Early humans and ancient knowledge of the sky
A village elder looks at the evening sky and says, "Rain will come soon." Farmers have watched the sky for thousands of years to plan their work, long before clocks or calendars.
Observation (निरीक्षण) means watching carefully and noting what happens. Early humans watched stars and used the observations to predict (अंदाज करणे) seasons and find directions.
Rule 1: Early humans and the sky — About 15–20 thousand years ago humans lived in caves, yet they watched the stars. Changing positions of star groups told them about the monsoon and winter, and the stars showed directions at night.
- Easy : Star positions told the season, like a calendar in the sky.
- Medium : Travellers at night found directions from the stars.
- Exam-type : People drew cave paintings of events like comets; many are found around the world.
Rule 2: Ancient Indian scientists — About 1500 years ago people believed the earth was still. Aryabhatta first explained that the earth goes around the sun. Bhaskaracharya II headed the Ujjain observatory and studied eclipses and phases of the Moon.
- Easy : Aryabhatta: the earth goes around the sun.
- Medium : Bhaskaracharya II: studied eclipses and the Moon's phases at Ujjain.
- Exam-type : They had no telescopes; they used only observation and mathematics, and results match modern values.
No telescope was needed: careful observing, predicting and calculating gave early knowledge of planets and eclipses.
Thinking early humans knew nothing about the sky. They knew a lot by observing.
Memory clue: Aryabhatta = Around the sun (the earth goes Around).
Spot the difference
| Person | Contribution |
|---|---|
| Early humans | Used stars for seasons and directions |
| Aryabhatta | Earth goes around the sun |
| Bhaskaracharya II | Studied eclipses and the Moon's phases |
Practice questions
- 1How did early humans find directions at night?
- 2What are cave paintings?
- 3Who first explained that the earth goes around the sun?
- 4Where did Bhaskaracharya II head the observatory?
- 5Cave life was about 15–20 thousand years ago and Aryabhatta's time about 1500 years ago. Which is older?
- 6Why could early people predict the monsoon without a clock or telescope?
- 7Why is it wrong to say ancient scientists needed telescopes?
- 8A student says people knew nothing about the sky before telescopes. Give two facts to correct this.
Show answers (try them yourself first)
- 1From the stars.
- 2Pictures drawn on cave walls, some showing events like comets.
- 3Aryabhatta
- 4Ujjain
- 5Cave life, since 15,000–20,000 is far more than 1500.
- 6By watching the changing positions of star groups.
- 7They used only observation and mathematics.
- 8They predicted seasons from stars, and Aryabhatta explained the earth goes around the sun.
Sky and space
On a hill the air feels thinner and climbing is tiring. The blue sky above is only the colour of sunlight scattered by air. Far above it, an astronaut sees a black sky even in the day.
The atmosphere (वातावरण) is the thick envelope of air around the earth. The sky (आकाश) is the space around the earth up to the end of the atmosphere. Beyond it begins space (अवकाश), which is empty and endless.
In the figure : In the figure a cloud (about 2 km) and an aeroplane (about 10 km) are in the sky. Air ends at about 100 km; above the dashed line is space, where a spacecraft in orbit at about 400 km moves. The figure is not to scale.
Rule 1: Air becomes rarer with height — Air is dense at sea level and rarer on high mountains. At about 100 km it is very rare; beyond, there is none.
- Easy : Mount Everest is about 8.8 km and aircraft fly at about 10 km, both inside the sky.
- Medium : A plane at 10 km still has 100 − 10 = 90 km of air-filled sky above it.
- Exam-type : A spacecraft at 400 km is 400 − 100 = 300 km above the end of air, so it is in space.
Rule 2: Why the sky looks blue — The atmosphere has gases, dust and water-vapour particles. They scatter blue colour of sunlight more, so the day sky appears blue.
- Easy : In the day the sky is blue because blue light is scattered more.
- Medium : Beyond the atmosphere there are no particles, so no scattering.
- Exam-type : So the sky looks black there, even in the day.
In the figure : In the figure, in A sunlight meets the particles of the atmosphere, which scatter blue colour more, so the sky looks blue. In B there are no particles beyond the atmosphere, so there is no scattering and the sky looks black.
