The variation in the length of daytime and nighttime across seasons is primarily caused by the Earth's revolution around the Sun combined with its axial tilt of approximately 23.5 degrees. As the Earth orbits the Sun, different parts of the planet receive varying angles and intensity of sunlight, leading to the changing lengths of day and night. This axial tilt, combined with the elliptical shape of the Earth's orbit, results in longer days during summer and shorter days during winter in each hemisphere, fundamentally influencing climate and weather patterns throughout the year.