Curiosity · Chapter 12 — Earth, Moon, and the Sun

Rashmika cycles to school in Kanniyakumari every morning. She notices that the shadows of coconut trees are long in the morning and short in the afternoon. She thinks the Sun must be moving across the sky. But she has also learnt that the Earth moves around the Sun. So which is it: does the Sun move, or does the Earth?

What we see. Every day the Sun appears in the East, climbs across the sky and sets in the West. It certainly looks as if the Sun is moving.

A ride on a merry-go-round. Sit on a merry-go-round facing outwards and ask a friend to turn it slowly anticlockwise (as seen from above). Look around. The trees, buildings and people around you seem to move past you, in the opposite direction, clockwise. Fix your eyes on one tree. It comes into view on your left, moves across in front of you and disappears on your right. Of course the tree has not moved at all. It only seems to move because you are turning.

Apparent motion. When we are moving, objects that are still can appear to move the other way. This is called apparent motion. You have felt it in a train: when your train starts gently, the trees and poles outside seem to slide backwards, and for a moment you may even think the platform is moving.

Back to the Sun. Now apply the same idea to the sky. Suppose the Earth is turning, and we are carried round with it. Then the Sun, which is far away and not going round us, would seem to move across the sky in the opposite direction to our turning. That is exactly what happens. The Sun appears to move across our sky because we watch it from the Earth, which is turning around itself.

An ancient insight. Aryabhata, the great Indian astronomer of the fifth century CE, explained this with a lovely comparison in his book Aryabhatiya: a person in a boat moving forward sees the trees on the bank moving backwards; in the same way, the stars, which are fixed, appear to move westwards to people on the turning Earth.

Why doesn't it feel as if we are moving? The Earth turns very smoothly and steadily, and the air turns with it. In a smoothly moving train with closed windows you also cannot feel the motion, and you feel it only when the train speeds up, slows down or jerks.

A spinning top, a ceiling fan and a ball spun on a finger all turn around themselves. The Earth does the same thing, all the time.

Rotation. The Earth spins, or rotates, on its own axis. The axis of rotation is an imaginary line through the Earth's centre, joining the geographic North Pole and South Pole. In rotation, every part of the Earth moves in a circle around this line. The Earth completes one rotation in about 24 hours.

Which way? Seen from above the North Pole, the Earth rotates anticlockwise. On a map this means it turns from West to East.

A globe and a torch. Stick a small sticker on a globe to mark where you live. In a dark room, shine a torch on the globe from about 1.5 m away. Notice that only half of the globe is lit; the other half is dark. On the real Earth, the half facing the Sun has day and the other half has night.

The day-night cycle. Now slowly turn the globe from West to East, as seen from above the North Pole. Your sticker moves into the lit half (sunrise), across it (day), into the dark half (sunset) and through it (night), and then comes back to the start. One full turn is one complete cycle of day and night. So the rotation of the Earth causes day and night.

How fast? The Earth turns through a full circle, 360 degrees, in about 24 hours. So it turns through 360 ÷ 24 = 15 degrees every hour, or 1 degree every 4 minutes.

Seeing the Earth turn. In the nineteenth century, Léon Foucault hung a very long pendulum with a heavy bob from a high ceiling. Over the hours, the direction of its swing slowly turned, showing that the Earth was turning beneath it. A Foucault pendulum 22 m long now hangs in the Constitution Hall of the new Parliament building in New Delhi.

A precise measurement. Aryabhata gave the time for one full rotation of the Earth (measured against the stars) as about 23 hours 56 minutes 4.1 seconds in modern units, remarkably close to today's accepted value.

Sunrise in Arunachal Pradesh can come nearly two hours before sunrise in Gujarat. When it is afternoon in India, it is night in the USA. Both facts follow from the Earth's rotation.

The view from the Equator. Imagine standing on the Equator facing North, with your back to the South. East is on your right and West on your left. As the Earth turns from West to East, it carries you along:

• Morning: your part of the Earth is just turning into the sunlight. The Sun appears low on your right, in the East.
• Noon: you have been carried round so that the Sun is high overhead.
• Evening: you have turned further. The Sun sinks out of view on your left, in the West, and soon the stars appear.

