Previous Year Question

The seasonal variation in the length of day and night results from the combined effect of Earth’s revolution around the Sun and the approximately 23½° tilt of Earth’s rotational axis. Option (d) represents this combined condition.

Option (a) is incorrect: Earth’s rotation on its axis produces the daily cycle of day and night. The part facing the Sun experiences daylight, while the part turned away from the Sun experiences night. Rotation alone does not explain seasonal changes in the duration of daylight.

Option (b) is incorrect: Earth’s revolution around the Sun is necessary, but the elliptical shape of its orbit is not the principal cause of seasonal variation in the length of day and night. Axial tilt is the decisive factor. Earth is relatively close to the Sun in early January, yet the Northern Hemisphere experiences winter at that time, showing that distance from the Sun is not the principal cause.

Option (c) is incorrect: Latitude determines the magnitude of variation experienced at a location. The variation is very small near the Equator and increases towards the poles. Latitude, however, does not itself generate the annual change; the underlying cause is Earth’s revolution with a tilted axis.

Option (d) is correct: As Earth revolves around the Sun, its axis remains tilted in nearly the same direction. During one part of the year, the Northern Hemisphere is tilted towards the Sun and experiences longer days and shorter nights. About six months later, the Southern Hemisphere is tilted towards the Sun and experiences longer days.

If Earth’s axis were perpendicular to its orbital plane, a given latitude would not experience such seasonal variation in the lengths of day and night.

Therefore, seasonal variation in day and night length is caused by the revolution of Earth on a tilted axis.