Earth: Shape, Motions, Latitude and Longitude

Almost every question on this chapter starts from one simple picture: Earth is a slightly squashed ball that spins. The squashing, the spinning and the way it goes round the Sun together explain latitude, longitude, day, night, seasons and time. So let us first fix the shape and size, and then the spin.

Earth is not a perfect sphere. It is an oblate spheroid: flattened at the poles and bulging at the equator. The reason is rotation. The faster a point spins, the more it is flung outwards, and the equator spins fastest, so the equator bulges. Scientists call the true shape a geoid.

Why do we say 'about'? Because the numbers differ slightly between textbooks and satellite surveys. For the exam, remember the direction of the difference: equatorial measures are always larger than polar ones. That is why a person standing at the equator is about 21 km farther from Earth's centre than a person standing at a pole.

Now the spin. Rotation is Earth's spinning on its own axis, an imaginary line that runs through the North Pole, the centre and the South Pole. Earth turns from west to east. That is why the Sun, Moon and stars appear to rise in the east and set in the west. Seen from above the North Pole the turn is anticlockwise; seen from above the South Pole it is clockwise.

The second motion of Earth is revolution: its yearly trip round the Sun along an orbit that is slightly oval (an ellipse). Rotation gave us day and night; revolution, together with one more ingredient, the tilt of the axis, gives us the year and the four seasons. Nothing else is needed to understand this entire topic.

Seasons need two things together: (1) the axis is tilted by 23.5 degrees, and (2) the axis keeps pointing the same way, towards the Pole Star, all through the year (parallelism of the axis). As Earth goes round the Sun, first the Northern Hemisphere and then the Southern Hemisphere leans towards the Sun. The leaning half gets the Sun higher in the sky and longer days: that is its summer.

A solstice (the Sun 'stands still') is the day when the Sun's overhead position reaches its farthest point north or south and turns back. An equinox ('equal night') is the day when the Sun is exactly over the equator and day and night are equal. The dates move by a day here and there, so questions say '21 March' or '23 September' as the nearest round dates. In the Southern Hemisphere every season is reversed: when it is summer in India, it is winter in Australia.

The year is not exactly 365 days; it is about 365 days and 6 hours. Those extra six hours add up to one full day in four years. To correct this we add 29 February and make a leap year of 366 days. Divisible by 4 means leap year, but a century year (1900, 2100) is a leap year only if it is divisible by 400. So 2000 was a leap year, 1900 was not; 2024 was a leap year and 2028 will be the next.

To tell someone where a place is on a round Earth, we draw an imaginary net of lines over it: lines running east-west are parallels of latitude and lines running north-south are meridians of longitude. First, latitude. Think of it as 'how far north or south of the equator'.

Latitude is the angle, measured at Earth's centre, between a place and the equator. The equator is 0 degrees, the poles are 90 degrees North and South. All parallels are full circles that run east-west and never meet; they get smaller towards the poles, and the pole itself is just a point. One degree of latitude is about 111 km everywhere.

These are the heat zones (thermal zones). They are drawn on one idea: how steeply the Sun's rays fall. Near the equator rays fall almost straight down and heat a small area strongly; near the poles they fall slantwise, spread over a large area and pass through more air. So temperature falls as you move away from the equator. Remember, the zones are based on the Sun's angle only; real climate also depends on height, sea, winds and ocean currents.

The Tropic of Cancer (23.5 degrees N) cuts across India almost through the middle, so the country lies partly in the Torrid Zone (south of the line) and partly in the Subtropical or northern warm-temperate belt. It crosses eight Indian states, listed here from west to east.

Latitude tells us how far north or south a place is. To pin the place down we also need 'how far east or west'. That is longitude, and it brings in the most useful idea in this chapter: because Earth turns, longitude is time. Read this topic slowly; every time question rests on one line: 15 degrees equals 1 hour.

Longitude is the angle, measured at Earth's centre, between a place's meridian and the Prime Meridian (0 degrees, Greenwich). It runs from 0 to 180 degrees East and 180 degrees West. Meridians are half-circles joining the two poles; all are equal in length, they are farthest apart at the equator and meet at the poles. There are 360 meridians if we count every whole degree.

The Prime Meridian passes through the Royal Observatory at Greenwich, London. It was fixed as the world's zero line at an international conference in Washington, D.C. in 1884. On land it crosses the UK, France, Spain, Algeria, Mali, Burkina Faso, Togo and Ghana. Its opposite half, the 180-degree meridian, runs mostly through the Pacific Ocean and forms the basis of the International Date Line.

15 degrees = 1 hour; 1 degree = 4 minutes. Earth turns 360 degrees in 24 hours, so 360 / 24 = 15 degrees per hour. Because Earth turns west to east, places to the east see the Sun first, so their clocks are ahead; places to the west are behind. Greenwich (0 degrees) is the reference: its time is GMT (Greenwich Mean Time), now used with UTC.

Now our own clock. India stretches about 29 degrees from Gujarat to Arunachal Pradesh, which means the Sun rises nearly two hours earlier in the east than in the west. A different clock in each town would be chaos, so the whole country follows one Indian Standard Time (IST), based on the meridian of 82 degrees 30 minutes East. This meridian passes close to Mirzapur in Uttar Pradesh. Since 82.5 x 4 = 330 minutes, IST is 5 hours 30 minutes ahead of GMT/UTC. If Radhika in Delhi calls a friend in London when it is 12 noon in India, it is 6:30 AM there (in winter, standard time).

The world is divided into 24 standard time zones, each about 15 degrees wide, but real zones follow country borders, so some countries use half-hour or 45-minute offsets (India UTC+5:30, Nepal UTC+5:45), and some large countries have several zones (Russia has 11). China, though very wide, uses a single time. Now the International Date Line (IDL): it follows roughly the 180-degree meridian through the Pacific, with bends to avoid splitting islands and countries (around the Bering Strait, Fiji, Tonga and Kiribati). Crossing it, you change the calendar date by one day.

Finally, the great circle. It is any circle on Earth's surface whose centre is Earth's own centre, so it cuts the globe into two equal halves. The Equator is the only parallel that is a great circle; every meridian, joined to its opposite meridian (0 and 180, 90 East and 90 West), also makes a great circle. All other parallels are small circles. The shortest path between two places on the globe lies along a great circle, which is why long-distance flights between India and the western USA curve far to the north and look strangely bent on a flat map.

Now let us use latitude and longitude on our own country, because this is where the exam is most generous. India lies in the Northern and Eastern Hemispheres, with the Tropic of Cancer cutting across its middle and the 82°30′E meridian giving it its clock. Learn the extents and the extreme points once, and a whole group of questions becomes free marks.

Notice something clever: the north-south and east-west spreads are both about 30 degrees, but the distances are different (3,214 km against 2,933 km). The reason is that a degree of latitude is always about 111 km, while a degree of longitude becomes shorter as you move away from the equator. India's east-west stretch lies mostly in higher latitudes than its southern tip, so its longitudinal degrees are shorter.