Deep dive · Science

Eclipses: shadows, science and stories

When the day suddenly turns dark, or the full Moon turns copper-red, it is natural to feel afraid. So people everywhere told stories about eclipses, and slowly they also learned to work them out. An eclipse is a game of shadows that plays out when the Sun, the Earth and the Moon line up. Here you can move the Moon yourself to see when an eclipse is total and when it is partial, read stories from around the world, and find out when India's next eclipses come and how to watch them safely.

After this you can explain

which way the Sun and the Moon rise, and why the Moon rises later each day;
the two parts of the Moon's shadow, and why one eclipse is total and another only partial;
what changes in the sky, step by step, during a solar eclipse;
why the Moon looks red in a total lunar eclipse;
why there is no solar eclipse at every new moon, and no lunar eclipse at every full moon;
the story of Rahu, and how Aryabhata explained eclipses;
how to watch an eclipse safely, and when India's next eclipses are.

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1Rising and setting: which way to look

Before eclipses, it helps to know how the Sun and the Moon seem to move across the sky. The Earth spins from west to east, so the Sun, the Moon and the stars all appear to rise in the east and set in the west. But the Sun does not rise due east every day. It rises due east only around 21 March and 23 September, the equinoxes. In June the point where it rises shifts north of east, and in December south of east. The point where it sets shifts north or south of west in the same way.

The Moon also rises in the east and sets in the west, but not at the same time each day. Moving along its orbit, the Moon gets about 12° farther east of the Sun every day. To bring it back to the horizon the Earth has to turn about 12° more each day, and turning 1° takes 4 minutes. That is why the Moon rises on average about 50 minutes later each day. The delay is not the same every day: depending on the season and how steeply the Moon's path meets the horizon, it can be anything from about half an hour to more than an hour.

This distance from the Sun also decides how much of the Moon's bright side we see, its phase. From new moon to full moon the Moon grows (waxes); from full moon back to new moon it shrinks (wanes). In India the two halves of the month are called shukla paksha and krishna paksha.

East to west: one day's sky

Imagine you stand in Jaipur facing south. East is then on your left and west on your right. Drag the time slider, or press Play: the Sun and the Moon rise on the left and set on the right. Pick a date above and watch where the Sun rises. The second slider changes the Moon's age, the days since new moon. The faint circles show where the Moon will be at this same time on the next few days: a little farther east each day.

Date

Time

Days after new moon

Sunrise
Sunset
Moonrise
Moonset
Moon's phase
Next day

Times are local solar time, in which the Sun stands due south at 12 noon. In Jaipur, clocks on Indian Standard Time run about half an hour ahead of it, and the gap changes a little through the year. The picture leaves out the 5° tilt of the Moon's orbit and the Sun's motion over the month, so all times are approximate.

Phase and rising time

At new moon the Moon rises and sets with the Sun, with its dark side towards us, so we do not see it. The half Moon of first quarter rises around noon and shines high in the south at dusk. The full Moon rises in the east around sunset and stays up all night. The half Moon of last quarter rises around midnight and is seen until morning. These are average times; they change a little with place and season.

Why eclipses keep their hours

A solar eclipse can happen only when the Moon passes right in front of the Sun, at new moon, and then the Moon is in the daytime sky with the Sun. So a solar eclipse is always seen by day. A lunar eclipse happens at full moon, when the Moon is opposite the Sun: as the Sun sets, the Moon rises. So a lunar eclipse is seen at night, and everyone who has night and the Moon above the horizon sees it at the same time.

The western edge first

The Sun and the Moon both cross the sky from east to west every day, but the Moon falls a little behind, because it is moving east along its orbit. So in a solar eclipse the Moon comes in from the west and first covers the Sun's western edge. Facing south at noon in India, west is on your right; that is why, in the solar eclipse picture further down, the Moon comes in from the right. In a lunar eclipse too the Moon, moving east, enters the Earth's shadow with its eastern edge first.

Checkpoint

1In June, where does the Sun rise as seen from Jaipur?
2Roughly when does the full Moon rise?
3Why does the Moon rise about 50 minutes later each day, on average?

