Why do days change in length?

Days (the amount of sunlight) change in length because the orientation of earth's axial tilt in relation to the sun changes throughout the year. Take the extreme cases of the north and south poles. At the summer soltice in the north, the north pole is experiencing daylight throughout the day, and the south pole is experiencing days on end without sunrise. As you move from the poles toward the equator, the effect at both hemispheres lessens little by little (for example, the farther south you are between the arctic circle and the equator the shorter your period of daylight; in the south the farther north you go between the antarctic circle and the equator the longer your period of daylight) until at the equator, every day throughout the year is almost exactly half sunlit and half with the sun below the horizon. If you refer to solar days, the exact length of time between two successive transits of the sun (where the sun is highest in the sky on a given day) the solar days are changing length a little throughout the year, and this is complicated to explain. Greenwich Mean Time, now more commonly referred to as Universal Time (UTC) has been defined to give the average length of a solar day as 24 hours. To make matters a little more complicated, the velocity of earth's rotation has changed very slightly since the original standards were set, so there is a need to add a leap second to the year now and then. Few people in the general population are aware of these leap seconds, or their accurate interpretation.