Class 7 Science Chapter 12: Earth, Moon, and the Sun

A Puzzle on the Way to School

Coconut tree shadows are long in the morning and shorter in the afternoon. The easy explanation is that the Sun moves across the sky during the day. But in Grade 6 you learnt that the Earth moves around the Sun, so which of the two is really moving?

Start With Something You Can Ride

Everyone knows that the Sun comes up in the East and goes down in the West. To see what that really tells us, go back to a merry-go-round. Sit on it facing towards the outer side, and ask somebody to turn it slowly in the anti-clockwise direction.

What You See

Look around you while it turns. The things around you appear to be moving, and they appear to turn about you in the contrary direction to your own. You are going anti-clockwise, and they appear to go clockwise.

Watch One Tree

Now fix your gaze on one tree in particular, or a building, ahead of you. It appears to turn around you, clockwise, and it is not in your view all the time. It comes into your view at your left-hand side and moves out of your view on your right-hand side.

Nobody Believes the Trees Moved

That is the important part. You know perfectly well that the tree stayed where it was. What moved was you, and the motion you saw in the tree was apparent motion. The same mistake is available to anybody watching the sky.

Now Apply It

From the Earth we see the Sun rise in the East, cross the sky westward, and vanish in the West. Is the Sun moving in the sky, or is the Earth turning around itself so that the Sun only appears to move?

The Answer

The Sun only seems to move, because we are watching it from an Earth that is turning around itself. Rashmika shadows got shorter towards noon not because the Sun travelled, but because she did.

Things That Turn on Themselves

Which way is the Earth turning? Think first of things you have watched spin. A top spinning around its spindle. A ceiling fan. A ball you have spun on a table. Each turns about a line through itself and stays in one place while doing it.

The Earth Does the Same

In the same way, the Earth spins, or rotates, about its own axis out in space. The axis of rotation of the Earth passes through its geographic North Pole and its South Pole.

How Long It Takes

The Earth completes one rotation in about twenty-four hours. That single fact is the reason a day is the length it is, and the reason you cannot see the whole sky at once from one place.

And Which Way Round

Direction needs a point of view, so the book gives one. When viewed from a point above the North Pole, the Earth turns in the anti-clockwise direction, which is from West to East.

Why That Direction Makes Sense

Put it beside the merry-go-round and the East-West business falls into place. Turning towards the East brings the Sun into view in the East, exactly as turning anti-clockwise brought the tree into view on your left.

Naming the Motion Properly

Rotation has a careful definition. It is the motion in which every part of an object travels in a circle about an imaginary line that passes through it, and that line is called the axis of rotation.

Notice What the Definition Demands

ALL its parts move in circles, and the line passes THROUGH the object. That is what separates a spinning top from a ball thrown across a room, and it is why the Earth rotating and the Earth travelling round the Sun are two different motions.

Try It on a Globe

Take a globe to stand for the Earth and put a small sticker on it to mark your own location. Looking down from above the North Pole, rotate the globe slowly in the anti-clockwise direction.

What the Sticker Does

Watch the sticker. It turns around and finally returns to where it began, which completes one rotation. In the real world that journey takes about twenty-four hours, and you make it every day without feeling a thing.

How It Was First Shown

Believing the Earth turns is one thing; demonstrating it simply is another. In the middle of the nineteenth century, Leon Foucault used a long pendulum, and gave the first simple demonstration of the rotation of the Earth.

The Foucault Pendulum

A pendulum of that kind, named a Foucault pendulum in his honour, is a long string carrying a heavy bob, hung from a high ceiling. One of them is twenty-two metres long. It hangs from a skylight in the Constitution Hall, which is part of the new Parliament building at New Delhi. There it stands for the idea of India set against the vastness of the cosmos.

Scenario:

Three cousins watch the sunset from a terrace. The youngest says the Sun is clearly moving, since you can watch it sink. The middle one says that is nonsense. The Earth goes round the Sun, she says, so it must be the Earth that moves. The eldest says they are both wrong about the reason. The sunset, she says, has nothing to do with the Earth going round the Sun. She offers to settle it with a merry-go-round at the park next morning. The youngest says a ride at a park can hardly decide what the Sun is doing.