2Light and shadow: how an eclipse happens

Hold your hand in front of a torch and its shadow falls on the wall. The same thing happens in the sky. The Sun shines with its own light, while the Earth and the Moon shine by reflecting it, and each of them always casts a long shadow behind it.

When the Moon comes exactly between the Sun and the Earth, its shadow falls on the Earth, and the people in that shadow see the Sun covered. That is a solar eclipse. When the Earth comes between the Sun and the Moon, the Earth's shadow falls on the Moon. That is a lunar eclipse.

Two parts of the shadow

Behind the Moon there is a dark core, the umbra, from where the Sun looks completely covered. Around it lies a paler shadow, the penumbra, from where only part of the Sun is covered. The umbra narrows as it goes and ends in a point about 374,000 km behind the Moon. The penumbra keeps spreading.

A happy coincidence

The Sun is about 400 times wider than the Moon, but also about 400 times farther away. So in the sky the two look almost the same size, and the Moon can just cover the Sun. The Moon is drifting away by about 3.8 cm a year. In hundreds of millions of years it will no longer cover the whole Sun, and total solar eclipses will stop.

Total, annular, partial

The Moon's distance keeps changing, and so does the Sun's: in January the Sun looks about 3.4 per cent bigger than in July. If the tip of the umbra reaches the ground, people there see a total eclipse. If the Moon is far away and the tip ends before the ground, the Moon looks smaller than the Sun and a ring of light is left around it: an annular eclipse. People in the penumbra see a partial eclipse. Now and then one eclipse is total in some places and annular in others; that is a hybrid eclipse.

A small umbra, a huge penumbra

On the ground the umbra's track is at most a few hundred kilometres wide, while the penumbra covers thousands. So many people see a partial solar eclipse, but a total one only from a narrow strip. The Moon's motion and the Earth's spin sweep the track along fast, so totality lasts only minutes at any one place.

Checkpoint

1During a solar eclipse, in what order are the three?
2If the Moon is at its farthest and lines up exactly, what kind of solar eclipse is seen?
3How can the small Moon cover the huge Sun?

3Solar eclipses

A solar eclipse always comes at new moon, because only then is the Moon on the Sun's side of the sky. The Moon touches one edge of the Sun and slowly slides across it. The partial part can last more than an hour; totality lasts only minutes.

The four contacts

First contact is the moment the Moon first touches the Sun's edge. Second contact is the moment the Sun is fully covered (in an annular eclipse, the moment the ring forms), and third contact is when light returns. Fourth contact comes when the Moon leaves the Sun completely, and the eclipse is over. In a partial eclipse the Sun is never fully covered, so there are only the first and fourth contacts. Hindi almanacs call the start sparsh and the end moksha.

Baily's beads and the diamond ring

The Moon's edge is not smooth: it has mountains and valleys. Just before totality the last sunlight shines through those valleys as a string of bright beads, called Baily's beads after the English astronomer Francis Baily. When only one bead is left, it blazes beside the faint glow like a diamond on a ring.

The Sun's crown

The moment the Sun is covered, a milky white glow spreads around it. This is the corona, the Sun's outer atmosphere, normally lost in the Sun's glare. Pink-red prominences may show at the edge. The sky turns dark enough for bright stars and planets to appear, the air feels a little cooler, and in many places birds fall quiet as if evening had come.

How long?

At any one place totality never lasts more than about seven and a half minutes. Of all eclipses from 2000 BCE to 3000 CE, the longest total eclipse comes on 16 July 2186, at about 7 minutes 29 seconds. An annular eclipse can last longer, sometimes more than 12 minutes.

What eclipses gave science

1868: a new element in the Sun

The total solar eclipse of 18 August 1868 was seen from India, and astronomers from several countries came to watch it. That day the light of the Sun's prominences showed a yellow line that matched no known element. The Frenchman Jules Janssen studied the prominences from Guntur. Later the Englishman Norman Lockyer took the yellow line as the mark of a new element and named it helium, after Helios, the Greek name for the Sun. Helium was found on Earth in 1895.

