once you know where north lies, the other three are fixed.

The four in between

Four directions are often too rough. A temple may lie neither straight north nor straight east of you, but somewhere between the two. For that we use the intermediate directions: northeast, southeast, southwest and northwest, written NE, SE, SW and NW. Each sits exactly between the two cardinal directions it is named after. With eight directions you can describe almost any position clearly, which is why sailors and weather reports have used all eight for centuries.

Why symbols exist

On the map of a small town a map maker can draw the little buildings. On the map of a country there is no room at all. A post office drawn as a post office would swallow half a district. So a small agreed mark stands in instead. The same is done for schools, bridges, wells, canals, roads and railway lines. Symbols let a great deal of information sit in a small space.

One agreed set

A symbol is only useful if you and the map maker mean the same thing by it, so somebody has to decide. In India that body is the Survey of India, a government organisation. It has fixed a standard symbol set for maps of this country. A broad gauge railway now looks the same on every Indian map, and so do a metalled road and a state boundary. Other countries made their own choices, so read the symbol box first.

Scenario:

A panchayat has opened a health centre and wants every household to be able to reach it. Two volunteers each make something to help. Sunita draws a careful picture of the village from memory, with every house shown from the front, the way it looks when you walk past. Imran draws the village as if seen from above. He marks north with an arrow and writes 1 cm = 100 m in the corner. For the marks he uses a small cross for the health centre, a circle for a well and a double line for the main road. Both sheets are pinned up beside the panchayat office. Within a week, people stop looking at one of them.

Analysis:

Imran's sheet is a map and Sunita's is a picture. The difference is not care or neatness, which Sunita has in plenty. It is that Imran's drawing answers the three questions a map has to answer. Distance: 6 cm from a house to the cross means 600 m of walking, because each centimetre stands for 100 metres. Direction: the arrow lets a reader turn the paper until it matches the real village. Symbols: the cross, the circle and the double line each carry a meaning that is written down. Sunita's fronts of houses are how the village looks from the road, which is exactly what a person standing there can already see.

Activity:

Measure two distances on Imran's scale in your notebook: 3.5 cm and 8 cm. Then find one real distance in your own village or locality, pace it out, and work out how long that line would be on a map drawn at 1 cm = 100 m.

Title:

Map your own room to scale

Safety:

Measure furniture edges carefully and do not climb on anything to reach a corner.

What You Notice:

The hard part is not the drawing but the choosing. You will find you cannot fit everything, and deciding what to leave out is exactly the work a real map maker does.

The Survey of India, the body that fixes map symbols for this country, has been mapping India since 1767, which makes it one of the oldest scientific offices in the land.

Different countries use different symbol sets, so a symbol that means a well on an Indian map may mean something else entirely on a map printed abroad.

You cannot flatten a piece of orange peel without tearing it, and for exactly the same reason no flat map of the round Earth can be perfectly correct everywhere.

The three components of a map — D-D-S — Distance, Direction, Symbols. How far, which way, and what is that. A map that cannot answer all three is only a picture.

Reading the directions clockwise — Never Eat Soggy Wafers — North, East, South, West, going clockwise from the top of the map.

Not a flat world

A town or a district can be drawn on paper honestly enough, because a small piece of the Earth is nearly flat. The whole planet is a different matter. The Earth is very nearly a sphere. It is not perfectly round, being slightly flattened at the two poles, but the difference is small and we shall treat it as a sphere throughout. That shape is the reason mapping the whole Earth is harder than mapping a city.

Why a flat map can never be perfect

Here is a test you can do with an orange. Peel it so that you have three or four large pieces of skin. Now try to press those pieces flat on a table. You cannot, unless you tear them at the edges. A sphere simply does not become a flat surface without damage. Every flat map of the world has the same problem, so every one of them stretches or squashes something. Map makers choose what to spoil.

