Consider the following statements:

• Assertion (A): The axis of the Earth is inclined to the plane of the ecliptic at an angle of 66½°.

• Reason (R): All the parts of the globe don't have equal day and night throughout the year.

The correct answer is:

The axis of the Earth is inclined (Correct)The Earth's axis is not perpendicular to its orbital path (the plane of the ecliptic). It is tilted at an angle of 23.5° from the perpendicular. Consequently, it makes an angle of 66.5° ($90° - 23.5°$) with the plane of the ecliptic itself. Reason (R): Unequal day and night durations (Correct)Because the Earth is tilted as it revolves around the Sun, different parts of the Earth lean toward or away from the Sun at different times of the year. In Summer (for a given hemisphere), that hemisphere leans toward the Sun, causing the "Circle of Illumination" to cover more than half of the latitude, leading to longer days. In Winter, it leans away, leading to shorter days. Only at the Equator or during the Equinoxes (when the tilt is sideways relative to the Sun) are day and night approximately equal. 

Why R explains A:

The very phenomenon of "unequal day and night" is the direct physical consequence of the 66.5° inclination. If the Earth's axis were perfectly vertical (90° to the ecliptic), the Circle of Illumination would always cut every latitude exactly in half, and every place on Earth would have equal 12-hour days and 12-hour nights all year round. Thus, the variation described in R exists because of the tilt described in A.