UPSC Prelims 2013 Q14: Geography – Astronomy & Earth - Sun System
Variations in the length of daytime and night - time from season to season are due to
- (a) the earth's rotation on its axis
- (b) the earth's revolution round the sun in an elliptical manner
- (c) latitudinal position of the place
- (d) revolution of the earth on a tilted axis
Answer: (d) revolution of the earth on a tilted axis
Explanation
Why day and night vary in length through the year
Two things together cause it:
the Earth's axis is tilted by 23.5 degrees, and the Earth revolves around the Sun.
| Option | Verdict | Reason |
|---|---|---|
| (a) The earth's rotation on its axis | Incorrect | Rotation causes day and night, not their changing length |
| (b) The earth's revolution round the sun in an elliptical manner | Incorrect | Revolution without tilt would give equal days all year |
| (c) Latitudinal position of the place | Incorrect | Latitude decides how much the length varies, not why |
| (d) Revolution of the earth on a tilted axis | Correct | Tilt plus revolution |
Key dates:
| Date | Event in the Northern Hemisphere |
|---|---|
| 21 June | Summer solstice: longest day |
| 22 December | Winter solstice: shortest day |
| 21 March and 23 September | Equinoxes: day and night equal everywhere |
Answer: (d) revolution of the earth on a tilted axis.
Why each option is right or wrong
- (a) Incorrect. Rotation gives day and night, not their seasonal variation.
- (b) Incorrect. The elliptical path alone does not change day length.
- (c) Incorrect. Latitude affects the amount of change, but is not the cause.
- (d) CORRECT. Revolution on a tilted axis causes the variation in day and night.
Key point
Varying day length and seasons: revolution of the Earth with its axis tilted 23.5 degrees. Rotation gives day and night; solstices and equinoxes mark the extremes.
Remember
Tilt + revolution = changing day length.
Common mistakes
- Choosing rotation, which only causes day and night.
- Thinking the elliptical orbit (distance from the Sun) causes seasons.
- Forgetting that at the Equator day and night stay nearly equal.
Exam tip
Rotation = day and night. Revolution + tilt = seasons and changing day length.
Sources
- NCERT Class VI — The Earth: Our Habitat, Ch 3 (Motions of the Earth)
- NCERT Class XI — Fundamentals of Physical Geography, Ch 9 (Solar Radiation, Heat Balance and Temperature)