What a globe is

A globe is a sphere with a map drawn on its surface. The one in your classroom is probably made of plastic, metal or stiff card, and turns on a stand. It need not show the Earth. There are globes of the Moon, of the planet Mars, and of the night sky with its stars and constellations. What they share is the shape.

Why a globe tells more truth

A globe shares the Earth's shape, so it can carry the Earth's geography without the stretching a flat sheet forces on it. Distances and shapes come out closer to right. The price is convenience. You cannot fold a globe into a bag, and you cannot print it in a book. So we use both, and remember which one is telling the fuller truth.

Two numbers find a place

Picture a big market where the shops stand in tidy rows, each shop the size of its neighbour. You want a friend to meet you at one particular stationery shop. Saying near the middle will not do. Saying the seventh shop in the fifth row from the entrance will, because two numbers together pick out exactly one shop. That idea is the whole foundation of locating places on the Earth.

A chessboard does the same job

A chessboard is marked with letters a to h along one side and numbers 1 to 8 along the other. Every square therefore has a name of its own, such as d2 or d4. Players use those names to write down games: moving a pawn two squares forward from d2 is recorded as d2 to d4. Nobody needs a picture of the board to follow it. A system like this is called a system of coordinates, and the Earth has one too.

The poles and the Equator

Spin a globe and watch it. Almost everything on it moves, but two points do not. Those two fixed points, at the top and the bottom, are the North Pole and the South Pole. Exactly halfway between them runs a circle, marked on every globe, called the Equator. The poles and the Equator are where all measurement of latitude begins.

What latitude measures

Imagine standing on the Equator and walking towards one of the poles. With every step your distance from the Equator grows. Latitude is the measure of that distance. Two places at the same latitude are the same distance from the Equator, even if one is in Africa and the other in India. Latitude says nothing about how far east or west you are, only how far from the middle.

Parallels of latitude

Stop anywhere on that walk and draw an imaginary line running east and west, keeping the same distance from the Equator all the way round. Such a line has a name: a parallel of latitude. It closes on itself, making one complete circle round the Earth. There are as many of them as you care to draw, and a globe usually prints one every fifteen or twenty degrees.

Why the circles shrink

Look carefully at a globe and compare the circles. The Equator is the largest of them all. The parallels above and below it are smaller, and they keep getting smaller as you move towards either pole. This follows from the shape of a ball: a slice taken near the middle is wide, and a slice taken near the top is narrow. So all parallels are parallel, but they are certainly not all the same length.

Degrees of latitude

Latitude is written in degrees rather than kilometres, because a degree works the same on any sphere. By agreement, the Equator is latitude nought degrees. From there the figure rises as you travel away from it, and a letter is added to say which way: N for north of the Equator, S for south. So 20 degrees N and 20 degrees S are equally far from the Equator, one above it and one below.

Ninety is the limit

How far can the figure go? To ninety, and no further. The North Pole is at 90 degrees N and the South Pole at 90 degrees S, written 90°N and 90°S. There is no 100 degrees of latitude, because by ninety you have run out of Earth to walk across. Anything beyond the pole is simply the other side, coming back down again with a smaller figure.

Latitude and climate

Latitude is tied to climate, which is one reason it matters so much. Near the Equator the climate is generally hot, and such places are called torrid. As latitude increases and you move away from the Equator, it grows more moderate, or temperate. Closer to either pole it becomes very cold, or frigid. In Science you will learn why this pattern exists, and why the seasons change through the year.

Scenario:

A school has two Class 6 sections preparing for an exhibition on the Earth. Section A is given a large flat world map and Section B a globe on a stand. Both are asked the same three questions. Which parallel of latitude is the longest? How far is Greenland from the Equator compared with India? And what is the shortest route from the northern tip of Norway to Alaska? Section A measures with a ruler on their map. They report that Greenland is enormous, larger than India, and that the Norway to Alaska route must cross Europe and Asia. Section B turns their globe, measures with a piece of string, and reports something quite different.

