Source: The Earth: Our Habitat – NCERT Class VI Geography, Pages 18–22 Class: VI
Two Types of Motion 🌍
| Motion | What It Means | How Long It Takes | What It Causes |
|---|
| Rotation | Movement of the earth on its own axis | About 24 hours | Day and night |
| Revolution | Movement of the earth around the sun in a fixed path or orbit | 365¼ days (one year) | Change of seasons |
🔑 The fixed path is called the orbit; the plane formed by the orbit is the orbital plane.
🔑 The earth's axis is an imaginary line inclined at 66½° to the orbital plane.
Rotation and Day and Night ⚙️
- The earth rotates from west to east; one rotation = 24 hours = one earthday (daily motion).
- Due to the earth's spherical shape, only half gets sunlight at a time – the lit half has day, the dark half has night.
- The circle of illumination divides day from night on the globe; it does not coincide with the axis.
What if the earth did not rotate? ⚠️ One side would have endless day and heat; the other endless darkness and cold – life would not have been possible.
Revolution and the Leap Year ⚙️
- Revolution is the movement of the earth around the sun; it takes 365¼ days (one year).
- The saved 6 hours accumulate to 1 extra day every 4 years, added to February (29 days) = leap year of 366 days.
- The earth moves in an elliptical orbit and stays inclined in the same direction throughout.
🔑 Seasons are caused by revolution + fixed axial inclination – together, not separately.
Seasons 🌍
Seasons (summer, winter, spring, autumn) change as the earth's position around the sun changes.
4.1 Summer Solstice (21 June)
- Northern Hemisphere tilts towards the sun; direct rays fall on the Tropic of Cancer.
- Northern Hemisphere: summer – longest day, shortest night; polar areas near Arctic Circle get ~6 months of daylight.
- Southern Hemisphere: winter – nights longer than days.
- This position = Summer Solstice.
4.2 Winter Solstice (22 December)
- South Pole tilts towards the sun; direct rays fall on Tropic of Capricorn (23½° S).
- Southern Hemisphere: summer – longer days, shorter nights. Northern Hemisphere: winter.
- This position = Winter Solstice. (Christmas falls in summer in Australia.).
4.3 Equinox (21 March and 23 September)
- Direct rays fall on the equator; neither pole tilts towards the sun.
- Whole earth experiences equal days and equal nights – this is called an equinox.
Common Confusions ⚠️
| Confusion | Clarification |
|---|
| Rotation vs Revolution | Rotation = spinning on its own axis (causes day/night). Revolution = going around the sun (causes seasons). |
| Earthday vs Year | Earthday = time for one rotation = 24 hours. Year = time for one revolution = 365¼ days. |
| Summer Solstice vs Winter Solstice | On 21 June, the Northern Hemisphere has summer (longest day). On 22 December, the Southern Hemisphere has summer (Northern has winter). |
| Equinox vs Solstice | Equinox = equal day and night everywhere (21 March, 23 September). Solstice = one hemisphere has longest day, the other has longest night. |
| Leap Year confusion | A leap year has 366 days, not 365. The extra day goes to February (29 days, not 28). |
Memory Aids 💡
| What to Remember | Mnemonic / Trick |
|---|
| Rotation causes day and night | 💡 Think: a spinning top Rotates in one spot – just like the earth spinning gives us day and Rising (sunrise) and setting sun. |
| Revolution causes seasons | 💡 Think: the earth goes on a long Road trip around the sun – the changing scenery on the trip = changing seasons! |
| Summer Solstice = 21 June = Northern Hemisphere summer | 💡 "June = Joyful summer up North!" – the North leans in to get more sun in June. |
| Leap year = every 4 years = 366 days | 💡 Every 4 years, February takes a leap and gets one extra day to catch up! |
| 66½° inclination of earth's axis | 💡 The earth stands a little tilted – not straight, not too tilted – just 66½°, like someone leaning slightly to one side while standing. |
Exam Corner 🎯
Directly from The Earth: Our Habitat – Exercise Questions (Pages 21–22):
| Question | Topic It Tests | Reference |
|---|
| Q1(a) What is the angle of inclination of the earth's axis with its orbital plane? | Angle of inclination = 66½° | Page 18, Ex. Q1a |
| Q1(b) Define rotation and revolution. | Definitions of both motions | Page 18–19, Ex. Q1b |
| Q1(c) What is a leap year? | Leap year definition and why it occurs | Page 19, Ex. Q1c |
| Q1(d) Differentiate between Summer and Winter Solstice. | Comparison: date, position, hemisphere affected | Pages 20–21, Ex. Q1d |
| Q1(e) What is an equinox? | Equinox definition, dates, equal day/night | Page 20, Ex. Q1e |
| Q1(f) Why does the Southern Hemisphere experience Winter and Summer Solstice at different times than the Northern Hemisphere? | Fixed axial inclination – when one hemisphere leans in, the other leans away | Pages 20–21, Ex. Q1f |
| Q1(g) Why do the poles experience about six months day and six months night? | Axial inclination during revolution – pole facing sun for half the orbit | Page 20, Ex. Q1g |
| Q2(a) The movement of the earth around the sun is known as – (i) Rotation (ii) Revolution (iii) Inclination | Revolution = movement around the sun | Page 21, Ex. Q2a |
| Q2(b) Direct rays of the sun fall on the equator on – (i) 21 March (ii) 21 June (iii) 22 December | 21 March (and 23 September) – Equinox | Page 20, Ex. Q2b |
| Q2(c) Christmas is celebrated in summer in – (i) Japan (ii) India (iii) Australia | Australia – Southern Hemisphere has summer on 22 December | Page 20, Ex. Q2c |
| Q2(d) Cycle of the seasons is caused due to – (i) Rotation (ii) Revolution (iii) Gravitation | Revolution | Page 19, Ex. Q2d |
| Q3(a) A leap year has ___ number of days. | 366 | Page 19, Ex. Q3a |
| Q3(b) The daily motion of the earth is ___. | Rotation | Page 18, Ex. Q3b |
| Q3(c) The earth travels around the sun in ___ orbit. | Elliptical | Page 19, Ex. Q3c |
| Q3(d) The sun's rays fall vertically on the Tropic of ___ on 21st June. | Cancer | Page 20, Ex. Q3d |
| Q3(e) Days are shorter during ___ season. | Winter | Pages 20–21, Ex. Q3e |
Abbreviation Legend
| Abbreviation | Full Form |
|---|
| NH | Northern Hemisphere |
| SH | Southern Hemisphere |
| N.P. | North Pole |
| S.P. | South Pole |