So the Sun appears to rise in the East, cross the sky and set in the West, because we turn from West to East.

East sees the Sun first. Because the Earth turns from West to East, places further east turn into the sunlight earlier. In India, sunrise happens first in the north-eastern states and last in the far west. India stretches over about 29 degrees from west to east. At 4 minutes per degree, that is about 29 × 4 = 116 minutes, nearly two hours. (The exact difference on a given day also depends on how far north or south each place is.)

Day here, night there. The USA is on the other side of the globe from India. When India faces the Sun, much of the USA faces away from it. So when it is day in India, it is night in most of the USA, and when people in India sleep, it is daytime there.

Why clocks show one time in India. Since sunrise varies across the country, India uses one common clock time, Indian Standard Time, for the whole country. A sundial shows local Sun time, which differs from place to place; that is why a correction is needed to read Indian Standard Time from a sundial such as the Samrat Yantra.

Shadows through the day. When the Sun is low in the East in the morning, a coconut tree's shadow is long and points West. As the Sun rises higher, the shadow shortens. Near noon it is shortest. In the evening it grows long again, now pointing East. Rashmika's shadows were telling her that the Earth was turning.

If the turning Earth makes the Sun seem to cross the sky, should it not make the stars seem to move too? Let us check on a clear night.

Watching the Big Dipper. On an evening between March and May, find the Big Dipper (Saptarishi) in the northern sky and, if you can, the Pole Star (Dhruva Tara). Note your place and the date. Draw the position of the Big Dipper compared with the Pole Star, or with a fixed tree or building, and write the time. Look again two hours later, and again two hours after that, and draw it each time. You will find that the Big Dipper has moved round in a circle about the Pole Star, while the Pole Star itself has hardly moved.

Why the Pole Star stays still. The Earth's axis of rotation, extended into the sky from the North Pole, points very close to the Pole Star. As the Earth spins about this axis, the Pole Star lies almost on the axis, so it seems nearly fixed. All the other stars seem to go round it in circles, just as a point on the edge of a spinning wheel goes round while the centre stays put.

How far do they turn? The Earth turns 15 degrees each hour, so the stars appear to turn round the Pole Star by 15 degrees each hour. In two hours the Big Dipper turns through 30 degrees; in 12 hours, half a circle.

Rising and setting. Stars that are not near the Pole Star rise in the East and set in the West, just like the Sun. The Moon also rises in the East and sets in the West every day, for the same reason: the Earth turns from West to East.

Star trails. If a camera on a stand keeps its shutter open for a long time at night, each star draws a curved line on the photo. These arcs are called star trails. Pointing the camera towards the Pole Star gives circles of trails all centred near it, a beautiful picture of the Earth's rotation.

Using the stars. Because the Pole Star stays nearly fixed in the north, travellers and sailors in the Northern Hemisphere used it for centuries to find direction at night. Some communities also used the appearance of certain stars to mark seasons; the Bhil and Pawara people of the Tapi valley watched for particular star patterns to know that the monsoon was near.

If you look East at sunset in December and again in June, you will see different stars rising. Why should the night sky change through the year?

Revolution. While spinning on its axis, the Earth also travels around the Sun. Motion of one object around another is called revolution. The path it follows is its orbit. Seen from above, the Earth's orbit is nearly a circle. (In side-view pictures the orbit looks like a flat oval, but that is only because we are looking at it from the side.) The Earth completes one revolution in about 365 days and 6 hours.

Why the night sky changes. At night we look out from the side of the Earth that faces away from the Sun. As the Earth moves round its orbit, this night side points towards different parts of space at different times of the year. So the stars we see at night, and the stars that rise just after sunset, slowly change from month to month. Stars that are behind the Sun, as seen from the Earth, cannot be seen at all at that time, because they are up in the daytime sky.

A little earlier every night. The Earth goes about one degree further along its orbit each day. Because of this, any particular star rises about 4 minutes earlier each night than the night before. Over a month this adds up to about 2 hours, and over a whole year to a full 24 hours, which brings the star back to the same time.