Analysis:

Section A has been misled by the map, not by carelessness. A flat sheet cannot carry a sphere without stretching, and on most world maps the stretching grows worse as latitude rises. Greenland sits at a high latitude and India near the Equator, so Greenland is inflated and India is not. Section B's globe has the Earth's own shape, so string laid on it gives a fair comparison, and Greenland turns out to be much smaller than it appeared. The route question exposes the same flaw. On the globe, the parallels near the top of the Earth are short circles, so a path over the far north is far more direct than the flat map suggests. The longest parallel question is the one both sections can answer, because the Equator is drawn as the widest circle on the globe and also as the line of measurement on the map.

Activity:

Take a ball, a piece of string and a marker. Draw a circle round its widest part, then two more circles nearer the top. Measure all three with the string and write the three lengths in your notebook in order. You have just measured why the Equator is the longest parallel.

Title:

Peel an orange, and then a ball of parallels

What You Notice:

The peel refuses to lie flat, and the circles get shorter the nearer you draw them to the pole. Those two observations are the whole reason latitude is measured in degrees on a globe rather than in centimetres on paper.

Safety:

Use a marker, not a knife, for drawing on the ball, and ask an adult to peel the orange if the skin is tough.

Every place on the Equator sits on the longest of all the parallels, so a journey right round the world is longer there than anywhere else on Earth.

The parallel at 90 degrees is not a circle at all. It has shrunk to a single point, which is why a pole is a dot on a globe and not a line.

Advanced chess players record a whole game using nothing but a letter and a number for each square, which is the same two-number idea that fixes a place on the Earth.

Telling latitude apart from longitude — Latitude is a ladder — The rungs of a ladder lie flat and level, one above the other, exactly as the parallels of latitude do.

The three climate belts by latitude — Hot, mild, cold — Torrid near the Equator, temperate in the middle, frigid near the poles. The names follow the temperature in order.

The shortest way from pole to pole

Put a finger on the North Pole of a globe and travel to the South Pole by the shortest route you can find. Now do it again, but set off in a different direction. You could pass through Europe and Africa, or you could pass through Asia instead, and the distance would be exactly the same. That is not true of latitude, where a circle near a pole is far shorter than the Equator. It is a clue that the two measurements are built differently.

Meridians of longitude

Each of those routes is called a meridian of longitude. Each is a half-circle. It stops at the far pole rather than carrying on round. Because every one of them makes the same journey, they are all the same length. They are also not parallel, whatever the parallels do: the meridians crowd together as they approach a pole and meet there. At the Equator they are as far apart as they ever get.

Why longitude also measures time

The Earth spins on its axis, which you will study in Science. Put a desk lamp beside the globe as a stand-in for the Sun and turn the globe slowly eastward. Watch one country pass into the light while another passes out of it. At any moment it is breakfast time somewhere, lunchtime somewhere else, and the middle of the night in a third place. So measuring how far round the Earth a place lies also measures the time there. That is a thread we pick up in the next topic.

Choosing a starting line

Latitude had a natural place to start. The Equator is halfway between the poles, and nobody has to agree to it. Longitude has no such gift. There is no natural middle going round the Earth, because every meridian is exactly like every other. So a starting line has to be chosen, and whoever uses it must agree to the same choice. That chosen line is called the Prime Meridian, and its longitude is nought degrees.

Greenwich, 1884

In 1884 a number of nations agreed that the meridian passing through Greenwich would be the international standard. Greenwich is an area of London, in England. That is why the Prime Meridian is also called the Greenwich Meridian, and why it is marked 0° on globes and atlases everywhere. Notice what kind of fact this is. It is not a discovery about the Earth. It is an agreement between people, and it could have been settled elsewhere.

Measuring longitude in degrees

Longitude is measured in degrees, as latitude is, and it counts how far you travel east or west from the Prime Meridian. From nought degrees the figure rises in both directions up to a hundred and eighty, and a letter is added to say which way: W for west, E for east. In round figures, New York lies near 74°W, Delhi near 77°E and Tokyo near 140°E. Those values are rounded off, not exact.