Sky and space
In the app you can move this figure with your finger.
Open in the appSky ends where air ends (about 100 km). Space begins above and is endless.
Thinking the sun, stars and moon are in the sky. They lie in space, far beyond.
Memory clue: Sky = kites and clouds; Space = Sun, stars and spacecraft.
Spot the difference
| Point | Sky | Space |
|---|---|---|
| Air | Present | Absent |
| Examples | Cloud, kite, aeroplane | Sun, stars, Moon, spacecraft |
| Colour of the day view | Blue | Black |
Practice questions
- 1What is the atmosphere?
- 2At about what height does the air end?
- 3Name two objects in the sky.
- 4Name two objects in space.
- 5Name two spacecraft or missions sent into space.
- 6Why does the sky look blue in the day?
- 7How far above the end of the air is a spacecraft orbiting at 400 km?
- 8Why does an astronaut in space see a black sky even in the day?
Show answers (try them yourself first)
- 1The thick envelope of air around the earth.
- 2About 100 km
- 3Cloud and aeroplane (also kite, bird).
- 4Sun and stars (also Moon, comets).
- 5Chandrayaan and Mangalyaan (also Aditya L-1).
- 6Particles in the air scatter the blue colour of sunlight more.
- 7400 − 100 = 300 km
- 8There are no particles beyond the atmosphere to scatter light.
Telescopes, stars and constellations
On a clear night away from city lights you see hundreds of tiny shining dots. Most of them twinkle like the sun's small cousins. A few steady ones are planets that only borrow the sun's light.
A constellation (नक्षत्र / तारकासमूह) is a group of stars with a fixed figure. A telescope (दुर्बीण) shows far objects larger and clearer.
Rule 1: Telescopes — Telescopes were first used to observe the sky in 1609, about 400 years ago. They showed craters on the Moon, sunspots and four satellites of Jupiter. Later telescopes and instruments using X-rays, ultraviolet and infrared rays, and spacecraft, gave much new information.
- Easy : Early telescope: craters on the Moon.
- Medium : Early telescope: sunspots and four satellites of Jupiter.
- Exam-type : Modern instruments use X-rays, ultraviolet and infrared rays.
Rule 2: Planets and stars — Among the many night objects, only eight, the planets, revolve around the sun. The rest are stars, self-luminous like the sun.
- Easy : Sun is a star: it makes its own light.
- Medium : A planet goes around the sun; there are eight of them.
- Exam-type : A farmers' proverb links the Swati nakshatra with timely rain and a rich crop.
In the figure : In the figure the pole star can be seen from the northern hemisphere (A) but not from the southern hemisphere (B), because the earth is a huge sphere and comes in the way.
Rule 3: Constellations and hemispheres — The groups of stars change place from east to west but their figures stay the same. The part of the earth above the equator is the northern hemisphere and below is the southern. We cannot see the whole sky from one place.
- Easy : A constellation's shape does not change, only its position.
- Medium : The pole star is seen from the north but not from the south.
- Exam-type : Observe from a dark place on a cloudless night; look first at stars and planets in the west, which set soon after sunset.
In the figure : In the figure a constellation is a group of stars with a fixed figure. Between 8 pm and 11 pm it moves from east to west in the sky, but its figure stays the same.
Only 8 objects are planets; all the other shining dots are stars.
Calling every bright dot a star. Planets look similar but only borrow light.
Memory clue: Star = Self-light, Planet = Pulled around the sun.
Spot the difference
| Point | Star | Planet |
|---|---|---|
| Own light | Yes | No |
| Revolves around the sun | No | Yes |
| Example | Sun | Earth |
Practice questions
- 1When were telescopes first used for sky observation?
- 2How many planets revolve around the sun?
- 3What is a constellation?
- 4Name two things the early telescopes showed.
- 5Which part of the earth is the northern hemisphere?
- 6Why can we not see all constellations from one place?
- 7Why is the pole star not visible from the southern hemisphere?
- 8Asha says all points of light in the night sky are stars. Correct her.