Example. Padmashree sees the Orion constellation nearly overhead at 8 pm. Tomorrow it will be overhead about 4 minutes earlier, at about 7:56 pm. A month later it will be overhead at about 6 pm.

Same date, same sky. After one full revolution the Earth is back at the same place in its orbit. So a group of stars that rises at midnight on 21 June will again rise at midnight on about 21 June next year.

Leap years. The extra 6 hours each year add up to about one whole day every four years. That is why we add an extra day, 29 February, in a leap year.

In north India, June is scorching and December is cold. Days are long in summer and short in winter. Yet in Australia, December is summer! What causes seasons, and why are they opposite in the two halves of the Earth?

The tilted axis. The Earth's axis is not upright compared with its orbit; it is tilted, by about 23.5 degrees. The axis keeps pointing in the same direction in space (towards the Pole Star) all year as the Earth goes round the Sun. So in one part of the orbit, the Northern Hemisphere leans towards the Sun, and six months later, on the other side of the orbit, it leans away.

June in the Northern Hemisphere. Around June, the Northern Hemisphere is tilted towards the Sun. Two things happen:

• Sunlight is more concentrated. Because the Earth is round and our half leans towards the Sun, the Sun's rays strike our ground more steeply. The same amount of sunlight falls on a smaller area, so each part is heated more. Try shining a torch straight down on a table and then at a slant: the slanted beam spreads over a larger, dimmer patch.
• Days are longer. As the Earth rotates, places in the Northern Hemisphere spend more than 12 hours in sunlight and less than 12 hours in darkness.

More intense sunlight for a longer time each day gives summer in the Northern Hemisphere. At the same time the Southern Hemisphere is tilted away, gets slanting sunlight for less than 12 hours a day, and has winter.

December. Six months later the Earth is on the other side of the Sun. Now the Northern Hemisphere leans away: it has winter, with short days and slanting sunlight, while the Southern Hemisphere has summer. That is why a cricket team touring Australia in December should pack summer clothes.

Two wrong ideas. Many people think seasons happen because the Earth is nearer the Sun in summer. But the tilt changes our distance from the Sun by very little, and the Earth's orbit is so close to a circle that its distance from the Sun changes only from about 147 million km to about 152 million km. In fact the Earth is closest to the Sun in early January, during the northern winter. If distance caused seasons, both hemispheres would have summer together, which they do not.

So: seasons are caused by the tilt of the Earth's axis together with its spherical shape.

You may have noticed that in summer it is still light at 7 pm, while in winter it is dark by 6 pm. Day length changes through the year in a regular pattern.

Solstices and equinoxes in the Northern Hemisphere.

After the summer solstice, days slowly get shorter until the winter solstice; after that they get longer again. In the Southern Hemisphere everything is reversed: 21 June is their shortest day and 22 December their longest.

At the poles. Around June the North Pole is tilted towards the Sun, so as the Earth rotates it stays in sunlight for all 24 hours. In December it stays in darkness for all 24 hours. At the North Pole, the Sun rises around the March equinox and stays in the sky, circling round, for about six months, setting around the September equinox. Then comes about six months of darkness. The South Pole has the opposite pattern. This is why polar regions are called lands of the midnight Sun.

At the Equator. On the Equator, day and night are each about 12 hours long all through the year, and the Sun's rays fall fairly steeply in every month. So there is little change of season there. The southern states of India, which are fairly close to the Equator, also feel the seasons much less than north India. Nearness to the sea, hills and winds also affect local weather, as you learn in social science.

Why do the poles stay cold even with 24-hour sunshine? At the poles the Sun stays low in the sky, so its rays are very slanted and spread out, and much of the sunlight is reflected by the white ice and snow.

Watching it yourself. Note the time of sunrise and sunset from a calendar or newspaper on the 21st of each month for a year. Work out the day length each time and plot it. You will see it rise to a maximum in June and fall to a minimum in December.

Hold your thumb up at arm's length and close one eye. You can hide a friend's whole head behind your thumb, even though your thumb is much smaller than a head. In the same way the small Moon can sometimes hide the huge Sun.

