Q. Variations in the length of daytime and nighttime from season to season are due to (UPSC Prelims 2013)
Answer: revolution of the earth on a tilted axis
Notes: The correct answer is [D] revolution of the earth on a tilted axis.This is a fundamental concept in physical geography that explains why we experience seasons and why the length of a day changes throughout the year.Why "D" is the Complete AnswerTo change the length of day and night from season to season, two conditions must be met simultaneously:

1. Axial Tilt: The Earth’s axis is tilted at an angle of 66.5° with its orbital plane (or 23.5° from the perpendicular).
2. Revolution: As the Earth moves around the Sun, this fixed tilt causes different hemispheres to lean toward or away from the Sun at different times of the year.

If the Earth’s axis were perpendicular to its orbit, day and night would be exactly 12 hours each everywhere on Earth, all year round.

• [A] Earth's rotation: This causes the phenomenon of day and night itself (the 24-hour cycle), but it does not account for the variation in their lengths across seasons.
• [B] Elliptical orbit: While the Earth's orbit is elliptical, this primarily affects the distance from the Sun (Perihelion and Aphelion) and has a negligible effect on the length of day and night compared to the tilt.
• [C] Latitudinal position: This determines the length of day at a specific location at a given time (e.g., the Equator always has roughly 12-hour days), but it is a static factor. It doesn't explain why the length changes from season to season at that same latitude.