1919: light that bends

During the total eclipse of 29 May 1919, with the sky dark, astronomers photographed the stars near the Sun. They had shifted slightly from their usual places, by about as much as Einstein's theory had predicted: the Sun's gravity bends light passing close to it.

Checkpoint

1On what day can a solar eclipse happen?
2When can you see the Sun's corona?
3How many contacts does a partial solar eclipse have?

4Lunar eclipses

A lunar eclipse always comes at full moon, when the Earth is between the Sun and the Moon. The Earth is more than three and a half times wider than the Moon, so its shadow is big too, and the Moon stays in it for hours. Everyone on the night side of the Earth who has the Moon in the sky sees the eclipse at the same time. And it is completely safe to watch: with bare eyes, binoculars or a camera, as you like.

Why does the Moon turn red?

Even in a total eclipse the Moon does not vanish. Some sunlight bends as it passes through the Earth's atmosphere and reaches into the shadow. On the way the air scatters the blue light and lets the red and orange through, just as it does when the Sun rises or sets. So at that moment the Moon is lit by all the sunrises and sunsets on Earth.

A different colour each time

How red or how dark the Moon looks depends on the Earth's air. With a lot of cloud and dust, or dust and droplets of sulphur spread high up by a big volcano, the Moon can look very dark, almost black. With clean air it stays a bright copper.

Penumbral eclipses

Sometimes the Moon misses the umbra and only passes through the penumbra. It then dims a little, so little that you notice it only if you look carefully. Eclipse lists count these too, so when a calendar shows an eclipse, check what kind it is.

How long?

The Moon moves through the shadow at about 3,400 km an hour. At the Moon's distance the Earth's umbra is about 9,200 km wide, more than two and a half Moons. So totality can last more than an hour. On 31 December 2028 India will see about 71 minutes of totality, and the whole eclipse, from touching the penumbra to leaving it, will run for more than five and a half hours.

Checkpoint

1On what day does a lunar eclipse happen?
2Where does the red light on a fully eclipsed Moon come from?
3What do you need to watch a lunar eclipse?

5Why not every month?

The Moon goes round the Earth every month, so why is there not a solar eclipse at every new moon and a lunar eclipse at every full moon? Because the Moon's orbit is tilted by about 5° to the plane of the Earth's orbit. At most new and full moons the Moon passes a little above or a little below the Sun–Earth line, and its shadow falls on no one.

The two orbits cross at two points, called nodes. Where the Moon climbs from below to above is the ascending node; where it drops from above to below is the descending node. An eclipse can only happen when a new or full moon falls near a node.

Eclipse seasons

Twice a year the Sun passes close to a node. The 31 to 37 days around that moment, about 34½ on average, are an eclipse season. A month from new moon to new moon is shorter than that, so every season has at least one solar eclipse, and often a lunar eclipse two weeks before or after. That is why every year has at least two solar eclipses, and at most five.

The nodes move too

The nodes do not stay put. They slide backwards by about 19° a year and go all the way round in about 18.6 years. So the two seasons are not half a year apart but about 173 days, and they come about 19 days earlier each year. Press Play above to watch this year by year.

The Saros: an 18-year cycle

Babylonian astronomers knew that eclipses come back after about 18 years. In that time the Moon goes 223 times from new moon to new moon, returns to the same node 242 times, and comes closest to the Earth 239 times. All three counts finish at almost the same moment, after about 18 years 11 days 8 hours. Sun, Moon and node then line up almost as before, and a very similar eclipse follows. In those extra 8 hours the Earth turns a third of the way round, so the new eclipse falls about 120° farther west. After three Saros, about 54 years and a month, an eclipse returns to roughly the same region.

Checkpoint

1Why is there no solar eclipse at every new moon?
2At least how many solar eclipses happen each year?
3How long is the Saros cycle?

6Stories from around the world

Before anyone could work eclipses out, people understood them through stories. In some, a demon swallows the Sun; in others a dragon, a frog or a wolf. In some, the Sun and the Moon quarrel. People beat drums to drive the swallower away, or settled their own quarrels. Interestingly, many of the same cultures later learned to predict eclipses too.

