An eclipse is a shadow that finds you

It is the middle of the day. The sky is bright, the crows are noisy, the shadows under the neem tree are sharp and black.

Then, slowly, something starts to go wrong.

The light turns thin and strange, like an old photograph. The air cools. The shadows under the tree stop being blurry blobs and become hundreds of tiny crescents scattered on the ground. The crows go quiet and fly to their branches. A cow lies down. Somebody's street light, confused, flickers on.

And if you are standing in exactly the right place on Earth, the last sliver of the Sun vanishes, a ring of pearly light appears where the Sun used to be, and for two or three minutes it is night in the middle of the morning.

That is a total solar eclipse. And here is the surprising thing about it: nothing has exploded, nothing has been switched off, and nothing has gone wrong at all. Something very ordinary has happened. A shadow has landed on you.

Stand outside on a sunny day and look at your own shadow. You have made it by doing something very simple: you got in the way.

Light from the Sun travels in straight lines. It cannot bend around you to fill in the space behind you, so that space stays dark. That dark space is your shadow, and it is not a thing lying on the ground — it is a long invisible tunnel of darkness stretching away from you, which only becomes visible where it hits something.

Now hold that thought, because it is the whole lesson:

• Every object lit by the Sun trails a shadow behind it. Your body. A tree. A building. A mountain.
• Earth has one. It is a cone of darkness about 1.38 million kilometres long, pointing away from the Sun.
• The Moon has one too. It is a much thinner cone, only about 374,000 kilometres long.

An eclipse happens when one of those two cones lands on the other body — and somebody is standing where it lands.

Hold your hand up in front of a small torch in a dark room and look at the shadow on the wall. Now do it again with a big frosted table lamp. The second shadow has a soft, fuzzy grey edge.

That fuzzy edge is telling you something important. A shadow has two parts:

• The umbra is the properly dark middle. Stand in the umbra and the light source is completely hidden.
• The penumbra is the grey fringe around it. Stand in the penumbra and you can still see part of the light source peeping past the edge of the blocker.

The words come from Latin: umbra means "shadow" and paene means "almost", so penumbra is the "almost-shadow".

Both kinds of eclipse come in two flavours for exactly this reason. Stand in the Moon's umbra and you get totality. Stand in its penumbra and you only get a partial eclipse: a bite taken out of the Sun.

Once a month, at new moon, the Moon passes between Earth and the Sun. The side of the Moon facing us gets no sunlight, which is why a new moon is invisible from Earth.

Most months the Moon slips past a little above or below the Sun, and nothing happens. But occasionally it passes exactly in front, and then its shadow cone reaches all the way down and touches Earth.

If you are standing where the umbra touches, the Moon covers the Sun completely: a total solar eclipse. If you are standing where the penumbra sweeps over, part of the Sun is still showing: a partial solar eclipse.

The umbra's footprint is tiny — usually a spot around 160 kilometres across, less than the distance from Delhi to Agra. The penumbra's footprint is thousands of kilometres wide. That is why partial eclipses are common and totality is so rare: the dark spot is small, Earth is enormous, and the spot races across the surface in a few hours.

Here are the three safe ways. All of them share one rule: your back is to the Sun and you are looking at a piece of paper.

1. The pinhole card. Take two pieces of stiff card. Make one clean pinhole in the first with a pin (not a big ragged hole — a small neat one). Stand with the Sun behind you, hold the pinhole card up, and let the light fall on the second card held about a metre away. A little disc of light appears: that is an image of the Sun, about 9 mm across at one metre. During a partial eclipse it will be a crescent. Move the cards further apart and the image grows: at 5 metres it is about 47 mm across, and dimmer.

2. The kitchen colander. Hold a colander, a slotted spoon or even a piece of card with twenty pinholes in it, and let the sunlight through onto the ground. Every hole makes its own image of the Sun. During an eclipse you get a spray of dozens of crescents. This is easily the best crowd-pleaser, and it is free.

