Apparent Movement of the Sun
From a moving train trees seem to run backward but they do not move. In the same way the sun does not move; our earth does.
The sun appears to shift north or south in a year; this is the apparent movement of the sun. In reality the sun does not move anywhere.
Key idea 1 : Apparent movement — The place of sunrise on the horizon changes daily and in a year seems to shift north or south. In reality the sun does not move anywhere, hence it is called apparent movement. The earth’s revolution and the tilt of the earth’s axis are the two causes of this appearance.
Key idea 2 : Dakshinayan and Uttarayan — From 21 June to 22 December the place of sunrise keeps shifting south; this period is Dakshinayan. From 22 December to 21 June the sun keeps shifting north; this is Uttarayan.
Key idea 3 : Two kinds of apparent motion — In science we study the east-west daily apparent motion of the sun, due to rotation. In geography we study the north-south annual apparent movement, due to revolution and axial tilt. In both the sun only appears to move. Seasons occur only with reference to the northern and southern hemispheres.
- Easy : Dakshinayan when? — 21 June to 22 December.
- Medium : Uttarayan when? — 22 December to 21 June.
- Exam-type : Cause of daily apparent motion? — Rotation.
In the figure: The sun’s apparent position over the year.
21 June → south; 22 Dec → north.
The sun does not really move north-south; it only appears so.
Memory clue: Dakshinayan = June→Dec; Uttarayan = Dec→June.
Spot the difference
| Period | Name | Sun’s direction |
|---|---|---|
| 21 June to 22 Dec | Dakshinayan | Southward |
| 22 Dec to 21 June | Uttarayan | Northward |
Practice questions
- 1What is the apparent movement of the sun?
- 2What are the two causes of the apparent movement?
- 3What is Dakshinayan?
- 4What is Uttarayan?
- 5State the difference between daily and annual apparent motion.
- 6After 22 December in which direction will sunrise shift?
- 7Does the sun really move? Explain.
- 8In which regions do seasons occur?
Show answers (try them yourself first)
- 1The sun appearing to shift north/south in a year.
- 2Revolution and axial tilt.
- 321 June to 22 Dec; sun southward.
- 422 Dec to 21 June; sun northward.
- 5Daily = east-west due to rotation; annual = north-south due to revolution and tilt.
- 6North.
- 7No; it only appears to move.
- 8In the northern and southern hemispheres.
Perihelion, Aphelion, Equinox and Solstice
Running around a ground we are near a friend standing at one point and far at another. The earth is sometimes near and sometimes far from the sun.
The earth revolves in an elliptical orbit. Perihelion is the nearest position and aphelion the farthest. On an equinox day and night are equal, and on a solstice the sun’s rays fall vertically on the Tropic of Cancer or Capricorn.
Key idea 1 : Perihelion and aphelion — The earth’s orbit is an ellipse with the sun at one of the two centres, so distance is not constant. In the first week of January the earth is nearest (perihelion, about 147 million km); in the first week of July farthest (aphelion, about 152 million km). The difference is not great so it does not affect seasons. Seasons are due to revolution and axial tilt.
Key idea 2 : Equinox — As the earth revolves, on two days of the year the equator receives vertical rays: 21 March and 23 September. Then both poles are at the same distance from the sun and the circle of illumination divides all parallels equally, so day and night are equal (12-12 hours) everywhere. This is an equinox. 21 March is the spring (vernal) equinox and 23 September the autumnal equinox. Equinox or solstice dates may vary by a day.
Key idea 3 : Solstice — When a pole of a hemisphere inclines most towards the sun, the 23°30′ parallel of that hemisphere receives vertical rays. On 21 June they are vertical on the Tropic of Cancer and on 22 December on the Tropic of Capricorn; these two days are solstices. Rays are never vertical on any parallel between Tropic of Cancer and North Pole or between Tropic of Capricorn and South Pole.
- Easy : Perihelion when? — First week of January.
- Medium : Equinox dates? — 21 March, 23 September.
- Exam-type : Vertical rays on 21 June? — Tropic of Cancer.
In the figure: Where vertical rays fall through the year.
21 Mar, 21 Jun, 23 Sep, 22 Dec = E-S-E-S.
It is wrong that distance causes seasons; tilt and revolution do.
Memory clue: Equinox = 12-12; solstice = Cancer/Capricorn.
Spot the difference
| Day | Vertical rays | Name |
|---|---|---|
| 21 March | Equator | Spring equinox |
| 21 June | Tropic of Cancer | Solstice |
| 23 September | Equator | Autumn equinox |
| 22 December | Tropic of Capricorn | Solstice |
Practice questions
- 1What is perihelion?
- 2What is aphelion?
- 3Why does this distance change not affect seasons?
- 4What is an equinox? Write the dates.
- 5What is a solstice? Write the dates.
- 6Why are day and night equal everywhere on an equinox?
- 7Why are rays never vertical north of the Tropic of Cancer?
- 8Where do vertical rays fall on 22 December?
Show answers (try them yourself first)
- 1Earth’s nearest position to the sun (January).
- 2Earth’s farthest position from the sun (July).
- 3Because the difference in distance is small.
- 4Day-night equal; 21 March, 23 September.
- 5Vertical rays on Cancer/Capricorn; 21 June, 22 Dec.
- 6Because the circle of illumination divides all parallels equally.
- 7Because the earth’s axis is tilted 23°30′.
- 8On the Tropic of Capricorn.