Where 180 meets itself

Count eastward from Greenwich and you reach 180°E. Count westward instead and you reach 180°W. Look at a globe and you will find that these are the same line, on the far side of the Earth from Greenwich. Since one meridian cannot sensibly carry two names, the letter is dropped and it is written simply as 180°. It is the only meridian besides the Prime Meridian that needs no letter.

Two numbers, one place

Now both coordinates are in hand. Latitude says how far from the Equator, and longitude how far from the Prime Meridian. Together they pick out one point on the Earth, exactly as a row and a shop number pick out one shop. So writing Kolkata down as roughly 22°N and 88°E is a complete answer to the question of where Kolkata is. Either number alone would leave a whole circle of possibilities.

The grid

Draw the parallels and the meridians on a globe together and they cross one another all over its surface. That criss-cross pattern is called a grid, and the lines that make it are grid lines. On most globes and atlases they are printed in a pale blue so that they do not hide the land beneath. The grid is not on the Earth itself. It is a set of imaginary lines that we agreed to put there, and it is what makes finding places possible.

India's own prime meridian

Greenwich was not the first line anyone used, and several prime meridians came before it. Many centuries before Europe settled the question, India had one of its own. It was called the madhya rekhā, which means middle line, and it passed through the city of Ujjayinī, known today as Ujjain. That city kept a reputation for astronomy across many centuries, and the astronomer Varāhamihira was working there about 1,500 years back.

A reference for Indian astronomy

Indian astronomers understood latitude and longitude, and understood why a zero meridian was needed at all. Once fixed, the Ujjayinī meridian became the reference for calculations throughout Indian astronomical texts. Old records name several cities as lying on it. Some sit very close to the line and others a little to one side. There is a good reason for that. Measuring longitude accurately needs accurate timekeeping, and clocks then were not what they are now.

Four hemispheres

A hemisphere is half the Earth, and the two great lines each cut it in half a different way. The Equator divides it into the Northern Hemisphere and the Southern Hemisphere. The Prime Meridian divides it into the Western Hemisphere and the Eastern Hemisphere. Every place on Earth therefore lies in two hemispheres at once, one from each pair. India lies in the Northern and the Eastern.

Where India sits on the grid

Put the two coordinates together for the whole country and you can state its extent. India's latitudes run roughly from 8°N to 37°N, and its longitudes roughly from 68°E to 97°E. Those four figures fence in the country on the grid. They also explain a good deal at once: being wholly north of the Equator and well east of Greenwich places India firmly in the Northern and Eastern Hemispheres.

Scenario:

A school in Ujjain is preparing an exhibition on Indian astronomy. The pupils learn that their own city once carried India's prime meridian, the madhya rekhā, and that old astronomical texts list several cities as standing on that line. When they check a modern map, some of those cities sit almost exactly on the meridian through Ujjain, near 75.8 degrees East. Others are ten or twenty kilometres to one side. One pupil concludes that the old texts were simply careless. Her teacher asks the class to think about what measuring a longitude actually requires before they decide that.

Analysis:

The pupil's conclusion is too quick. Measuring a latitude can be done by watching the sky from one place, but measuring a longitude compares the time at two places at once, and that demands an accurate clock. Indian astronomers understood latitude, longitude and the need for a zero meridian perfectly well. What they did not have was precise timekeeping, and the further two places are apart in longitude the more a small timing error shifts the answer. So a city listed as lying on the line may genuinely have been measured as carefully as the tools allowed and still be twenty kilometres off. The astronomy was sound; the clock was the limit. There is also a second point worth making at the exhibition. Ujjain being remembered this way shows that the idea of a prime meridian was not invented at Greenwich in 1884. It was chosen there. India had made the same choice, for its own line, many centuries earlier.

Activity:

Find Ujjain in your atlas and write down its longitude to one decimal place. Then list two cities within one degree of that longitude, north and south of it, and work out roughly how far each lies from the line.