3. The mirror. Cover a small flat mirror with paper that has a hole about 5 mm across cut in it. Stand in a doorway or a shaded veranda, catch the sunlight on the mirror and bounce it onto a wall in the shade, at least 5 metres away. A surprisingly big image of the Sun appears on the wall. Never point the mirror at anyone's face, and never look into the mirror yourself.

And the one that needs no equipment at all: look under a leafy tree. The gaps between overlapping leaves are already pinholes. During a partial eclipse the ground under a neem or a peepal tree is covered in crescents. This one always makes people gasp.

Now turn the whole picture around.

Two weeks after new moon comes full moon, when the Moon is on the opposite side of Earth from the Sun. And behind Earth, pointing straight away from the Sun, is Earth's own shadow cone — 1.38 million kilometres of darkness, more than three and a half times further out than the Moon.

Most full moons, the Moon sails past a little above or below that cone. But occasionally it goes straight in. The Moon does not disappear; it turns a deep, dusty copper red, and stays that way for up to about an hour and three quarters.

That is a total lunar eclipse, and it is much easier to see than a solar one for two reasons:

1. Everybody on the night side can see it. The Moon itself is what changes, so anyone who can see the Moon at all sees the eclipse — roughly half the planet at once.
2. It is completely safe to look at. The eclipsed Moon is around ten thousand times fainter than a full moon, which is itself perfectly harmless. Use your eyes, binoculars, a telescope, a camera — anything you like.

People muddle these two constantly, and photographs in newspapers get captioned wrongly all the time. Here is how to never get it wrong again.

Ask yourself one question: what is the dark thing sitting in front of?

In a solar eclipse the dark disc is in front of a brilliant white Sun in a daytime sky. Everything around it is bright. It only lasts minutes. And everyone who can see it is in the same narrow strip of the world.

In a lunar eclipse the Moon is hanging in a night sky full of stars. There is no glare. It goes red rather than black. It takes hours. And half the world can watch it together.

There is a third giveaway that never fails: the shape of the edge.

Here is a puzzle that should be bothering you.

The Moon goes round Earth once a month. So once a month it passes between us and the Sun (new moon), and once a month it passes behind us (full moon). If everything lay flat in one plane, we would get a solar eclipse and a lunar eclipse every single month, like clockwork.

We do not. We get somewhere between four and seven eclipses of all kinds in a whole year, and most of those are partial or penumbral. Why?

Because the Moon's path around Earth is tilted. It is tipped by about 5.1 degrees compared with the path Earth takes around the Sun. So most months the Moon passes above the Sun, or below it, and its shadow misses Earth entirely — flying off into empty space.

A tilted circle crossing a flat one must cut through it in exactly two places. Picture a hula hoop pushed through a tabletop at a slant: it crosses the table's surface at two points on opposite sides.

Those two crossing points are called the nodes of the Moon's orbit, and they are where all eclipses live.

• If the Moon happens to be at a node and it is new moon → solar eclipse.
• If the Moon happens to be at a node and it is full moon → lunar eclipse.
• If it is a node but the wrong phase, or the right phase but nowhere near a node → nothing at all.

Both things have to be true at once, and that is why eclipses come in bunches a few times a year rather than every month. These bunches are called eclipse seasons, and there are two of them a year, roughly six months apart.

Here is a lovely detail for anyone who grew up hearing about Rahu and Ketu: in classical Indian astronomy those two names are given to exactly these two points, the north and south nodes of the Moon's orbit. The old stories say Rahu and Ketu swallow the Sun and Moon. The geometry says eclipses happen at the nodes. Astronomers in India worked out centuries ago that these were two descriptions of the same two places in the sky.

Go outside and look at the full Moon. Now look at the Sun — no, don't. But you know how big it looks: about the same.

That is astonishing, and there is no reason for it.

The Sun is about 400 times wider than the Moon. And it happens to be about 400 times further away. Those two 400s cancel out almost exactly, so the two discs look almost exactly the same size from Earth — both about half a degree across.

Nothing in physics required this. The Moon is not the right size because of anything. It is a coincidence, and it is the reason total solar eclipses exist at all.