1 · India

Rahu and a sip of amrita

The Adi Parva of the Mahabharata tells how the gods and the asuras churned the ocean together and amrita, the nectar of immortality, rose from it. While the gods were drinking it, a danava named Rahu sat among them disguised as a god and began to drink. The amrita had reached only his throat when Surya and Soma, the Sun and the Moon, recognised him and told. Narayana cut off his head with the discus. Ever since, there has been a long quarrel between Rahu's head and the Sun and the Moon, and to this day it swallows them.

The Bhagavata Purana tells the same story; there Vishnu takes the form of Mohini to serve the amrita. The Bhagavata says Rahu attacks the Moon and the Sun on full-moon and new-moon days. Later tradition called the severed head Rahu and the body Ketu, and counted both as shadow planets.

What science says

The story holds a truth: eclipses really do happen only at full moon and new moon. Indian astronomers later linked Rahu and Ketu to the two nodes of the Moon's orbit, exactly the points near which eclipses happen.

2 · Arabia (Madinah)

Not for anyone's death

On the day Ibrahim, the son of the Prophet Muhammad, died, there was a solar eclipse, and people said it was a sign of mourning. According to a hadith in Sahih al-Bukhari, the Prophet said that the Sun and the Moon are not eclipsed for anyone's death or life; when you see an eclipse, pray and call upon God.

What science says

An eclipse has nothing to do with anyone's life or death. It comes from the motions of the Sun, the Moon and the Earth, and it can be worked out hundreds of years ahead.

3 · Babylon (today's Iraq)

A king for a few days

In ancient Mesopotamia some eclipses were read as bad omens for the king. To turn the danger aside, a substitute king was put on the throne for a while, and the real king lived in hiding. When the danger had passed, the substitute was often put to death, so that the bad omen ended with him.

What science says

This is one reason the Babylonians watched the sky closely and kept writing eclipses down. From centuries of such records they came to see how eclipses repeat, and began to predict them.

4 · Greece and Lydia (today's Türkiye)

The eclipse that stopped a war

The Greek historian Herodotus writes that the Lydians and the Medes had been at war for five years. In the sixth year, in the middle of a battle, day suddenly turned into night. Both armies took fright, stopped fighting and made peace. According to Herodotus, Thales of Miletus had foretold this eclipse.

What science says

The eclipse is usually dated to 28 May 585 BCE. Many historians doubt that Thales really predicted it.

5 · Scandinavia

A wolf behind the Sun

In the Norse myths of northern Europe a wolf called Sköll chases the Sun across the sky, and another called Hati chases the Moon. That is why the Sun and the Moon never stop but keep on running. The old poems say the wolves will catch them when the world ends, and they say nothing about eclipses. Many modern retellings link eclipses to the chase: the wolf has caught up with its prey.

What science says

The Sun and the Moon really never stop. The Earth's spin makes both cross the sky from east to west each day, and among the stars the Moon slides east by about its own width every hour.

6 · China

A dragon eating the Sun

In old China people believed a heavenly dragon was eating the Sun. They beat drums, gongs and pots to make a din, so that the dragon would take fright and let the Sun go. And every time, the Sun did come back.

What science says

At the Chinese court an eclipse was a serious matter, so its astronomers kept records of eclipses for centuries. Such old records still help scientists today, for example in measuring how much the Earth's spin has slowed.

7 · Vietnam

A frog swallowing the Sun

In a Vietnamese folk tale a giant frog swallows the Sun. Only its master can make the frog give the Sun back, and then the day returns.

What science says

The Moon's dark disc eats into the Sun from one side, so a partial eclipse really does look as if someone has taken a bite out of the Sun.

8 · Togo and Benin (West Africa)

End the quarrel, bring back the Sun

In a story of the Batammaliba people of northern Togo and Benin, an eclipse happens when the Sun and the Moon fight. To calm them, people settle their own old quarrels and make up with each other.

What science says

The Sun and the Moon do not fight; they just line up. But the lesson still works: an eclipse gets a whole village or city looking up at the sky together.