Show answers (try them yourself first)
- 1In 1609
- 2Eight
- 3A group of stars with a fixed figure.
- 4Craters on the Moon and sunspots (or four satellites of Jupiter).
- 5The part above the equator.
- 6The earth is a huge sphere, so we see only a part of the sky.
- 7The earth comes in the way.
- 8Eight of them are planets, which revolve around the sun.
Comets, meteors and galaxies
A burning firecracker leaves a bright streak for a moment. A shooting star is like that in the sky: a rock burns brightly as it rushes through air. A bicycle's light looks tiny from far away, just like stars look tiny from the earth.
A comet (धूमकेतू) is a body of dust, frozen gases and ice. A meteor (उल्का) is the streak of light of a burning rock. A galaxy (दीर्घिका) is a huge cluster of billions of stars.
In the figure : In the figure a comet goes round the Sun in an elongated path. Near the Sun (A) the heat turns its frozen material into gas, which is thrown away from the Sun and forms a tail, so the tail points away from the Sun. Far from the Sun (B) it has no tail.
Rule 1: Comets — Comets orbit the sun in elongated paths. Near the sun the heat turns frozen material into gas, which forms a tail pointing away from the sun. Halley's comet takes 76 years for one round.
- Easy : The comet's tail always points away from the sun.
- Medium : Far from the sun the comet has no tail.
- Exam-type : A child who sees Halley's comet at age 10 can see it again at 10 + 76 = 86.
In the figure : In the figure a rock from space (A) rubs against the air and burns; the streak of light (B) is a meteor. A very big rock can reach the ground and form a crater (C), as at Lonar lake.
Rule 2: Meteors and craters — Rocks left by comets can be pulled by the earth's gravity. Rubbing against air makes them burn: a meteor (shooting star). A very big rock forms a crater; the salty Lonar lake in Buldhana district was formed this way.
- Easy : Rock + friction with air → burns → meteor.
- Medium : A very big rock reaches the ground and makes a crater.
- Exam-type : Lonar lake in Buldhana is such a crater.
In the figure : In the figure a galaxy is a huge cluster of billions of stars arranged in a spiral. The Sun is only one star in one arm of our galaxy, the Akashganga (Milky Way).
Rule 3: Galaxies — The smoky white band in the night sky is the Akashganga (Milky Way). The sun is one of its stars. Light travels 3 lakh km in a second but needs about 1 lakh years to cross it.
- Easy : Light goes 3 lakh km in 1 second, so in 10 seconds 30 lakh km.
- Medium : Sunlight takes 15 crore km ÷ 3 lakh km per second = 500 seconds, about 8 minutes, to reach the earth.
- Exam-type : Our solar system (sun, eight planets, their moons, asteroids, comets) is a tiny part of the Akashganga. The nearest galaxy, Andromeda (Devyani), is slightly bigger. All known galaxies together are the known universe.
Comet tail points away from the sun. Meteor = burning rock. Galaxy = billions of stars.
Thinking the tail trails behind the comet. It points away from the sun, whichever way the comet moves.
Memory clue: Tail Turns Away (TTA) from the sun.
Spot the difference
| Term | Meaning |
|---|---|
| Comet | Dust, frozen gas and ice; tail near the sun |
| Meteor | Streak of a burning rock |
| Galaxy | Billions of stars in a spiral |
Practice questions
- 1What is a comet made of?
- 2In which direction does a comet's tail point?
- 3How many years does Halley's comet take to go round the sun once?
- 4What is a meteor?
- 5How is Lonar lake formed?
- 6What is the Akashganga?
- 7How many seconds does sunlight take to reach the earth if the distance is 15 crore km and light goes 3 lakh km per second?
- 8Ravi says a comet's tail trails behind it like a kite's tail. Correct him.
Show answers (try them yourself first)
- 1Dust, frozen gases and ice.
- 2Away from the sun.
- 376 years
- 4The streak of light of a rock burning in the air.
- 5By a very big rock from space that hit the ground and made a crater.
- 6Our galaxy, a huge cluster of billions of stars; also called Milky Way.
- 715 crore ÷ 3 lakh = 150000000 ÷ 300000 = 500 seconds.