Analysis:

The eldest is right, and the ride does decide it. Take the youngest first. He is reporting honestly: the Sun does appear to sink. But apparent motion is exactly what a moving observer sees, and the merry-go-round is the cleanest case of it. Sitting on it facing outward while it turns anti-clockwise, the objects around appear to turn clockwise, and a chosen tree comes into view on the left and leaves on the right. Nobody believes the trees moved, so everybody accepts that a turning observer sees a turning world. Now the middle cousin. She has the right fact and the wrong application. The Earth does travel around the Sun, but that takes a year and cannot account for anything happening in an evening. What explains sunset is the other motion. The Earth ROTATES on its own axis, about a line through its North and South Poles, once in about twenty-four hours. That turn is anti-clockwise seen from above the North Pole, which is from West to East. Turning towards the East is what brings the Sun into view there in the morning and carries it out of view in the West at night.

Activity:

Stand facing a wall with a clear mark on it, then turn slowly on the spot to your left and watch the mark travel across your view. Write down which side it entered and which side it left, and one sentence saying what the Sun does that matches it.

Title:

Try it yourself: a globe and a sticker

Safety:

Hold the globe over a table, not over your feet, and keep the stand steady.

What You Notice:

The sticker travels round and comes back to its starting place. That is one complete rotation, and it stands for about twenty-four hours. Turning the other way would be wrong. The Earth turns anti-clockwise from that viewpoint, which is west to east.

A 22-metre Foucault pendulum hangs in the new Parliament building at New Delhi.

The Earth takes about 24 hours to turn round once on its own axis.

On a merry-go-round, a tree enters your view on the left and leaves on the right.

Anti-clockwise from above the North Pole. That is west to east.

You turn one way and the world seems to turn the other. Every time.

From One Turn to Two Halves

The Earth rotates once in about twenty-four hours, from west to east. That single fact has a consequence you live by: day and night. To see the connection clearly, the chapter asks you to build the whole thing on a table.

What You Need

A globe to stand for the Earth, with a small sticker marking your own location on it. A torch to stand for the Sun. And a dark room, which matters more than it sounds, because the point is to see where the light does NOT reach.

Setting It Up

Place the torch about one and a half metres away from the globe and switch it on, so that the light falls on the globe as sunlight falls on the Earth. Keep the torch steady. In this model the torch does not travel; only the globe turns.

The First Thing You See

Look at the globe from the side. Half of it is lit by the torch and the other half is in darkness. This is not a trick of the set-up. A ball lit from one side can only ever be lit on one side.

Naming the Halves

The half of the globe that is lit is having day. The half in darkness is having night. At every moment, half the Earth is in daylight and the other half is in the dark, and that is true of every day of the year.

Now Turn It

Rotate the globe slowly from west to east, which is the direction the real Earth turns. Keep your eye on the sticker, not on the globe as a whole. The sticker is you, and you are about to watch your own day happen.

Sunrise

The sticker moves out of the dark half and into the lit half. In the real world, that is sunrise at your location. Nothing about the Sun changed at that moment. You arrived.

Noon and Afternoon

The sticker carries on through the brightest part of the lit half and then starts to move towards the far edge of it. The light arriving at the sticker grows more slanting as it goes, which is why evening sunlight looks so different from midday sunlight.

Sunset

Then the sticker moves out of the lit half and into the dark half. That is sunset. Carry on turning and the sticker crosses the whole dark half, which is your night, and arrives back at sunrise. One rotation, one day and one night.

A Useful Side Result

Watch which part of the globe the light reaches first as you turn it from west to east. For India, the light of the torch reaches eastern India before it reaches the west of the country.

So the Sun Rises Earlier in the East

That is exactly what happens in the real country. Sunrise comes earlier in Arunachal Pradesh than in Gujarat, because the Earth turns towards the east and the eastern places move into the sunlight first. Nobody has to arrange it; the direction of the turn settles it.

Standing on the Equator

Now swap the model for a view from the ground. Imagine standing on the Equator, facing North. East is then on your right-hand side and West is on your left. Hold that arrangement for a whole day.

Morning, Noon, Evening

In the morning you see the Sun on your right-hand side, in the East. At noon it is overhead. By evening it has moved to your left-hand side, in the West. Then it disappears below the horizon and the stars come out.

Why That Order, and Not the Reverse

It follows from the turn. You are being carried eastward. So the Sun comes into view on the eastern side and leaves on the western side. It is exactly as the tree on the merry-go-round entered on the left and left on the right.