9 · Greenland (Inuit)

The Moon chasing the Sun

In an Inuit story the Sun goddess Malina and the Moon god Anningan quarrelled. Malina smeared soot on Anningan's face and ran away. Ever since, Anningan has chased her across the sky. Once in a while he catches her, and that is a solar eclipse.

What science says

The Moon moves across the sky faster than the Sun and passes it every month. But its orbit is tilted, so it only covers the Sun once in a while.

What India's astronomers said

Aryabhata (499 CE)

In the Aryabhatiya, Aryabhata wrote that in a solar eclipse the Moon covers the Sun, and in a lunar eclipse the Earth's great shadow covers the Moon. He needed no Rahu to explain an eclipse.

Varahamihira (6th century)

In the Brihat Samhita, Varahamihira questions the Rahu story directly: if Rahu has a body, or is only a head moving on a fixed path, how can he eclipse the Sun and the Moon when they are 180° away from him? He then says plainly that in a lunar eclipse the Moon enters the Earth's shadow, and in a solar eclipse the Moon, coming from the west, covers the Sun from below like a cloud.

Brahmagupta (628 CE)

Brahmagupta could calculate the times of eclipses, yet he criticised Aryabhata for not accepting that Rahu causes them. Science and tradition were argued over openly in that age.

Eclipses and our customs

Many families in India keep sutak during an eclipse: they do not cook or eat, many temples close their doors, and afterwards people bathe and give alms. These are matters of faith and tradition, and every family keeps them in its own way. In science, nothing in an eclipse's light is poisonous. There is only one real danger, looking straight at the Sun, and the next section shows how to avoid it.

Checkpoint

1What did Aryabhata give as the cause of a lunar eclipse?
2Which part of the Rahu story agrees with science?

7Watch safely: the next eclipses in India

Looking straight at the Sun is always dangerous, eclipse day included. Even in a partial eclipse the uncovered part of the Sun can burn the retina at the back of the eye. It does not hurt, so you do not notice, and the damage can be permanent. A lunar eclipse is different: watch it without a worry.

Make a pinhole at home

Make a small, round hole in a piece of card or thick paper with a needle or the tip of a pen. Stand with your back to the Sun and hold the card over your shoulder so that light through the hole falls on a sheet of white paper half a metre or more away. A small, upside-down image of the Sun appears on the paper; the farther the paper, the bigger the image. During an eclipse you will clearly see the bite taken out of it.

A sieve, your criss-crossed fingers, or the gaps between leaves do the same job. During an eclipse the ground under a tree can be dotted with hundreds of little bitten Suns.

The next eclipses in India, up to 2034

This list has only the eclipses that can be seen from somewhere in India. Times are Indian Standard Time and approximate; in your town they may differ by a few minutes.

Solar eclipses
Date	What India sees	Where and when
2 August 2027	Partial	Over nearly all of India, in the afternoon. The south sees more of the Sun covered and the north less; Delhi only a little, from about 15:54 to 17:03. The track of totality crosses Spain, North Africa and Arabia, not India.
1 June 2030	Partial	North and north-east India, late morning; Delhi from about 10:18 to 12:53, a small part of the Sun. Not seen from Mumbai, Chennai or Bengaluru.
21 May 2031	Annular, in south India	Along a strip of Kerala and Tamil Nadu, including Kochi, Madurai and Rameswaram: a ring of about 5 minutes between about 13:00 and 13:16. Thiruvananthapuram and Kanyakumari lie outside the strip. Partial in the rest of the country.
3 November 2032	Partial	A little of the Sun covered in north and north-east India, in the morning; Delhi from about 9:36 to 10:52.
20 March 2034	Total, in Jammu and Kashmir and Ladakh	Around Srinagar and Leh, about 2 minutes of totality between about 17:22 and 17:25, with the Sun low over the horizon. Partial in the evening across the rest of the country; in Delhi most of the Sun will be covered.