Apparent size. How big an object looks to us, its apparent size, depends on both its real size and its distance. The Sun is about 400 times wider than the Moon, but it is also about 400 times farther away. So the Sun and the Moon look almost the same size in our sky. This lucky match lets the Moon cover the Sun exactly.

What is a solar eclipse? The Moon revolves around the Earth. Sometimes the Moon passes directly between the Sun and the Earth, and blocks the Sun's light from reaching part of the Earth. This is a solar eclipse. It can happen only on a new Moon day, when the Moon is between the Earth and the Sun.

• Where the dark central part of the Moon's shadow touches the Earth, a small region, the whole Sun is hidden. People there see a total solar eclipse. For a few minutes, day turns almost into night, birds go quiet and stars may appear.
• In a much wider region around it, only part of the Sun is hidden. People there see a partial solar eclipse, as if a bite has been taken out of the Sun.

Because the Earth rotates and the Moon moves along its orbit, the Moon's shadow races across the Earth. So totality at any one place lasts only a few minutes. As the Moon begins to move off, a brilliant point of sunlight may flash at its edge, looking like a diamond ring, and daylight returns.

Why not every month? The Moon's orbit is slightly tilted compared with the Earth's orbit. On most new Moon days the Moon passes a little above or below the line joining the Sun and the Earth, so its shadow misses the Earth.

Why can't Venus or Mercury block the Sun? They are larger than the Moon but much, much farther away, so they look tiny. When Venus passes in front of the Sun, it appears only as a small black dot crossing the Sun's face, a rare event called a transit of Venus.

Watch safely! Never look at the Sun directly, even during an eclipse, and never through sunglasses, binoculars or a telescope; it can damage the eyes and cause blindness. Use special solar eclipse glasses, project the Sun's image with a pinhole or a small mirror onto a wall (with a teacher), or join a viewing event at a planetarium or astronomy club.

No need to fear. Long ago, eclipses frightened people and many superstitions grew up around them. Ancient Indian texts such as the Surya Siddhanta gave methods to calculate when eclipses would happen. Today scientists travel across the world to study eclipses.

Sometimes, on a full Moon night, a dark shadow slowly creeps across the bright Moon, and the whole Moon turns a dull coppery red. What is happening?

What is a lunar eclipse? The Earth, lit by the Sun, casts a long shadow into space on the side away from the Sun. As the Moon revolves around the Earth, it sometimes passes through this shadow. Then the Earth blocks sunlight from reaching the Moon. This is a lunar eclipse. The Earth is between the Sun and the Moon, so a lunar eclipse can happen only on a full Moon night.

• When the whole Moon is inside the Earth's dark shadow, it is a total lunar eclipse. The Moon does not vanish; it glows dark red, because a little sunlight is bent through the Earth's atmosphere and reaches it, coloured red as in a sunset.
• When only part of the Moon is in the Earth's shadow and the rest is still bright, it is a partial lunar eclipse. The edge of the Earth's shadow on the Moon is curved, which also tells us that the Earth is round.

Why do so many people see a lunar eclipse? In a solar eclipse, the Moon's small shadow touches only a narrow strip of the Earth, so a total solar eclipse is seen from only a small region. In a lunar eclipse, the Moon itself goes dark, so everyone on the night side of the Earth who can see the Moon sees the eclipse, about half the planet at once. A total lunar eclipse can also last for over an hour, much longer than a total solar eclipse.

Not every full Moon. Just as with solar eclipses, the tilt of the Moon's orbit means that on most full Moon nights the Moon passes a little above or below the Earth's shadow.

Finding eclipses. Free sky software such as Stellarium can show when the next solar or lunar eclipse will be visible from your town.

Indian astronomy. The Kodaikanal Solar Observatory in the Palani hills, set up in 1899 and run by the Indian Institute of Astrophysics, has recorded the Sun for over a hundred years. M. K. Vainu Bappu, called the father of modern Indian astronomy, built major telescopes at Kavalur (now named after him) and near Nainital, discovered a comet and travelled the world to study solar eclipses.