The Moon Does It Too

The same reasoning covers more than the Sun. The Moon too appears to come up in the east and set in the west, and for the same reason: we are watching it from a rotating Earth.

What the Model Does Not Show

Be careful about what one torch and one globe can prove. The model explains day and night from rotation alone. It says nothing yet about seasons, or about why days are longer in summer, because nothing in this set-up is tilted.

The Idea Worth Keeping

Day and night are not something that happens TO you from outside. They are the record of your own journey around the axis of the Earth, out of the light and back into it, once every twenty-four hours.

Scenario:

Ritu is in Dibrugarh, in the far east of India, and her cousin Mehul is in Dwarka, in the far west. They agree to ring each other at the moment the Sun comes up, and Ritu rings first. Mehul complains that Ritu phone must be fast, because the Sun cannot possibly rise at two different times on the same day. Their grandmother says the clocks are both right and so are both sunrises. Mehul then suggests that Dibrugarh must be closer to the Sun, which is why it is lit sooner.

Analysis:

The grandmother is right, and Mehul explanation is not. Build the answer from the globe and torch model. In a dark room, a torch about one and a half metres from a globe lights exactly half of it, because light coming from one side can light one side only. The lit half is having day and the dark half is having night, at the same moment. Turn the globe from west to east, which is the direction the real Earth rotates, and the sticker marking a location crosses from the dark half into the lit half, which is sunrise there. Now the whole point: as the globe turns west to east, the light reaches eastern India before the west of the country, so Dibrugarh crosses into the light first and Dwarka crosses in later. Both sunrises are real and both happen at the correct local moment. Mehul idea about distance does no work at all. The two cities are at almost exactly the same distance from the Sun, and nothing in the explanation needs distance. Only the direction of the turn matters.

Activity:

Look up the sunrise time today for one city in the far east of India and one in the far west. Write both times down, work out the gap in minutes, and add one sentence saying which direction of turn produced that gap.

Title:

Try it yourself: making a day with a torch

Safety:

Keep the torch pointed at the globe and never into anybody eyes.

What You Notice:

Exactly half the globe is lit. So half the Earth is having day while the other half has night. The sticker entering the lit half is sunrise, and leaving it is sunset. One whole turn gives one day and one night.

Half of the Earth is in daylight at every single moment of the year.

Keep the torch about 1.5 metres from the globe, in a dark room.

The Moon appears to rise in the east and set in the west, just as the Sun does.

Half lit, half dark, always. You simply ride from one into the other.

Turning east to meet the Sun. So the east meets it first.

The Sun Was Only the Start

Rotation explains the Sun crossing the sky and it explains day and night. If the explanation is right, it should work at night too, and on things far beyond the Sun. The chapter sends you outside to check.

What to Look For

On a clear night between March and May, find two things in the northern sky. The Big Dipper, a pattern of bright stars also known as Saptarishi. And the Pole Star, also known as Dhruva Tara, which lies close to the Big Dipper and can be found from it.

How to Record It

Sketch how the Big Dipper and the Pole Star are placed, taking care to draw the orientation of the pattern and not just the stars. Then leave it two hours and sketch again. Repeat it every two hours through the night.

Why Sketching Beats Remembering

The change over two hours is large enough to see but slow enough to miss, and memory is a poor instrument for an orientation. A sketch from midnight laid beside a sketch from ten o clock settles the question in a moment.

The Result

The Big Dipper appears to swing round the Pole Star. Its orientation at the end of the night is nothing like its orientation at the start. The Pole Star, meanwhile, has stayed nearly in the same place all night.

One Star That Barely Moves

That asks for an explanation, and the explanation is about us rather than the star. The axis of rotation of the Earth points almost exactly at the Pole Star, as seen from the Northern Hemisphere.

What That Means for the View

Spin on your heel and look straight up along the line of your spin: the point right overhead barely shifts. Everything to the side of it swings round. The Pole Star sits almost exactly along the line the Earth is spinning about.

So Everything Else Goes Round

The Pole Star appears nearly stationary, and all the other stars appear to move around it. The Big Dipper is simply the easiest pattern to watch doing it, because it is bright and close to the Pole Star.

The Stars Did Not Move

Say the conclusion carefully. The stars have not been travelling across the sky all night. We have been turning under them, and the motion we recorded is apparent motion, exactly like the Sun crossing the sky by day.