Title:

Build a grid and look places up

What You Notice:

Finding a place from its coordinates is easier than finding the coordinates of a place, and both get much easier once the lines are drawn. You will also notice that your prime meridian works perfectly well, which is the whole point about Greenwich.

Safety:

Use a marker on the ball rather than anything sharp, and work on a surface that will not be spoilt by ink.

Greenwich was not the first prime meridian. Several others were used before it, and nations only settled on Greenwich as the international standard in 1884.

India used a prime meridian of its own long before Europe settled on one. Its name, madhya rekhā, means middle line, and it ran through Ujjayinī, the city called Ujjain today.

Ujjayinī held its reputation as a centre of astronomy across many centuries. Varāhamihira, a famous astronomer, was working there about 1,500 years back.

Telling meridians apart from parallels — Long lines are long ways down — Longitude lines run the long way, pole to pole. Latitude lines lie flat across, like the rungs of a ladder.

Which line makes which hemispheres — Equator: up or down. Meridian: left or right — The Equator gives Northern and Southern. The Prime Meridian gives Western and Eastern.

The Earth turns, so time moves

Set the globe spinning from west to east, which is the direction our planet really turns. Put a lamp beside it as a stand-in for the Sun. Watch one country move into the light while another moves out of it. At this very moment somebody is eating breakfast, somebody else is at lunch, and somebody in a third country is fast asleep. Nothing about their clocks is wrong. They are simply at different places on a turning Earth.

Fifteen degrees an hour

One full turn of the Earth takes 24 hours, and a full turn is 360 degrees. So divide: 360 degrees shared out over 24 hours gives 15 degrees for every hour, because 15 multiplied by 24 comes back to 360. That single number is the key to this whole topic. Every 15 degrees of longitude is worth one hour of time, and every hour of difference means 15 degrees of longitude between two places.

Eastward: add an hour

Now mark meridians every 15 degrees, starting from the Prime Meridian: 0°, 15°E, 30°E, 45°E and so on to 180°. Suppose it is 12 noon at Greenwich. Then a clock at 15°E reads 1 pm by local time, one at 30°E reads 2 pm, and one at 45°E reads 3 pm. Each step eastward adds an hour, because the Sun reached those places earlier. East is ahead.

Westward: take an hour away

Going the other way the rule simply reverses. With noon at Greenwich, it is 11 am local time at 15°W and 10 am at 30°W. Each step westward takes an hour off, because the Sun has not yet got that far. So the same moment is called by different names all round the Earth, and every one of those names is correct for its own meridian.

Two friends, one phone call

Two friends are on the phone in the late afternoon, one in Porbandar in Gujarat and the other in Tinsukia in Assam. The friend in Assam says the sun has set and it is dark outside. The friend in Gujarat is astonished, because it is still full daylight there. Neither is mistaken. Take the difference in longitude between the two towns as 30 degrees. Divide 30 by 15 and you get 2, so their local times differ by about two hours.

Local time becomes inconvenient

That example shows the trouble with local time. It is perfectly exact, and it is perfectly awkward. If every meridian kept its own clock, a train timetable would change along the length of the platform. A radio bulletin would reach half the country at the wrong hour. A country needs one answer to what time is it, not a different answer in every district.

Standard time

So most countries settle on a standard time, taken from one meridian that crosses the country. Everybody inside that country sets their clocks to it, whatever their own local time may be. In Assam, then, the local time and the standard time are not the same thing, and the Assam sunset in our phone call is exactly why. The standard clock is a shared agreement; the local time is astronomy.

Indian Standard Time

India's own is Indian Standard Time, usually shortened to IST. It runs 5 hours 30 minutes ahead of the local time at Greenwich, which is called Greenwich Mean Time or GMT. You can also write that as 5.5 hours. The arithmetic is worth doing once slowly. If it is 12 noon in London, add five hours to get 5 pm and another thirty minutes, so it is 5:30 pm in India.