• If the Moon looked a bit bigger, it would blot out the Sun easily and we would never see the delicate corona around the edge.
• If it looked a bit smaller, it could never cover the Sun completely and there would be no totality at all — just a bright ring, every time.

As it is, the Moon is just big enough, just. It covers the Sun's blinding disc and no more, leaving the faint outer atmosphere exposed for a few minutes.

People who have stood in the umbra almost all describe it the same way: not as a picture they saw, but as something that happened to them.

Here is what to expect, in the last minute before and the first minute of totality.

The light goes wrong. In the last ten minutes the daylight turns the colour of old steel. Colours drain out of clothes and walls. Your own shadow gets strangely sharp on one side, because the Sun has stopped being a disc and become a thin line.

Shadow bands. Sometimes, in the last minute, faint rippling stripes of light and dark run across pale ground or a white sheet, like sunlight on the bottom of a swimming pool. They are caused by our own turbulent air.

Baily's beads. As the last sliver closes, the Moon's edge is not smooth — it has mountains and valleys. Sunlight shines through the valleys and is blocked by the peaks, so the sliver breaks into a line of glittering dots. These are Baily's beads.

The diamond ring. The beads wink out one by one until a single one is left: a blazing white point on a dark ring of corona. It lasts a second or two and it is the picture everyone has seen.

Totality. The point vanishes. In its place: a black hole in the sky, ringed by the silver-white corona, streaming out in feathery petals. Often a few red flecks, called prominences, glow at the rim — those are fountains of gas bigger than Earth. Venus and Jupiter appear. The horizon glows orange in every direction at once, because you are standing in a shadow only 160 km wide and it is still sunny everywhere beyond it. That 360° sunset is the detail that people say they were not ready for.

For most of human history, an eclipse was terrifying. The Sun went out in the middle of the day for no reason anyone could name, and then came back. Every culture on Earth built a story around it: a dragon in China, a jaguar in the Amazon, a wolf in Norse tales.

In India, the story is about Rahu. In the old account, a demon sneaked a sip of the nectar of immortality; the Sun and Moon reported him; Vishnu beheaded him with a discus — but too late, because he had already drunk, so his severed head lived on as Rahu and his body as Ketu. Rahu chases the Sun and the Moon forever in revenge, and when he catches one, he swallows it. Since he has no body, it soon falls out again.

That is a good story, and it belongs to a lot of families. Many people in India today still fast during an eclipse, avoid cooking and eating, take a bath afterwards, and ask pregnant women to stay indoors.

If that happens in your family, the right response is not mockery. It is curiosity — and then evidence.

You do not need equipment, a trip or money to see an eclipse. You need to know when, and you need a piece of card.

Find out when. Eclipse dates and paths are published years in advance and are free to look up. Search for the year and "eclipse visible from India", or use NASA's eclipse pages or timeanddate.com. Note two things for your town: the date and local time, and the maximum percentage of the Sun that will be covered where you are.

Three that are already on the calendar: 31 December 2028, a total lunar eclipse fully visible across India; 21 May 2031, an annular "ring of fire" whose path crosses Kerala, northern Sri Lanka and the Andaman and Nicobar Islands; and 20 March 2034, a total solar eclipse whose path of totality crosses northern India, including Kashmir. If you are nine today, you will be about seventeen in 2034.

For a lunar eclipse. Nothing to prepare at all. Find a spot with a clear view of the sky in the right direction, go out about half an hour before it starts, and let your eyes adjust for ten minutes. Look for the curved edge of Earth's shadow creeping across, then the colour change. Binoculars make it better. A phone camera on night mode will usually capture the red.

For a solar eclipse. Prepare the day before: make a pinhole card and a colander projection, and borrow or buy ISO 12312-2 eclipse glasses from a planetarium or science centre. Check the glasses for scratches and pinholes before use, and throw them away if damaged. Then set up in an open spot with your back to the Sun, and watch the crescents appear on the paper.

Take other people with you. Half the fun of an eclipse is the moment someone who was humouring you suddenly goes quiet.