A Photograph of the Whole Thing

A camera can record this in a single picture. In astrophotography, the shutter is held open for a long time, so each star draws a line on the picture instead of a dot. These lines are called star trails.

What the Trails Show

Every trail is an arc, and all the arcs are parts of circles with the same centre, which is the Pole Star. The photograph in the book was taken at Mahuli, in Maharashtra, and it shows at a glance what the sketches showed over a whole night.

The Trails Measure the Turn

A longer exposure gives longer arcs, because the arc is just the record of how far the Earth turned while the shutter was open. The picture is not really a picture of the stars at all. It is a picture of our own rotation.

An Old Explanation

The idea is far from new. Aryabhata, in his work the Aryabhatiya, written in about the fifth century CE, explained the apparent motion of the stars with a comparison anybody who has been on a river can check.

The Boat

In Verse 9 of the Golapada he observes that a man in a boat moving ahead sees the stationary objects on the bank appear to move backwards. In just the same way, he writes, a person on the Earth sees the stationary stars appear to move towards the west.

Why That Comparison Is So Good

It picks a case where nobody is in any doubt about what is really moving. Nobody believes the trees on the bank are sliding upstream. Granting that, you must grant the same for the stars, and the boat and the merry-go-round are making the same point.

And a Number

Aryabhata also gave a figure. He put the time for one rotation of the Earth at 23 hours, 56 minutes and 4.1 seconds, which is impressively close to the value accepted today, and far more precise than the rough twenty-four hours we use for everyday purposes.

What to Take Away

One night of sketches, one long-exposure photograph and one verse from the fifth century all say the same thing. The sky is not turning. The ground under your feet is.

Scenario:

A school astronomy club puts a camera on a tripod, points it north and leaves the shutter open for two hours. The picture that comes back shows dozens of bright curved streaks, all bending the same way, with one short stub near the middle. A visitor says the photograph must have been edited, because stars are points of light and nobody has ever seen one as a streak. A club member says the streaks prove the stars travel in circles round the sky. The teacher says both of them have it wrong.

Analysis:

The teacher is right, and the picture is exactly what the sky does to a long exposure. Take the visitor first. Stars do look like points to the eye because the eye takes in only an instant. A camera with the shutter held open for two hours records everything that happened during those two hours in one frame, so each star is drawn as the whole path it appeared to take. Those lines are called star trails, and nothing has been edited. Now the club member. The arcs are real, but they are not the paths of the stars. Each arc records how far the EARTH turned while the shutter was open, which is why a longer exposure gives longer arcs and a very short one gives dots. The stars themselves stayed where they were. The stub near the middle is the giveaway. All the arcs are parts of circles sharing a single centre, which is the Pole Star. It draws almost nothing, because the axis of rotation of the Earth points very close to it in the Northern Hemisphere. That is also why a night of two-hourly sketches shows the Big Dipper swinging round it while the Pole Star hardly stirs.

Activity:

Find a star trail photograph in a book or a magazine, mark the centre of the arcs with a pencil dot, and write one sentence naming the star that belongs there and saying why its trail is the shortest in the picture.

Title:

Try it yourself: four sketches in one night

Safety:

Observe from a safe open place with an adult, and keep the torch beam low.

What You Notice:

The Big Dipper appears in a different orientation in each sketch, having swung around the Pole Star. The Pole Star stays nearly in the same place. The cause is the rotation of the Earth, about an axis that points very close to the Pole Star.

Aryabhata gave one rotation as 23 hours, 56 minutes and 4.1 seconds.

The Big Dipper is also known as Saptarishi, and the Pole Star as Dhruva Tara.

A star trail photograph turns every star into an arc centred on the Pole Star.

The axis points at it, so it does not move. Everything else goes round.

Sketch, leave it two hours, sketch again. The Dipper will have swung.

One Motion Was Not Enough

Rotation has explained a great deal: the Sun crossing the sky, day and night, the swing of the Big Dipper. But rotation happens in a day, so it cannot explain anything that takes months. For that the Earth needs a second motion, and it has one.

Revolution

besides turning on its own axis, the Earth also travels round the Sun. Revolution is the motion of one object around a second object, and the word is doing careful work here: around ANOTHER object, not around a line inside itself.

Hold the Two Apart

Rotation is about an imaginary line passing through the Earth, from pole to pole, and takes about a day. Revolution is about the Sun, a completely separate body, and takes about a year. The Earth is doing both at once, all the time.