Lunar eclipses
Date	What India sees	Where and when
21 February 2027	Penumbral	Before dawn, about 2:42 to 6:43; the Moon will only dim slightly. In the east the Moon sets before the end.
6–7 July 2028	Partial	Across the country; the umbra covers part of the Moon from about 22:38 to 1:00.
31 December 2028	Total	Across the country, from start to finish. Totality from about 21:46 to 22:57: a red Moon on the last night of the year.
21 December 2029	Total	Before dawn; totality from about 3:45 to 4:38. In the north-east moonset cuts off the end.
15–16 June 2030	Partial	Across the country; around midnight (about 0:03) roughly half the Moon is in the umbra.
10 December 2030	Penumbral	Across the country, from about 1:37 to 6:17; in the middle one edge of the Moon will look clearly dimmed.
25 April 2032	Total	Across the country. When the Moon rises only the faint penumbral phase is under way; the umbra begins at about 18:58, around moonrise. Totality from about 20:10 to 21:16.
18–19 October 2032	Total	Across the country, from start to finish; totality after midnight, from about 0:08 to 0:55.
14–15 April 2033	Total	Across the country, from start to finish; totality from about 0:17 to 1:07.
8 October 2033	Total	The Moon rises during the eclipse. Totality lasts from about 15:46 to 17:04, so only the far north-east (Arunachal, Nagaland, Manipur, upper Assam) sees its last minutes as the Moon rises. Where the Moon rises before about 18:06, the last partial phase is seen; in the west, as in Mumbai, the Moon rises later and only the faint penumbra is left.
3–4 April 2034	Penumbral	Across the country, from about 22:22 to 2:48.

The last total solar eclipse seen in India was on 22 July 2009, and the last annular one on 21 June 2020, when its track passed over Suratgarh in Rajasthan.

Checkpoint

1Which is a safe way to look at an eclipsed Sun?
2When and where will India see its next total solar eclipse?

8Check yourself

Checkpoint

1Why is the track of a total solar eclipse so narrow?
2At the Moon's distance, about how wide is the Earth's umbra?
3With which points in the sky did Indian astronomers link Rahu and Ketu?
4Compared with the year before, when does an eclipse season come?
5When are Baily's beads seen?
6What will a strip of south India see on 21 May 2031?

Glossary

Phase: How much of the Moon's bright side we see: a crescent, a half, a gibbous or a full disc. It depends on how far the Moon is from the Sun in the sky.
Waxing and waning: From new moon to full moon, about 15 days, the Moon grows each day (waxes; in India, shukla paksha). From full moon to new moon it shrinks (wanes; krishna paksha).
Solar eclipse: When the Moon comes between the Sun and the Earth and covers all or part of the Sun. Always at new moon.
Lunar eclipse: When the Earth comes between the Sun and the Moon and its shadow falls on the Moon. Always at full moon.
Umbra: The dark core of a shadow, from where the light source looks completely covered.
Penumbra: The paler shadow around the umbra, from where the light source looks only partly covered.
Total and partial eclipse: In a total eclipse the Sun or Moon is covered completely; in a partial one, only in part.
Annular eclipse: When the Moon is far enough to look smaller than the Sun, leaving the Sun's bright edge as a ring.
Hybrid eclipse: A solar eclipse that is total from some places along its track and annular from others.
Penumbral lunar eclipse: When the Moon passes only through the Earth's penumbra and dims slightly.
Contacts: The four moments of an eclipse: first contact (start), second and third (totality or the ring begins and ends), fourth (end).
Apparent size: How big something looks in the sky. A big thing far away and a small thing nearby can look the same size, like the Sun and the Moon.
Corona: The Sun's outer atmosphere, seen only during a total solar eclipse.
Baily's beads, diamond ring: The last sunlight shining through valleys on the Moon's edge just before and just after totality.
Nodes (ascending and descending): The two points where the Moon's tilted orbit crosses the plane of the Earth's orbit. At the ascending node the Moon climbs from below to above; at the descending node it drops.
Eclipse season: A stretch of 31 to 37 days, twice a year, when the Sun is near a node and eclipses can happen.
Saros: A cycle of about 18 years 11 days 8 hours, after which a very similar eclipse comes back.
Transit: When a small body such as Mercury or Venus crosses in front of the Sun as a small black dot.