The movement of the earth that causes variation in the length of the day and night is

Step 1: Analyze the question The question asks which movement of the Earth causes variation in the length of day and night.

Step 2: Explain the concepts

• Rotation: The Earth spinning on its axis once every 24 hours. This causes the alternation of day and night.
• Revolution: The Earth's orbit around the Sun, which takes about 365¼ days. Because the Earth's axis is tilted at 23½° and stays pointed in the same direction as it orbits, different parts of the Earth receive different amounts of sunlight at different times of the year. This causes variation in the length of day and night, and also the seasons.
• Traction: Not an Earth movement. It is a term for a pulling or dragging force (for example, glacial or river-bed transport).
• Coriolis: This is an effect of rotation that deflects winds and ocean currents (right in the Northern Hemisphere, left in the Southern Hemisphere). It does not affect the length of day.

Step 3: Work through it

• Rotation alone gives day and night every 24 hours, but if it were the only movement, every place would have equal day and night all year.
• The variation in length (longer days in summer, shorter in winter, such as at higher latitudes) exists only because the Earth revolves around the Sun with a tilted axis.
• Only at the equinoxes (around 21 March and 23 September) are day and night equal everywhere.

Step 4: Answer The movement that causes variation in the length of day and night is revolution, which is option B.

Exam tip: Remember: Rotation = Representing day and night (alternation); Revolution = seasons and Variation in length. If a question uses the word "variation" or "seasons," think revolution plus axial tilt. If it says "day and night occur," think rotation.