The Path Has a Name

The path along which an object revolves is called its orbit. So the Earth travels along its orbit around the Sun, and the Moon travels along its own orbit around the Earth.

What Shape Is That Orbit?

Here the chapter corrects a picture you may already carry. Viewed from the top, the orbit of the Earth round the Sun is nearly a circle. It is not a long stretched oval with the Sun pushed off to one end.

Then Why Did It Look Elongated?

Because of the point of view. The picture you met in Grade 6 was a SIDE view of the same nearly circular orbit, and a circle seen from the side always looks squashed into an oval. Hold a bangle flat in front of your eyes and tilt it, and you will see the same thing happen.

Why It Matters

This is not a small correction. A great many wrong explanations of the seasons depend on believing the orbit is a long oval. So it is worth fixing the shape in your mind now, before the seasons come up at all.

How Long One Revolution Takes

The Earth finishes one journey round the Sun in about 365 days, plus 6 hours. Those extra six hours are not a rounding error that can be dropped; they are the reason our calendars need adjusting.

Two Clocks, Two Motions

So the Earth keeps two kinds of time at once. One rotation gives about twenty-four hours, which we call a day. One revolution gives about 365 days and 6 hours, which we call a year.

A Consequence You Can See

The second motion has a result in the night sky. As the Earth moves along its orbit, the night side faces out in a different direction at different times of the year, so you are looking out at a different part of the sky.

The Stars After Sunset Change

Because of this, the stars seen in the sky after sunset gradually change over the course of a year. The sky of a December evening is not the sky of a June evening, and nothing has moved except us.

How to Check It Properly

The method matters, because rotation can easily fool you. Observe at a FIXED time of night, and compare days about a MONTH apart. If you observe at different times on the two nights, the turn of the Earth during the night will swamp the change you are looking for.

What You Will Notice

A pattern that stood high in the east after sunset last month will be further across the sky at the same hour this month. After several months it can be gone from the evening sky altogether, while new patterns have come into it.

Two Motions, Two Time Scales

That is the clean way to keep them apart. Rotation changes the sky in hours and brings the same stars back the following night. Revolution changes the sky over months and does not bring them back until the year comes round.

People Have Used This

Communities that depend on the weather have read the sky for a long time. The Bhil and Pawara communities of the Tapi Valley used certain star patterns as markers for the arrival of the monsoon.

Why That Works

It works because the evening sky is a reliable calendar. Since the stars seen after sunset depend on where the Earth has reached in its orbit, a particular pattern appearing in the evening sky marks a particular time of year, every year.

The Idea Worth Keeping

The Earth is doing two motions at once. One is about a line through itself and takes a day; the other is around the Sun along a nearly circular orbit and takes about 365 days and 6 hours. Between them they account for both the fast changes in the sky and the slow ones.

Scenario:

A grandfather who farms near the Tapi river tells his granddaughter that the monsoon is near. A particular group of stars has come into the evening sky, he says. She says that cannot be science, since stars have nothing to do with rain. He asks her to explain something, then. Why have the same stars arrived in the evening sky at the same time of year for as long as he can remember? She says the stars must be moving across the sky. She offers to prove it by watching them on two nights in one week, once at eight and once at eleven.

Analysis:

The grandfather practice is sound and the granddaughter test is badly designed. Begin with why his marker works. The Earth revolves around the Sun along its orbit, taking about 365 days and 6 hours for one revolution. As it travels, its night side faces out in a different direction at different times of the year, so the stars seen after sunset gradually change through the year. That makes the evening sky a reliable calendar: a given pattern appears after sunset when the Earth has reached a given place in its orbit, which happens at the same time every year. This is exactly the practice the Bhil and Pawara communities of the Tapi Valley followed, using certain star patterns as markers for the arrival of the monsoon. The stars do not cause the rain; they mark the date. Now her test. Comparing eight o clock with eleven o clock on nights in one week measures rotation, not revolution, because the Earth turns through the night and shifts the whole sky in hours. The slow change she wants to see is hidden by the fast one. The method in the chapter fixes this: observe at a FIXED time of night, on days about a MONTH apart.

Activity:

Note the time and sketch the three brightest patterns you can see an hour after sunset tonight. Put the sketch away, repeat it in a month at the same time, and write one sentence on what moved and which motion moved it.