Day Length and Polar Regions
In summer our day is long and in winter short. The same happens very strongly near the poles where the sun does not set for months.
21 June is the longest day in the northern hemisphere and 22 December the shortest. From the Arctic Circle to the north pole the sun is visible for 24 hours or more; at the equator day and night are equal all year.
Key idea 1 : Longest and shortest day — 21 June is the longest day and shortest night in the northern hemisphere. 22 December is the longest day and shortest night in the southern hemisphere, and the shortest day in the northern. On 21 June the northern hemisphere gets the most sunlight. At the equator day and night are 12 hours each throughout the year.
Key idea 2 : Polar regions — From the Arctic Circle to the north pole the sun is visible for 24 hours or more. At the north pole the sun is visible from 22 March to 23 September, six months. Between the Antarctic Circle and the south pole the same happens from 23 September to 21 March. In Norway the ‘midnight sun’ is seen in summer.
Key idea 3 : What are seasons — Seasons are decided on the basis of duration of sunlight, equinoxes and solstices. The equatorial region has no change of season so its climate hardly changes through the year. Beyond the equator in either hemisphere summers and winters follow one another. The occurrence of seasons one after another is the cycle of seasons. Generally there are two seasons, summer and winter, but in some places four are taken.
- Easy : Longest day in the northern hemisphere? — 21 June.
- Medium : How long is the sun visible at north pole? — Six months.
- Exam-type : Day length at equator? — 12 hours.
In the figure: Day length at various places.
As per the textbook 21 June is the longest day in the northern hemisphere and 22 December the shortest; in Norway the ‘midnight sun’ is seen in summer. (as per the textbook)
June summer in the north, December in the south.
It only means climate hardly changes there, not that it never changes.
Memory clue: Pole = 6 months day, 6 months night.
Spot the difference
| Place | Day-night | Seasons |
|---|---|---|
| Equator | 12-12 hours all year | No change |
| Beyond tropics to poles | Changes | Summer-winter |
| Poles | Six months day/night | Sharp contrast |
Practice questions
- 1Which are the longest and shortest days in the northern hemisphere?
- 2How are day and night at the equator?
- 3How long is the sun visible at the north pole?
- 4When is the sun visible for over 24 hours between the Antarctic Circle and the south pole?
- 5What is the cycle of seasons?
- 6Why do seasons not change in the equatorial region?
- 7In which season is cricket played in Australia and why?
- 8Compare 21 June and 22 December.
Show answers (try them yourself first)
- 121 June longest; 22 December shortest.
- 212 hours each all year.
- 322 March to 23 September; six months.
- 423 September to 21 March.
- 5Seasons following one another in a year.
- 6Because duration of sunlight is almost same through the year.
- 7December to March, when it is summer in the southern hemisphere.
- 821 June = long day in north; 22 Dec = short day.
Cycle of Seasons and the Living World
In winter we wear sweaters, birds go to far regions and different crops grow. Seasons affect all living things.
Because the earth’s axis is tilted, seasons and diversity occur. The cycle of seasons affects living beings, agriculture and bird migration.
Key idea 1 : If there were no tilt — If the earth’s axis were not tilted the same climate would have prevailed throughout the year and there would be no seasons. Each parallel would have one type of climate. It is the tilt that leads to seasons, change and diversity.
Key idea 2 : Seasons and living things — Even the regions between 66°30′ and 90° have certain plants and animals because of mild sunlight for a part of the year. Antarctica has penguins, seals and walruses; the northern polar region has reindeer, polar bears and Arctic foxes. People there also adapt. In severe cold when food is scarce many birds and animals migrate temporarily. Trees bear fruit in a particular season and agricultural seasons depend on local climate.
Key idea 3 : Four seasons in India — Changes in the atmosphere, vapour, wind and precipitation also influence seasons. India has rain in a specific period so we take four seasons: summer, rainy season, retreating monsoon and winter. Europe and North America also have four: summer, autumn, winter and spring.
- Easy : Where are polar bears found? — Northern polar region.
- Medium : Where are penguins found? — Antarctica.
- Exam-type : How many seasons in India? — Four.
In the figure: Axial tilt and life.
The Arctic tern flies south when winter becomes severe at the North Pole and returns north when summer arrives; as per the textbook it travels about 70,000 km a year. (as per the textbook)
Tilt = seasons = changes in life.
Not all animals can live in the same climate; each prefers a habitat.
Memory clue: Tilt → seasons → migration, farming, habitat.
Spot the difference
| Region | Animals | Reason |
|---|---|---|
| Antarctic | Penguin, seal, walrus | South polar habitat |
| Arctic | Reindeer, polar bear | North polar habitat |
Practice questions
- 1What would happen if the earth’s axis were not tilted?
- 2How does the cycle of seasons affect living things?
- 3Why are penguins not found at the north pole?
- 4Why do birds migrate?
- 5Which four seasons are taken in India?
- 6State the four seasons in Europe.
- 7What do you think is the reason for J&K’s summer and winter capitals?
- 8How do seasons affect agriculture?
Show answers (try them yourself first)
- 1Same climate all year; no seasons.
- 2Affects food, migration, fruiting, farming seasons.
- 3Because they live in a south polar habitat.
- 4Due to cold and lack of food.
- 5Summer, rainy season, retreating monsoon, winter.
- 6Summer, autumn, winter, spring.
- 7Jammu is likelier milder in winter; Srinagar is cold.
- 8Climate decides crop seasons.