Time zones on the world map

All these standard times are organised into time zones. Look at a world map of them and you will see broad bands running roughly north to south, each keeping one standard time. The bands follow the 15 degree meridians in a general way, which is what you would expect from the fifteen-an-hour rule. What you would not expect is their shape.

Why the zone lines bend

The lines dividing the zones are not straight. They wander, and they wander for a practical reason. Each zone has to respect the standard time that a country has chosen, so the boundaries tend to follow international borders instead of meridians. Cutting a country in half with a time zone would mean two halves keeping different clocks, which is precisely the inconvenience standard time was invented to avoid.

Plus and minus against GMT

On a time zone map you will find a figure printed inside many countries. It is the number of hours to add to GMT to get that country's standard time. Where the figure carries a minus sign, it is the number of hours to subtract instead. India's would be plus five and a half. The figures are a shorthand: once you know GMT, one small sum gives you the time anywhere.

Countries with many zones

It would be tidy if every country had one standard time, but some are simply too wide. The United States of America has six time zones and Russia has eleven. That eleven has a striking consequence. Travelling right across Russia from east to west, you readjust your watch ten times. Eleven zones have ten boundaries between them.

The International Date Line

Greenwich holds the Prime Meridian. Directly opposite it, near 180 degrees longitude, runs a line of a different kind: the International Date Line. The zones marked plus twelve and minus twelve meet each other there. Cross it by ship or plane travelling eastward and you subtract a day, so Monday becomes Sunday. Cross it travelling westward and you add one, so Sunday becomes Monday. It is only approximately at 180 degrees, because it deviates in places to avoid dividing a country into two different dates.

Scenario:

A school in Tinsukia, in Assam, joins an online quiz hosted by a school in Porbandar, in Gujarat. The organisers set it for 5 pm IST. The Assam pupils arrive to find it already dark outside. On screen, the Gujarat pupils sit in bright afternoon light. One Assam pupil argues that the quiz was unfair. He says it should have begun at 5 pm local time in each place. A teacher points out that both schools set their clocks to the same standard time. She then puts two questions to the class. How far apart are the two towns in longitude? And what would happen to the quiz if every town used its own local time instead?

Analysis:

The pupil has noticed something real. Take the longitude difference between the two towns as 30 degrees. Divide by 15 degrees an hour and their local times differ by about two hours. So at 5 pm IST the Sun really is much lower over Assam. The daylight the two schools see is genuinely different. What the pupil has not seen is the cost of his alternative. If each school worked to its own local time, 5 pm would mean two different moments. One school would log in about two hours after the other had finished. Nor would the trouble stop at two towns. Every town between them keeps a slightly different local time. There would be no single moment left to agree on. That is why India, like most countries, sets one standard time from a single meridian. IST is not a claim about the Sun. It is an agreement that lets a whole country keep one appointment.

Activity:

Find the longitudes of your own town and of Porbandar in an atlas. Work out the difference, divide by 15, and write down how many hours and minutes your local time differs from Porbandar's. Then write whether your local time is ahead or behind, and why.

Title:

A lamp, a globe and a day

What You Notice:

Half the globe is always lit and half is always dark, and no amount of turning changes that. You will also find that the arithmetic and the lamp agree: an eighth of a turn is 45 degrees and three hours, because each hour is 15 degrees.

Safety:

Let the lamp cool before moving it, and keep the flex where nobody can trip over it.

Travelling right across Russia from east to west you have to readjust your watch ten times, because the country is spread over eleven time zones.

Delhi and Bengaluru are far apart on a map, yet their longitudes are almost the same at about 77 degrees East, so their local times barely differ at all.

The International Date Line is not perfectly straight. It bends where it must, so that no country ends up living in two different dates at once.

Degrees to hours — Fifteen an hour — 360 degrees in 24 hours gives 15 degrees an hour. Divide a difference in longitude by 15 to get the difference in hours.

Which way the clock goes — East is early — Travel east and the local time is later, because the Sun reached there first. Travel west and it is earlier.