Title:

Try it yourself: the sky as a slow calendar

Safety:

Observe from a safe open place with an adult, and keep the torch beam low.

What You Notice:

The marked pattern sits in a different part of the sky at the same hour a month later. Over several months the evening sky changes altogether. The cause is the revolution of the Earth along its orbit. That makes the night side face a different direction through the year.

One revolution of the Earth takes about 365 days AND 6 hours.

Seen from above, the orbit of the Earth is nearly circular, not a long oval.

The Bhil and Pawara communities read certain star patterns as monsoon markers.

Rotation is about a line inside you. Revolution is around somebody else.

Same time of night, a month apart. That is how you catch the sky changing.

A Second Thing About the Orbit

The Earth revolves around the Sun along a nearly circular orbit in about 365 days and 6 hours, while rotating on its axis once a day. One more fact about that axis changes everything: it does not stand upright.

The Tilt

The axis of rotation of the Earth is not upright with respect to its orbit but tilted. And it maintains that tilt as it revolves around the Sun, pointing the same way all the year round rather than swinging about.

The Cause of the Seasons, Stated Properly

The tilt of the axis, together with the spherical shape of the Earth brings about the seasons. Both halves are needed. A tilted flat sheet would not do it, and an upright sphere would not do it either.

June: the North Leans In

Follow the Earth round. In June the Northern Hemisphere leans towards the Sun. Two separate things then follow, and both of them make it warm.

The First Thing: Light Spread Thinly or Thickly

Because the Earth is a sphere and that hemisphere is leaning towards the Sun, the same amount of sunrays falls on a SMALLER AREA there. The same heat concentrated into less ground means that ground is heated more.

See It With a Torch

Shine a torch straight at a wall and you get a small bright patch. Tilt the wall and the patch stretches out, covering more area and looking dimmer, although the torch has not changed. That is the whole idea.

The Second Thing: Longer Hours

The Northern Hemisphere also receives sunlight for more than twelve hours a day in June. So it is not only heated more intensely while the Sun is up; the Sun is up for longer. Together these give summer.

December: the Other Way Round

In December the situation is reversed for the Northern Hemisphere. The sunrays arrive spread over a larger area and the hours of daylight are fewer, so it is heated less. That is winter.

And the Other Half of the World

Because the two hemispheres lean opposite ways, the seasons and the lengths of day and night are reversed in the Southern Hemisphere. When it is summer in the north it is winter in the south, at the very same moment.

Now the Two Wrong Reasons

Two explanations are often given and both are wrong. The first says the hemisphere tilted towards the Sun is CLOSER to it and so warmer. The second says the orbit is an oval with the Sun off to one side, so the whole Earth is nearer in summer.

Why the First Is Wrong

The difference in distance produced by the tilt is very small compared with the distance to the Sun, far too small to produce a season. The cause is how the light is SPREAD, not how far it has come.

Why the Second Is Wrong

Here the chapter produces a fact that settles it. The Earth is actually CLOSEST to the Sun in January, which is the middle of winter in the Northern Hemisphere. If nearness caused summer, January would be the hottest month in the north, and it is not.

Four Dates Worth Knowing

The chapter marks four points in the year. Around 21 June the Northern Hemisphere has its longest day, called the summer solstice. Around 22 December it has its shortest day and longest night, the winter solstice.

And the Two Equinoxes

Around 21 March comes the spring equinox and around 23 September the autumn equinox, and on those days daytime lasts about twelve hours. They are the two moments when neither hemisphere is favoured.

At the Poles It Is Extreme

At the North Pole the Sun comes up in the East on 21 March and then stays above the horizon for six months, setting on 23 September. The South Pole has the opposite arrangement, so the polar regions get six months of sunshine followed by six months of darkness.

At the Equator It Is Even

On the Equator there are about twelve hours of light and twelve of darkness all the year, with little difference in the intensity of sunlight. This is why seasons are not very prominent in the southern states of India.

One Honest Complication

The tilt is the main cause, but it is not the only influence. Local geography and proximity to oceans also shape the weather a place actually gets, which is why two places at the same latitude can feel quite different through the year.

Scenario:

Two cousins video-call each other every December. One is in Shimla, wrapped in a blanket beside a heater; the other is in Melbourne, in a cotton shirt with the fan on. The Shimla cousin says the Sun must simply be further away from India at this time of year. His teacher mentions in class that the Earth is actually closest to the Sun in January. He decides the teacher must have misspoken, because that would make January the hottest month, which it certainly is not where he lives.

