Why does the length of day and night change with the seasons?

Because the earth is tilted at 23.5° and as it orbits the sun different parts of the earth are nearer to the sun, making it seem higher in the sky or lower in the sky. The closer you are to the the equator, the less of a difference there is in the length of the day throughout the year. By the December 21st, the northern hemisphere is tilted as far away as it can be, making the days shortest in the northern hemisphere and longest in the southern hemisphere. by June 21st, the northern hemisphere is tilted at the maximum towards the sun and the longest days are in the northern hemisphere.