- 8The tail is made by the sun's heat and always points away from the sun, not behind the comet.
Space missions, scientists and blind beliefs
When a cricket team prepares for a match, it practises, collects data about the pitch, and sends the best players. In the same way, scientists plan, build and send a spacecraft to collect data from space. That is a space mission.
A space mission (अवकाश मोहीम) is sending a developed spacecraft into space to collect information. ISRO (इस्रो) is India's space agency, famous for such missions.
In the figure : In the figure the time line marks ISRO's Chandrayaan-1 in 2008, Mangalyaan in 2014 and Chandrayaan-3 in 2023, as given in this lesson.
Rule 1: ISRO missions — Chandrayaan-1 (2008) mapped minerals on the Moon and first discovered water there. Then came Mangalyaan (2014) and Chandrayaan-3 (2023).
- Easy : Years between Chandrayaan-1 and Mangalyaan: 2014 − 2008 = 6.
- Medium : Between Mangalyaan and Chandrayaan-3: 2023 − 2014 = 9 years.
- Exam-type : From Chandrayaan-1 to Chandrayaan-3: 2023 − 2008 = 15 years (6 + 9).
Rule 2: Aditya L1 and NISAR — Aditya L1 was launched on 2 September 2023 from Sriharikota to study the sun from about 15 lakh km. NISAR, built by NASA and ISRO and launched on 30 July 2025, circles the earth every 12 days.
- Easy : NISAR takes 12 days per round, so 60 days make 60 ÷ 12 = 5 rounds.
- Medium : It gives information on forests, climate change, earthquakes, melting glaciers and disasters.
- Exam-type : Light from the sun reaches Aditya L1 at 15 lakh km in 15 lakh ÷ 3 lakh = 5 seconds.
Rule 3: Scientists and blind beliefs — Dr Vainu Bappu built a spectrometer at home and discovered a comet in July 1949. Dr Jayant Narlikar set up IUCAA in Pune. Observatories: Jantar Mantar (Jaipur), GMRT near Khodad (Junnar) and Hanle (Ladakh), the highest in the world. Celestial events are natural; they do not affect human life.
- Easy : A comet's appearance is a natural event; it does not bring danger.
- Medium : A falling meteor does not bring good luck; believing so is a blind belief.
- Exam-type : Dr Bappu's story shows a home-made instrument and curiosity can lead to a discovery.
Space missions, not stars or comets, give us real knowledge. Stars and comets do not decide our luck.
Believing a comet or meteor brings bad or good luck. They are natural events with no effect on life.
Memory clue: 08 → Moon, 14 → Mars, 23 → Moon again (Chandrayaan-1, Mangalyaan, Chandrayaan-3).
Spot the difference
| Mission | Year | Work |
|---|---|---|
| Chandrayaan-1 | 2008 | Mapped minerals; found water on the Moon |
| Mangalyaan | 2014 | Mission to Mars |
| Chandrayaan-3 | 2023 | Moon mission |
Practice questions
- 1What is a space mission?
- 2What is ISRO and what does it do?
- 3Which mission first discovered water on the Moon?
- 4Name the mission launched in 2014.
- 5What is Aditya L1 launched to study?
- 6What is the gap between Chandrayaan-1 and Chandrayaan-3?
- 7NISAR takes 12 days for one round of the earth. How many rounds in 60 days?
- 8A friend says a comet in the sky will bring disaster to the village. How will you reply?
Show answers (try them yourself first)
- 1Sending a developed spacecraft into space to collect information.
- 2It is India's space agency that plans and runs space missions.
- 3Chandrayaan-1
- 4Mangalyaan
- 5The sun
- 62023 − 2008 = 15 years
- 760 ÷ 12 = 5 rounds
- 8It is a natural event; it has no effect on human life, and believing otherwise is a blind belief.
Quick revision
- The sky extends as far as the atmosphere; space lies beyond it and is endless.
- Comets are dust, frozen gases and ice; a meteor is a burning rock; a galaxy has billions of stars.
- ISRO's missions: Chandrayaan, Mangalyaan, Aditya L1 and NISAR; celestial events do not affect our lives.