Analysis:

The teacher did not misspeak, and the January fact is the clearest evidence against the cousin own explanation. Work through it. The axis of rotation of the Earth is not upright with respect to its orbit but tilted, and it maintains that tilt all the way round. In December the Northern Hemisphere leans AWAY from the Sun. The same amount of sunrays then falls on a larger area there, and the hours of daylight are fewer than twelve. Together these make it cold in Shimla. At the same moment the Southern Hemisphere leans TOWARDS the Sun. In Melbourne the same rays land on a smaller area, and the Sun is up for more than twelve hours. That is why a fan is needed. The seasons are reversed between the hemispheres because the two halves lean opposite ways. Now the distance idea. If nearness to the Sun caused warmth, both cousins would be warm at once, and January would be the hottest month everywhere. Neither is true. The Earth comes nearest the Sun in January and it is deep winter in Shimla, so distance is not what produces the seasons. The cause is the tilt of the axis together with the spherical shape of the Earth.

Activity:

Find the sunrise and sunset times for your town today and work out the length of the day in hours and minutes. Do it again at the end of the month, then write one sentence saying whether your hemisphere is leaning towards the Sun or away from it.

Title:

Try it yourself: a torch, a ball and a tilted axis

Safety:

Keep the torch pointed at the ball and never into anybody eyes, and mind the sharp end of the needle.

What You Notice:

With the needle leaning, the top half of the ball catches the beam over a smaller patch at one position. At the opposite position the patch is larger. The torch never moved, and the distance barely changed. That is the mechanism of the seasons: the tilt together with the spherical shape.

The Earth comes nearest the Sun in January, which is winter in the north.

At the North Pole the Sun rises on 21 March and sets on 23 September.

Seasons are not very prominent in the southern states of India.

Tilt plus sphere. Take either one away and the seasons go.

Closest to the Sun in January, and that is the northern winter. So not distance.

Something Can Get in the Way

The Earth rotates, revolves and carries a tilted axis, and the Moon revolves around the Earth all the while. Sometimes these motions line three bodies up, and then one of them blocks the light reaching another.

Why Not Mercury or Venus?

Mercury and Venus also come between the Earth and the Sun at times. But they appear very small from here and never block the entire Sun. The Moon can. To see why, you need one idea first.

Apparent Size

Stand about five metres from a friend, close one eye and hold up your thumb towards their face. Your thumb appears to cover their whole head. Nobody thinks the thumb is bigger than a head.

What the Thumb Proves

How large something looks, its apparent size, depends on two things together: its real physical size together with its distance from you. Bring a small object close enough and it can hide a large one far away.

The Moon and the Sun

The Moon is very much smaller than the Sun, but it is also very much closer to us. Those two facts nearly cancel, so the Moon and the Sun have similar apparent sizes in our sky. That is what lets the Moon cover the Sun.

And the Planets

Mercury and Venus are larger than the Moon, but they are much farther away, so their apparent sizes are far smaller. When Venus passes between the Earth and the Sun it shows up only as a tiny black dot crossing the Sun, a rare event called the Transit of Venus.

The Solar Eclipse

Now the event itself. In a solar eclipse, the Moon comes between the Sun and the Earth and obstructs the sunlight. The Moon is the natural satellite of the Earth and revolves around it, while the Earth revolves around the Sun.

Total, and Partial

The shadow of the Moon reaches only a small area of the Earth, and from inside that area the Sun is completely blocked and it becomes dark. That is a total solar eclipse. From the surrounding regions only part of the Sun is blocked, which is a partial solar eclipse.

Why Only a Few Minutes

During totality it turns dark for only a few minutes. Two motions see to that: the rotation of the Earth and the orbital motion of the Moon both move the shadow across the surface, so no place stays inside it for long.

The Diamond Ring

As the Moon moves away, a brilliant point of sunlight appears at the edge of the dark disc while a thin bright rim is still visible. This is called the diamond ring, and then ordinary daylight returns.

The Rule That Matters Most

Directly viewing a solar eclipse must be strictly avoided. Even during the eclipse, the Sun can damage the eyes and cause blindness. This is not a caution about comfort. It is about permanent loss of sight.

Things That Are NOT Safe

Sunglasses are not safe. Binoculars and telescopes are worse, because they gather light and concentrate it. None of these should be used to look at an eclipse, however dim the Sun may appear at the time.

The Safe Way

The best course is to attend a viewing event organised by a planetarium or an astronomy club. They supply specialised eye protection and explain what is happening, so an eclipse can be watched safely and understood at the same time.

From Fear to Fieldwork

Historically, eclipses were feared and surrounded by superstitions, with rules about not eating, not cooking and not going out. Today scientists travel the world to observe them, because the few minutes of totality allow measurements that are impossible at any other time. Grahan is the Sanskrit word for an eclipse, and predictions of eclipses fill many ancient Indian texts. The Surya Siddhanta, set out in rhythmic shlokas, is the best known.

The Other Eclipse

Swap the order of the three bodies and you get the other kind. In a lunar eclipse the Earth blocks the sunlight from reaching the Moon, so the shadow of the Earth falls on the full disc of the Moon.

Total, and Partial Again

When the Moon is completely inside the shadow of the Earth, its bright disc turns dark red until it moves out, and that is a total lunar eclipse. When only a part of it lies in the shadow while the rest remains visible, it is a partial lunar eclipse.

And This One Is Safe

Here is the difference that matters in practice. Unlike the Sun, an eclipsed full Moon is perfectly safe to watch with the naked eye, because you are looking at reflected light from a dimmed Moon rather than straight at the Sun. The free Stellarium application will tell you when the next eclipse is visible from your location.

Where This Work Is Done in India

The Kodaikanal Solar Observatory, in the Palani hills in southern India, was established in 1899 and holds more than a hundred years of solar data. It is run by the Indian Institute of Astrophysics in Bengaluru. M. K. Vainu Bappu is often called the father of modern Indian astronomy. He set up telescopes at Manora Peak near Nainital and at Kavalur in Tamil Nadu, where the observatory is named after him. He studied stars, discovered a comet, and travelled to observe solar eclipses.

Scenario:

A partial solar eclipse is due at four in the afternoon, and four friends make their plans. Ravikiran borrows a pair of solar goggles from his school science club and reads the instructions on the packet. Jyothi sets up a mirror on a stand in the shade with her teacher, and projects the image of the Sun on a wall. Adithya says all that is fuss, steps into the yard and looks straight up at the Sun for a few seconds, reasoning that it is dimmer than usual today. Aruna goes to a programme at the local planetarium. Afterwards Adithya says nothing happened to him, so the warnings must be exaggerated.

Analysis:

Three of the four were careful and Adithya was not, and nothing about his experience makes the warning exaggerated. Take the rule first, because it is the most important thing in this topic. Directly viewing a solar eclipse must be strictly avoided. Even during the eclipse the Sun can damage the eyes and cause blindness. Damage to the eye can happen without pain at the time, so feeling fine afterwards is not evidence of safety. The chapter also names what does NOT count as protection. Sunglasses, binoculars and telescopes must not be used. The last two are worse than nothing, because they gather light and concentrate it. Now the three who did well. Ravikiran used specialised eye protection of the kind supplied for eclipse viewing. Jyothi never looked at the Sun at all; she looked at a projected image, with her teacher present. Aruna chose the course the chapter recommends most warmly, a viewing event organised by a planetarium or astronomy club. Such events provide specialised eye protection and explain the event as it happens. One more thing is worth saying to Adithya. A partial eclipse feels safe because it looks dim. That is exactly the trap. The Sun is no less damaging for being partly covered.

Activity:

Find out whether a planetarium, science centre or astronomy club near you runs eclipse viewing events, and write down what they say they provide. If there is none nearby, write the two safety sentences from this topic in your own words and pin them up at home.

Title:

Try it yourself: how a thumb hides a head

Safety:

Choose level ground, and keep away from traffic and open drains.

What You Notice:

At five metres the thumb appears to cover the whole head; at two metres it does not. Nothing about either object changed, so apparent size must depend on distance as well as actual size. That is why the Moon, far smaller than the Sun but far closer, can cover it in a solar eclipse.

A thumb at arm length can appear to cover a head five metres away.

When Venus crosses between the Sun and the Earth it is a tiny black dot.

The Kodaikanal Solar Observatory was established in 1899.

Moon in the middle: solar. Earth in the middle: lunar.

Never look at an eclipsed Sun. An eclipsed Moon is safe to watch.